Deep Space 845se55      July 2008

Last edited November 2011.
Some minor comments have been clarified.


Photo 1, The two 55W amps on my bench.
After a few months of research and development , I completed construction
of a pair of 55 watt class A monoblocs each using a pair of paralleled 845
triodes for single ended operation.  These amps were a very difficult
handcrafting challenge but I think my discerning customer who waited nine
months will never find any better sounding amplifiers with such natural clarity
and fidelity.  During tests one evening a friend and I were reduced to tears
with some good recordings. Only the best audio gear achieves a real emotional
impact similar to well performed live unamplified music which has always
been my “gold standard” for audio quality.

The amps also have excellent technical performance with wide bandwidth,
low distortion, low noise, and excellent damping factor.


Photo 2.
The first prototype monobloc had all the power supply components and
audio amp components on the one chassis. But total chassis weight
reached 42Kg, and it became extremely difficult to move easily, and parts
had to be crammed together too tightly to allow low chassis temperatures.
So I adopted the same principles I used in my 300W monobloc amps
to have two chassis per mono channel with power supply on one
and audio circuit on the other, with each chassis connected with very
heavy duty umbilical cabling.
This reduced weight problems and I could have all hot running resistors
clamped inside a heatsink on one end of the chassis top instead of
having them air cooled under-chassis, thus the amps stay cool even on
hot summer days. Access to all encased resistors is possible by unscrewing
the heatsink outer fins.

Each audio amp chassis weighs approximately 30Kg and is
520mm deep x 230mm wide x 280mm high.
Each power supply chassis weighs 16Kg and is
280mm deep x 200mm wide x 250mm high.
Total amplifier weight for two channels is about 92Kg.    

Rectifiers are all silicon, and the power supplies always run cool and can be
placed on the floor. The audio amp chassis can be placed on a bench or
equipment stand above the power supplies so the front on/off switch
can easily be reached, and you can keep an eye on the tubes.


Photo 3.
This  shows a close up view with 2 x KR Audio 845 output tubes,
3 x Sovtek EL84 triode drivers, and 1 x NOS AWV 6CG7 input.
Initial tests were done using "expendable" Chinese made 845 Shuguang type B.
These measured well and sounded well during tests.

The amps are made to work with any brand of 845.
The KR Audio 845 have cathodes needing 10V at 1A,
while Chinese 845 need 10V at 3.3A.
Thus KR tubes run cooler than the Chinese types, if they both have the
same anode voltage and anode current at idle, ie, same anode dissipation, Pda.
The KR845 may have a very slightly more detailed sound than the cheapest
type B Shuguang Chinese 845, which I think is the best Chinese 845.
I leave the final judgment on sound quality of KR tubes to other people.
The KR tubes certainly look better made than the Chinese 845 which are
a very close copy of the original RCA 845 and other old ancient brands.
Both measured very similarly low distortion levels, and both adjusted themselves
in my auto biasing circuit with the same biasing voltages and bias currents, and
both gave the same power outputs with the same circuit.
The power supply produces two 10Vdc supplied for each 845 cathode.
There are two choke input type filters, L + C, and there are 3 taps on the
two 13Vac power transformer cathode heater windings so the cathode
heating voltage can be finely adjusted for the correct level by choice of taps.
Chinese 845 require 3.3Adc, the highest current ever likely for any 845,
therefore the highest Vac tap is used to obtain 10Vdc. The KR may use
the lowest voltage tap for the lower heater current.

RCA or other brands of NOS are so rare now that it’s pointless trying to find any.
Because the KR tubes use 20 watts less to heat their cathodes, they
could be idled at a full 100 Watts of anode dissipation. But the KR 845
are more than 3 times the price of the Chinese types so I have set up the
KR to run at about 75 Watts of anode dissipation to ensure long tube life.
See the notes about the operating conditions below.


Photo 4.
The amp has its top cover removed in my workshop. You can see
the hand made heatsink to the left which encloses all the main hot
running resistors. Moving right you see a solenoid choke for the
CLC filter for the dc power applied to all input and driver tubes.
Under the solenoid are two potted chokes for the two choke input dc
cathode heating in each 845. In the center back is a large potted output
transformer with 72mm stack of 51mm GOSS E&I lams.
Center front shows some of the 470uF filter caps in main anode
supply rails and a 60H choke which is part of the 36mA dc anode
supply to the 3 x EL84 working in parallel triode mode.
The 845 'Johnson' tube sockets are recessed and there are 4
McMurdo 9 pin sockets for smaller tubes. White labels with
black lettering is used to indicate what goes where to avoid people
wearing and cursing because they cannot read tiny lettering in the gloom
of a listening room.


Photo 5.
"Beneath the bonnet"of each audio amp chassis, top left, you can see the
entry and terminations of the incoming umbilical cables. Towards the
right is a heatsink for three diode bridges for dc heater supplies,
then rail discharge resistances, then underside of Johnson tube sockets,
and far right you see the compact wiring of the 4 tube sockets for input
and driver stages. At bottom left is the active protection board to shut
down the amp if an output tube draws excessive anode current, ie,
malfunctions. Towards the right there is 60,000 uF in four caps for
845 heater filtering, then rail discharge resistances.
All wiring is genuine point to point wiring with mainly hardwood
terminal strips well sealed with varnish. Hookup wire has 1mm thick
PVC insulation and is multi strand copper chosen for very long term
reliability and with very generous current ratings and much with an
additional shrink wrap layer of insulation added where voltages in
wires are over plus or minus 500Vdc relative to 0V.

Right in the middle are two rows of terminals which allow a tech to
reconfigure the output transformer secondary to suit loudspeakers of
either 3-6 ohms or 6-or-more-ohms. Speakers above 6 ohms may be
used with the terminations set for 3-6, given excellent fidelity but
reduced power ceiling. 95% of listeners would find the 3-6 load match
to be excellent for any kind of speaker ever made, as long as 25 Watts
was enough power required.


Photo 6.
Here is the under chassis view of each power supply with its bottom
cover removed. There is a small 5VA auxiliary mains transformer, top LHS.
This supplies 12Vdc power to the active protection and dc turn on/off circuit.
The power transformer has a board on the RHS with 48 terminals for taps and
ends of windings. The range of voltages available allow for the use of many
different output tubes and configurations in future if 845 become scarce or
unavailable. Suitable alternative tubes are 4 x KT90 in parallel SEUL, using
the same output transformer but with a different winding arrangement on
the primary. Or two x 13E1 can be used, or various push pull arrangements.
So if ever there are no 845, the amps can be altered by a skilled tube technician,
and sonic purity and great sound can be maintained without compromise.
Instead of recycling the amps at the scrap metal dealer, if all the transformers
remain intact the amps can re-built into something else, and even a six pack
of EL34, 6L6GC, 5881 could be used.

But in 20 years, I may not be around, and the kind of willing and highly
skilled tradesmen like myself may not be available.


Photo 7.
This workshop picture shows the rear of an audio chassis (left) beside
its power supply (right). It would be difficult to make a mistake with
the umbilical cable plugs because one is painted bright red, and so is
its socket. If the absent minded audiophile makes a mistake by
reversing the plug positions, the amp cannot be turned on and no
damage is sustained at all.

On the amp chassis there is provision for bi-wiring or having two
speakers into the 4mm bind posts which are glued into a plywood
block to protect them from breakage, and ensure that connections
to speakers are only possible with leads that have 4mm banana plugs.
Binding posts that rely on a wire poked through a hole in the post and
then with a knob turned tight are an unreliable connection and possible
sonic horror so I won’t let anyone use them with my amps.
Quad thought the same way in about 1952 with their Quad-II amps
which sold in large numbers. Input socket is a Cardas RCA input for
a single ended input only, shown recessed on the left.

On the amp rear side to the right, the umbilical cables emerge and are
soldered into the amp to avoid connection confusion,
and the problem of having too many plug and socket connections.

Cable length is 1.5metres, so the power supplies may sit on the floor,
out of sight, out of mind, and away from any other gear, while the
audio chassis may be on a bench 900mm high to allow easy access
to the on-off switch, and to keep an eye on the tubes.

The rocker type on-off switch is recessed to avoid damage, and it
switches low voltage 12Vdc. The actual mains switching is done with
relays within the power supply. Thus 240V mains wiring is not brought
into the audio amp chassis and so there no diode switching noise spikes
or hums from where the on-off switch is so closely situated to the
audio input circuits.

Schematics.
I don't mind publishing schematics for free on line. People think I am crazy.
But nobody is going to copy what I have done and make a profit because
these amps will have a cost of production far in excess of most cheap nasty
toy like amplifiers which one can inspect around the Internet. The few
other manufacturers who supply similar power levels of Single Ended
triodes dare not publish the full details of their amps lest the secrets about
their shortcomings be displayed to everyone. They don’t want you to know
how they managed to get the cost of production to down to a very tiny
fraction of what they want you to pay. Nobody will copy my designs
of amps and be able to make a profit without employing a bean counter
to remove the quality to cheapen the cost of production, thus stealing and
ruining your music, and lessening the amp reliability.

I have never obtained a review from magazines such as Stereophile,
nor paid the huge sums to advertise in that magazine, and I make only
low volume productions. So copy cat makers are never going to copy me,
because they cannot offer a fake amp at a low price because the design
has become world famous and priced way above most peoples' ability to pay.

Warning. There are high voltage potential differences
of up to 2,000V within each amplifier when operational.
Only trained and experienced technicians should attempt
to examine the working circuits or build the circuits
shown in the schematics.


Sheet 1.
Input signals enter the input V1 6CG7 with both halves paralleled.
There is a high pass CR input filter with C1-R1 to give a pole at 5Hz.
This keeps out dc in sources and extremely low frequency signals.
The MJE350 transistor might seem to be quite out of place in a tube circuit,
but it acts as a passive component which supplies V1 anodes with a non
changing current or what is called a constant current source, CCS.
The CCS acts in a manner identical to an equivalent resistance of several
megohms to an imaginary supply voltage of thousands of volts. Such a
resistance and supply are utterly impractical, and not needed. It would
have been possible to use a pentode to perform the same function as the
MJE350, but in this case the MJE performs better than any tube.
Because the effective real dynamic collector resistance is so high, it cannot
impart any sonic signature in the signal path, apart from allowing V1 to
operate with less THD than if it had a "normal" resistance between the B+
and anode.
So with the MJE350, V1 anode load is effectively only the following cap
coupled biasing resistance R11, 180k. The Ra of V1 is about 5k0, and the
RL is 36 times greater, and when triodes are loaded with RL > 20Ra,
they give the best sound, and the lowest possible distortion measurements.
If you were to replace the CCS with a simple resistance of about 39k,
THD/IMD would maybe increase 3 fold. The THD of V1 is mainly all 2H,
but it will add to that of the output stage because both V1 and output stages
have the same phase of the 2H, so to minimize overall THD, the CCS
transistor helps to lower overall distortion, and maximize the voltage gain of V1.

Any brand of 6CG7 may used, and my favorite is Siemans NOS made in
Germany followed closely by NOS versions made in Australia before 1965.
Genuine NOS Siemans are hard to find and ruinously expensive.
The expense is because they have become rare, and not only because
they have a good sound reputation.
I suggest the Australian made AWV are very "fine wines" indeed, and
those who detect the German tubes are better might be drawn to that
conclusion because they paid more for them, and in an AB blind
comparison they might be surprised.

For greater input sensitivity, 6922/6DJ8 could be used for V1.
One would have to use 2k7 R4 grid resistors at each grid because the
6DJ8/6922 does tend to oscillate at around 200MHz if you parallel the two
halves without using two separate series grid R "stoppers".
The cathode biasing resistor, R5, would need to be reduced in value
until Ea measured about +120Vdc.

The 6CG7 is an evolution of the famous octal based 6SN7. Oz made samples
often used exactly the same anode, grid and cathode structures but just mounted
slightly closer, and a slightly lower anode Pda rating given for the smaller 9 pin size.
There was often a screen also fitted between each anode taken to pin 9 and 0V.
The 6CG7 technical character is identical to 6SN7 and it ensures the audio signal
is amplified very linearly, while maintaining excellent musicality, micro detail and
warmth, transparency etc that one enjoys with the best tubes when set as I show.
German 6CG7 and some Japanese 6CG7 were made with smaller anodes than Oz
mades or 6SN7, and had slightly high µ than Oz versions. But anything labelled
6CG7 will function well in a circuit designed for them.

8dB of global NFB is applied from the output transformer through R3&C2
to the top of R6.

C6, C7, R8, and R11 form a LF gain stepping network to optimize the LF stability
and fidelity.

V2,3&4 are triode connected EL84, each with individual cathode biasing.
Any brand of EL84 may be used, and you could have 3 different brands
together if need be because they each have their own cathode biasing network.
I've fitted Sovtek which sound well, and mixing up brands or using
3 x NOS EL84 may or may not make a change. Three are used to produce
what becomes ONE super triode with the ability to produce a maximum
of 164Vrms of signal output with less than 2% THD, and with good gain,
and wide bandwidth, and with combined Ra = 700 ohms only.
The use of a 60H choke plus 7k0 to supply Ia = 36mAdc to the 3 x EL84
provides a high ac impedance anode supply load which dissipates an extremely
small amount of ac power, so hence the excellent linearity, because like V1,
RL is many times Ra, and RL approaches a CCS.
The load seen by the 3 x EL84 is approximately 20k, and mainly due to
the grid biasing R28&R29 of 23.5k of the two following 845 grids.
Again, RL = 28 x Ra, thus ensuring minimal THD&IMD and maximum
fidelity.

There is zener diode shunt regulated Vdc for V1 supply to assist LF stability.
Any noise in the zeners is filtered by R21 and C9, and the CCS MJE350
prevents any other noise entering V1 anode circuit.


Sheet 2.
Each 845 is set up in conditions as follows :-  Ea = +1,060Vdc, Ia = 70mA,
cathode bias voltage = 150Vdc, RLa per tube = 12k0,
and so for both 845, the OPT load is 6k0.

The maximum drive voltage to 845 grids for clipping is up to approximately 110Vrms containing
1.4% 2H from the driver stage. The driver stage anodes applies the drive voltage to the network
of C16,17,18 and R23, 24, 25, 28. This network transfers the signal safely from the EL84 anodes
at +310Vdc to the 845 grids at -600Vdc.
Coupling caps are 2.2uF each and each rated at 1,000Vdc and the LF pole is at 9Hz.

The 845 anode current is supplied from two rails, one at +600Vdc, and the other at -624Vdc.
This unusual arrangement reduces the likelihood of arcing within the OPT between anode
windings and earth potential secondary windings. The filtering of the two rails is by
C-LR-C-R-C set up to give a damped LF pole. Many tests were done to ensure the
continual mains voltage level changes and LF switching noise caused by other users
connected to the mains supply does not create LF resonance signal which then appears
between grid and cathode of the output tubes, and therefore does not appear in the output
in excess of 0.5mV, even if mains noise level change is very  bad.
The overall noise performance despite the twin rail use is extremely good.
Despite so little global NFB these amps were the quietest tube amps I have ever built.

Two supplies of 10Vdc are applied to 845 cathodes XX and YY so that the small amount
of 24mV of residual 100Hz noise is balanced by R30&31, 32&33.


Sheet 3.
This shows the three simple heater dc supplies used for ALL tubes within the amp.
The L3, L4 chokes used in the choke input dc supplies for the 845 cathodes are
potted and do not cause any magnetic interference in the potted OPT on the same chassis.
L2 is a solenoid type of choke in a CLC filter, so the Vac across the choke is tiny,
and thus its change in magnetic field is negligible, so potting was not needed.


Sheet 4.
The main power supply chassis have all the above within to generate the
positive and negative Vdc rails for the 845 and other tubes, and 12Vdc
for relay switch on and protection circuits. All the ac cathode heating
voltages from the power transformer for the 845, EL84, and 6CG7
are conveyed in the umbilical cables to the amp chassis where they
are rectified to dc. Despite all heater rectifiers being on the audio
chassis, there is no resulting diode noise in the output. See notes
below sheet 6 about power transformer and iron core component
replacement.

A large number of rail voltage arrangements are possible.

FUSES.

Caution! The amp must be turned off and mains cables removed
from wall socket, and allowed 15 minutes rest after turn
off before changing any fuse !!!!!!!!!!!!!!!!!!!!!!!!!!

Fuses or fuse wire links for all windings are as follows:-

F1, Mains input, for 220V, 230V, 240V, 3A slow blow, type 3AG,
and accessible at rear of PSU by owner.
For 100V, 110V, 120V operation, mains fuse is 6A slow blow type 3AG.

All fuses below may only be replaced by a technically trained person.

F2, F3, F4. Three fuse wire links rated for 10A soldered into the underside
of the psu chassis and covered with black polyester sleeving for 2 x 845
ac cathode heater windings and one other cathode heater winding for
3 x EL84, and 1 x 6CG7.

F5, F6. Two x 3A slow blow, 3AG, soldered into place under psu chassis
for the two main HT rails of +660Vdc and -640Vdc, derived from voltage
doubler rectifiers.

R58 Provides some protection for the auxiliary small power transformer
under the psu chassis. This 1 watt resistor will burn out if the auxiliary
transformer is shorted.

845 Anode fuses, There are two 0.5A slow blow 3AG fuses
soldered between the bottom of R36 and each 845 anode in case
the anode current exceeds 0.6Adc.


Sheet 5.
Most tube power amps don't have any kind of active protection
against the unavoidable and inevitable eventual failure of one or more
output tubes during what is called a "bias failure" event.
I have had to repair very many "good" hi-end brand amplifiers that gave
a lot of trouble due to poor design, or through mishap caused by owners,
or malfunctioning speakers etc, etc, etc.
To avoid smoke in the listening lounge room, and collateral damage to
other parts within the amp, most amp makers do fit a couple of fuses.
But fuses provide only partial protection, and they don't always blow
when one wants them to, and owners are notorious for replacing them with
something convenient like aluminium foil from the kitchen or using a
2A fuse instead of 0.2A which encourages a faulty amp to burn the house down.

Active protection is needed to stop the smoke and damage and to
tell an owner when something is wrong, and if possible to shut down
the amp and prevent fuses blowing.

Nevertheless, there are a fair number of fuses fitted in these amps
and apart from the mains fuse they are all soldered into place because
fuse holders are notorious for not holding a fuse firmly and becoming
loose over time, and thus becoming intermittent especially with dc flow.
Fitting new soldered-in fuses is a painful exercise requiring a tech with
a soldering iron. The most likely problem in any tube power amp is
the sudden or gradual unwanted increase in the idling dc current flow
in each output tube. This current is usually called the anode bias current,
and it is controlled by the voltage between the grid and cathode.
But a tube can change its character as it ages or during some trauma
such as caused by a shorted speaker cable, and despite the biasing
voltage Vg-k, the bias current may increase to many times the idle value,
with dire results if not dealt with ASAP, within seconds.
The above simple circuitry will shut down the amp in 90% of bias failure
or tube failure cases, and if the failure was caused by some temporary fault,
it may be easily reset to go again simply by switching off, then on again as
explained in the text in the schematic above. Under normal operation, the 845
anode current is around 70mAdc at idle. This generates 150V across cathode
R34&R35. Should the anode current ever rise to about 102mAdc, there will
be a cathode bias voltage of 225Vdc, which means the Pda will have risen to
about 100W, and although this is the maximum rated Pda for 845, its plain
wrong in these amps and due to a fault condition. So to give the fault condition
it only takes an Ia increase of 32mAdc, or 45%, and you cannot rely on fuses
to blow with such a small amount of current change, so active protection is
the only reliable way to prevent Ia rising to perhaps 400mAdc in this amp.
400mAdc would damage the cathode bias resistors, and perhaps the OPT
primary winding if the condition lingered for too long, and I have seen this
happen in many amps brought to me for repair.
The protection circuits have utterly no effect on the sound.


Sheet 6.

The power transformer does not run very hot because the turns per volt ratio
gives a B-max of less than 0.9 Tesla, and wire sizes are generous and rated
for no more than 3 Amps per square mm. of copper wire section area.
The transformer is neatly layer wound with layered construction shown in the
drawn section through the winding bobbin.

I give a two year warranty on amp transformers. But if one were to fail,
and I was not around in future then there are no standard easily available
replacements for any iron cored wound components in these amplifiers from
any known commercial transformer winder.
All are custom wound and may have to be ordered as a special order from
perhaps Sowter Transformers located in the UK. Sowter would be the only
transformer maker I know who could produce a power transformer to
do fairly close to what is done by my originals. But I doubt they would like to
include the the many taps for alternative tube usage. Most other commercial
transformer winders HATE TAPS anywhere; they just cannot cope with the
routine levels of complexity and thought I put into all my work.
OR one might rebuild the two power supplies on ONE new chassis using a
larger single PT rated at 1.2KVA. This sounds like a lot, but it means the
single transformer would have the same core lamination tongue size of 51mm,
but have a GOSS stack height of 100mm instead of the 72mm now used.
Wire sizes would be thicker, but fewer turns per volt are used, and it’s
no more difficult to wind than winding a pair of trannies with cores now
rated at 650W for cool running.

An alternative is to rebuild the power supply on a new chassis to allow the
use of multiple power transformers which may be available as trade stock
from Hammond Engineering, through the Australian dealers, EVATCO in
Queensland, and thus avoid custom winding any transformers.
Other toroidal transformer alternatives are possibly available from Tortech
in Sydney but I cannot say that the Hammond wound in Canada or toroids
wound in Australia will be silent running like the existing transformers
because their de-fault Bac = 1.2Tesla, and these products are NOT potted.

In the event that a transformer is damaged and cannot be used, it is possible
for a suitably trained tech to disconnect it from the circuit, unscrew the
external screws, and remove it off the psu chassis.

The terminal board and top sealing layer of resin and sand concrete seal can
be removed with chisel and hammer and remaining loose dry sand surrounding
the transformer can be drained out. The transformer should be able to be
removed from its pot and thus the pot can be re-used.
To dismantle the wound transformer all bolted angles and bolts are all removed.
The grain oriented silicon steel core can be salvaged by heating the transformer
in a wood fire to a dull red to vaporize all plastics in the construction,
then allowing it to cool down for a few hours. The burnt wire is cut away
for re-cycling, and laminations should all fall easily apart and will be ready
for re-use. The heating will NOT affect the core magnetic properties.
A new transformer is then wound using a new plastic bobbin to suit a
70mm stack of 51mm tongue laminations so that it will fit inside the pot.
The newly wound transformer must be varnished while being wound or
after with a soak and bake method. A new terminal board is made and fitted.
After testing the new transformer it is re-assembled back into its pot and clean
dry sand used to fill the pot except for the last 15mm. The sand must be
thoroughly vibrated and settled. Spray-can varnish is applied to the sand surface
and next day the top 15mm of fill can be done using a 50-50 mix of epoxy
(fibreglassing) resin and sand. This seals the pot and prevents dry sand running
out. The spray varnish prevents liquid resin soaking into dry sand below.
The completed tranny is re-installed into the chassis and connected up and
tested.
At present, nobody I know in Australia has the competence
to do such work on transformers except the gentleman who
made the amps.

Once outside the warranty period, transformer replacement is expensive
but I think you may find I am cheaper to employ than the hi-end makers
when a power transformer fails.

The power transformers and other wound components such as chokes and
OPT have been designed to run cool, and all windings have fuses, and active
protection is used against output tube failure.

After winding so many power and output transformers and chokes during
the last 12 years of commercial operation, not one has failed,
so I have never needed to repair any of my work.

Much time has been spent on making the iron cored items in these amps.


Sheet 7.
The output transformer is layer wound and varnished with Wattyl 7008
polyurethane two part varnish generously applied to windings as the
transformer was wound. It is potted in a zinc plated sheet steel pot and
filled around with dry sand or roof pitch as chosen.
See the notes above about transformer replacement.
The output transformer is more difficult to wind than the power
transformer because of the high number of layered fine wire turns around
the large size core size.
There is no known ready made replacement type available from any
commercial winding specialist although one may possibly be ordered
as a special from Sowter Transformers located in the UK.

The dynamic anode resistance of the two 845, Ra, in parallel,
is 1,100 ohms and when this Ra is in parallel with an anode load of
6,000 ohms, the source resistance = 930 ohms.
Primary inductance is over 40H at 150mA dc current.
The -3dB response drop at LF and due to primary shunting inductance
occurs at 4Hz at loud levels normally used.
At the 50 Watt output level the onset of core saturation occurs just under
20Hz.
At 50W, the HF -3dB point is above 30kHz, all with zero global NFB.
Some of the slight sag in HF is due to the stability network of R37&C23
beginning to load the amp above 50kHz. Shunt Capacitance from the
primary anode terminal No1 to the secondary is less than 3,000pF.
Leakage Inductance has slightly less attenuation effect than the shunt
capacitance, and the resonance between leakage L and Shunt C is
above 30kHz. Primary winding resistance = 95 ohms for 2,760 P turns,
using 0.45mm dia copper wire, about 0.52mm oa dia with enamel.
Secondary winding resistance = 0.124 ohms for 5 x 72 turn secs in parallel,
0.9mm Cu dia wire. Sec winding resistance referred to primary = 182 ohms,
so total Rw = 95 + 182 = 277 ohms at the primary input.
Winding loss percentage = 100% x 277 / 6,277 = 4.44% with 6,000 ohms
anode load with the OPT secondaries set up for 4.1 ohms or 6.4 ohms.

LOAD MATCHES AVAILABLE.
5 x 72 turn parallel secs give 72 turns to match for 4.1 ohms,
allowing speakers nominally above 3 ohms.
4 x 90 turn parallel secs give 90 turns to match for 6.4 ohms,
allowing speakers nominally above 6 ohms.
4 x 72 turn parallel secs in series with 2 x 36 turn parallel secs
give 108 turns to match 9.2 ohms, allowing speakers nominally
above 8 ohms.

The signal current density in every secondary winding remains equal for
the first two arrangements of secondaries but slightly higher for the 9.2 ohm set up.
With such a high maximum power of 55 watts into 4 ohms, there is usually
enough power for any speaker regardless of its nominal impedance.
The 6.4 or 9.2 ohm setting should only ever be used if speakers have under
86dB sensitivity, 1W, at 1M and have impedance above
6 ohms and 8 ohms respectively.

All speakers including ESL types by Quad such as the ESL57, ESL 63, 989,
2805 should be tried with the 4 ohm setting before committing to the trouble of
changing the setting to a higher one.
Many older high impedance speakers of say 16 ohms are much more sensitive
than more modern types, and therefore require a tiny amount of signal voltage
and power to play very loud, so there is no need to change the impedance
matching to the 9.2 ohm setting, and the 4 ohm setting will be very adequate.
Most people will never use more peak power over 10 watts with average
power well below this figure, often only 0.5W per channel. Because the amp
measures so well at 55 watts, at an average power of 2 watts the distortion
is well below audibility, see the notes below.


Sheet 8.
Six chokes per channel are used for filtering and to prevent the heat
losses using alternative methods of filtering or active regulation.
No solid state chip regulators are used because they become
unreliable when used in circuits with such high voltages lurking about.
There are a few simple zener diodes for basic shunt regulation, but used
so they are under no heat stress. The main positive and negative voltage
rails of over +600V and below -600V have a C-LR-C-R-C type of filter.
Each C is formed with 2 x 470uF in series to make 235uF, and total C
per rail = 705uF. There is a resonance between the 4H choke and following
235uF at 5.2Hz, but the added 100 ohms in series plus the following
additional 100 ohms plus 235uF act to damp the peak in the resonance,
thus making each voltage rail less liable to vary at LF below 10Hz
due to the unavoidable mains voltage level changes that occur continuously,
and typically of +/- 20mV up to +/-200mV.
I have found the steady average value of mains voltage can be from 235Vac
on a cold winter evening to +255Vac on days with little mains load, with
247Vac being most common here in Canberra.

Many high end brand amps have been designed to run in the USA with a
nominal 110Vrms mains or 220Vrms. Many such amps have fixed bias
and tubes run unnecessarily hot even with the correct mains voltage present.
But here the mains voltage is often 250Vac, and with fixed bias these amps
often overheat badly when tube Pda rises close to or above the rated maximum
Pda. Jolida and ARC amps suffer in this manner badly, and need to have
alterations made to their PSU to prevent the B+ from exceeding the capacitor
voltage rating and to stop the tubes exceeding their Pda levels. Many owners
have thanked me for the efforts I have made in this regard.
But in my amps, there are tapped windings to allow for worst case voltage
variations.

The SE55 work just fine where mains = 250Vac, but even if mains = 220Vac, they
will still work fine even though Ea and Ia in tubes is reduced and Pda is reduced
thus giving 10% less maximum power.


Sheet 9.
This shows the arrangement of resistors used within the aluminium
heatsink at the rear end of the amp chassis. Cheap ceramic bodied wire
wound types are used with their dissipated power being well below
the rating for the resistance chosen. These are readily and cheaply
available from many suppliers. A total of about 50 Watts is dissipated
in the resistors shown, and to keep them all cool and thus more reliable,
the heatsink was built up to enclose the resistors behind a removable
cover fitted with many fins and which springs tightly against the
enclosed resistors. They are glued to the 3mm thick aluminium
heatsink fixed plate with Selleys 401 engineering grade silicone with
a temperature rating of 200C. The screwed cover can be removed by
removing the 9 x 4mm metric machine screws. White heatsink paste
is used between resistors and the cover. While it is possible than
resistors might fail, my experience tells me it is not likely that they
will fail, unless the current flow is 10 times the value in them as used.
Resistors usually fail by fusing open, and they may be prized off the
heatsink with a chisel, and a new one fitted. It is a messy job to
replace any resistors, and a tech needs to know what he is doing,
but at least the resistors are easily accessible once exposed to view.
While it may have been wiser to use much more expensive aluminium
bodied resistors each screwed to a heatsink, there was no need, and the
method used is entirely adequate, and any fused resistors are cheap to
source and easy easy to replace.


Sheet 10.
Here there is the layout for rugged cables used to get power from the power
supplies to the amp chassis. When the amp chassis are examined with
a copy of the above, just exactly how everything is set up becomes
less confusing.
The octal plugs at the ends of cables are permanently connected.
All wires are soldered into the hollow pins of the plugs.
There is no access to the wire ends leading into the hollow pins of the plugs.
If a pin is broken off a plug, the whole plug is made useless. The only solution
is to cut the plug off, and rewire a new octal plug onto the lead as shown above.
A cheap type of octal plug from RS components could be used to make a new plug.
The original plugs were made using only the bottom base from an 8 pin octal tube.
This had the sides ground off so the base fits neatly inside a 30mm long piece of
PVC electrical wiring conduit tubing with about 25mm internal dia.
The central keying spigot of the plug has a 4mm threaded rod inserted to reinforce
the spigot which otherwise will all too easily be broken off accidentally by a
careless owner, leaving no way to correctly locate the plug into the socket at
the power supply, and therefore promoting many bad tempered experiences while
trying to make the amplifiers work. Both plugs MUST be plugged in correctly for
the amp to be able to be turned on.

There are some inbuilt safety features of the umbilical cables.
While plugged in there is little danger. The danger from a shock is no worse than
any normal 240V wall socket plug used for many other household appliances.
It is impossible to turn on the amps with only one octal plug plugged in,
or with plugs reversed, i.e, and red into black socket, black into red socket.
However, if somebody were to wrench one of the power supply cables from
the power supply while the amps are turned on, the amps will turn off immediately.
If any person were to immediately grab the pins of the plugs after removing them,
then they would be connected to the live stored voltages within capacitors in the
amplifier chassis. Diodes have been placed to prevent the flow of current
from these capacitors, see D1, D2, sheet 2, thus preventing a shock.
The 845 cathode heater supplies are biased at the cathode voltage of -450V
but at turn off there is a relay to reduce this voltage to less than 40V within
less than 0.5 seconds, so it would be very unlikely to experience any kind of
accidental shock unless one tried desperately to do so.

It would be unwise to allow a pet dog to chew on cables. Most animals will get a
stern message from a wire they chew, and learn to leave them alone.
The cabling used is particularly rugged industrial grade cabling with thicker
PVC insulation than used for high power 240Vac rated cables. Indeed the
cabling is normally used for 415Vac 3 phase high power supplied to industrial
electric motors. The highest voltages are carried in the two thick black cables
while very low voltages are carried by the orange cable.

For all things we cherish, my practiced care is my best insurance.
It was impossible easily build such amplifiers to be fully child safe.
Little hands will reach to touch anything. However, the tube heat will cause
pain before a burn is sustained, and children soon learn the danger of
anything hot. If there is any doubt, supervise children, if not, make sure
the amps are secured so a bench, and additional mesh screens made and
fixed to prevent child access or amp movement. When I was a child,
I don't recall I cause grief to my parents by upsetting the many dangerous
things in our lounge-room. When I did eventually become curious about
hot vacuum tubes in open-backed radio sets, I was about 15 years old,
not 15 months old.

Place the power supplies on the floor behind the amps and speakers and
well away from any likelihood of being tripped over by passing traffic.
Although these amps have been made fairly ruggedly, they don't like
being dropped, or pulled off a bench. Metalwork can easily bend and
dent in falls because of the weight. When moving the amps, turn them
off and wait until they have cooled down for 10 minutes.
The umbilical cables can be unplugged, and coiled up and tied up to
the rear carry handle on the amps.
Don't let trailing cables get under your feet, or catch on anything.


Sheet 11.
So how clean is the signal from these amplifiers?
I'd like to say it is much cleaner than most other tube amplifiers.
Do you ever wonder why the nitty-gritty technical aspects of tube amplifiers
are hardly ever mentioned as you surf around the Internet?
Does anyone else bother to publish curves like these? Ever wonder why?

There is usually too much to be ashamed of if makers told you the whole story,
and because the technical character of the amps concerned is usually very poor.
Many makers realize that nobody cares about the technical aspects as long as
the sound is good. Of course many makers know you can't tell what is really
good sound, and that you won't notice if THD is 1% instead of only 0.1%.
But I know good sound is only possible if the amps are technically very good.
The ability of the general public to discern the best sound has always been
poor. The human ear is a frightfully deceiving instrument. However,
many of my clients never assume anything, and operate by comparing
everything and then they soon realize what good sound should be like.
We all can see the difference between seeing a real object, an average
photograph of it, and an excellent photograph. If we can see differences
in photographs, we should hear differences in sound systems, taste
differences between chefs, and between wines.

A large part of my living is earned by re-engineering very poorly designed
hi-end brand amplifiers made by brainless designers who are eternally
optimistic and in constant denial about the woeful aspects of their products.

NOISE. As supplied, the 845 amps have extremely low noise level that can
only just be heard if an ear is held tight against a midrange speaker.
Noise of any kind measures less than 0.35mV.
Maximum power of 55 watts to 4 ohms is 14.83Vrms.
Official SNR = -92dB, ie, noise is 1/42,000 of the maximum V0, un-weighted.

DISTORTION. The graphs above in sheet 11 show THD levels between
half a Watt and clipping and for various load values. The vertical and
horizontal axies are both logarithmic so it is easier to see low distortion
levels at low power levels.
THD is mainly 2H, with some 3H, 4H and 5H well below the 2H.

The amps will comfortably give huge sound levels into any type of speaker
over 3 ohms including ESL. THD is about 2% at an "illegal" power output
of up to 60 watts at just past clipping.
THD is 0.5% just under clipping, but the first 10 watts of power at any
load over 3 ohms produces less than 0.15%. The least THD at all levels
occurs with a 7 ohm load, due to the natural second harmonic distortion
cancellation that occurs between the driver and output triode amplifier stages.
THD artifacts generated during normal loud listening never rise above 0.05%.
IMD artifacts consist of those harmonics related to the second harmonics
of fundamental tones. Such "2H" related IMD harmonic products are the
subjectively "least worst" type of distortions.

If you have 1.414Vrms at the output with 4 ohms, the power level is 0.5 Watt.
If speakers with average sensitivity produce 89dB SPL for 1 Watt, the total
of 1 Watt from both channels combined gives 89dB SPL. Probably your wife
will tell you to turn it down! She will find average levels of 84dB to be just right.

But at 1Watt into 4 ohms, THD < 0.04%, and THD is 0.0008 Vrms or 0.8mV rms.
If we could listen to that 0.8mV of distortion played through your 89dB/W/M
speakers and without the wanted undistorted music, we would find the sound
of the distortion would not be audible, or about as loud as nervous mouse
sneaking across the floor to get past a sleeping cat.

Way back in about 1953, exhaustive tests by the BBC and others revealed
that trained listeners were unable to discern any distortion present if it
measured below 0.5% and if the system response had hi-fi bandwidth.
It must be remembered that in 1953, transducers such as microphones,
speaker drivers, record cutter head amps and record playing and tape
recording and radio transmission and reception introduced 10 times the
0.5% on a routine basis, and with noise levels that might wake the dead.
Nowadays we have the artifacts created by solid state and digital processing,
and the music still battles to get through all that recording gear.

From the above graphs for various loads, notice that the distortion is low
where we listen to music, but it increases with power. If one were able
to accurately record a gunshot, and replay this recording through these
amps to try to reproduce the sound as it was heard without ear muffs,
the amps would be forced into clipping, ( maximum possible power levels )
and into high distortion, but it is of no importance because such noise is
so short lived, and only noise, and not musical, and the distorted noise
won't alter the idea that someone may have been shot.
So the high ceiling level of power is good for "transients" and not much else.
Nobody is going to listen to deafening levels of anything musical for very long.
We like to sit at least 20 metres away from the local orchestra in a hall because
the sound among the players can be deafeningly loud, and never how Beethoven
or Mozart intended out among the audience. And by the way, to get the sound
of a pistol shot in a room properly reproduced without an amp or speaker clipping,
a wall full of speakers and 1,000 watts would be needed.
A single grand piano played as loudly as possible sounds overwhelming up close,
but majestic when some distance away. With 50 watt amps, if we want that majesty,
it is up to us to have sufficient speaker sensitivity or get more powerful amps.
Majestic bundles of cash are required, and perhaps a change of wife as well though :-).
The use of TWO 845 per channel raises the power ceiling and lowers the
distortion compared to when only one 845 is used per channel.
This should allow complex orchestral music to be enjoyed,
not just a lone cello or piano.

BANDWIDTH. At ordinary loud levels bandwidth is 5Hz to over 50kHz,
which is slightly better than the amp with no global NFB, as quoted above.

DAMPING FACTOR. Non technical people might think about mops and
buckets of water when the words "damping factor" are used.
Not one amplifier ever made is perfect. Amplifiers are like a car which
gets you from A to B, but the more passengers you have the slower the
car goes. The perfect car would behave the same with 4 people aboard or
with just the driver. Now amplifiers don't actually "slow down" like a car
might when the load increases, but the output voltage from any amp will
drop in level as soon as a speaker "load" is connected, and continue to
drop as more speakers are connected. We want a minimum amount of
voltage drop because all speakers are made to operate ideally with a
non changing amplifier voltage and regardless of the signal frequencies.

The lower the number of ohms in the speaker, the greater the voltage
drop will be. This is confusing. The more people in the car, the harder the
car works to drive uphill for a given speed. But in electronics, the less
ohms there are, the harder the amp works. The God Of Triodes invented
audio electronics and made the rules so difficult and absurd compared
to common sense thinking so that fools wouldn't try to muck around
with electrical things. The basics about ohms, current and voltage can be
learnt by Googling, and experimenting with basic resistors and gear at
home on rainy Sundays, using a low voltage ac supply of 50Hz and a
voltmeter, and armed with knowledge about Ohm's Law.
So, you should learn the output voltage of an amp is like the car speed
with no passengers. The passengers are like the Amps of current, as
as Amps are increased, like increasing passengers, the voltage is less, or
speed is lower.
The ohms are a calculated quantity, and there is not an easy equivalent
quantity in a car which people discuss, hence ppl get muddled by
electronics. 

Suppose an amp makes 6Vrms without any speaker connected so that's
the measured output voltage without a "load". So there is no current
flow between the speaker terminals with no load. Let us connect a
5 ohms speaker, and let us suppose the amp voltage drops down to 5 volts.
From this, we know 5Vrms across the 5 ohm speaker load gives current
of 1 Amp. This is calculated using Ohm's Law,  I = E / R,
where I = current in amps, E = voltage in volts, and R = resistance in ohms.
Now the voltage drop at the amp terminals was from 6 volts to 5 volts,
with current changing from zero amps to 1amp.
The amplifier "output" resistance, Rout behaves as if we had a perfect
unchanging voltage source but with a concealed mystery series resistor
between this perfect source and the output terminal. This resistance is
called Output Resistance, Rout.
We need to know what the mystery resistance value is, and we may calculate
it very easily with Ohm' Law.
I = E / R, so R = E / I and in this case,
Rout = Voltage change caused by load / amplifier current change due to load.
So Rout = ( 6V - 5V ) / (1A - 0.0A ) = 1V / 1A = 1 ohm.

There is in fact no real 1 ohm R or perfect voltage source within the amplifier,
but the amp acts exactly as if there is such an Rout present and in series with a
perfect voltage source with extremely low output resistance. We carry the
concept into our minds by considering model of what is present,
even though we really have a bunch of tubes and an output transformer.

There is a ratio between the amplifier Rout, "output resistance",
also known as the Zout, "output impedance" and the speaker ohm value,
and it is called the Damping Factor, or DF.
The Damping Factor = speaker load ohms / Rout. The higher this number,
the better, but it should be at least above 4.0, and preferably 10.0.
But suppose the amp Rout was 1 ohm, then DF = 4 / 1 = 4, and this is
barely acceptable. With 6 volts without any load, and variations of 3 to 30
ohms, and the speaker voltage will vary from between 4.5 volts to 5.8 volts,
and the change in acoustic levels is only 1.5dB. Usually a good speaker
designer would have assumed all amplifiers have Rout = 1 ohm or less.
A DF = 10 would be even better though, so with speaker = 4 ohms,
Rout would need to be 0.4 ohms. Any increase of DF above 10 will
cause no perceptible change to the speaker response.

But suppose the amplifier had a DF = 0.5. That would mean that output
resistance Rout = twice speaker ohm value. And speakers vary in ohm
value at different frequencies, and typically between 3 ohms and 30 ohms
for what might be a nominal "4 ohm speaker", which averages 4 ohms
between the most energetic band of musical frequencies between 100Hz
and 1 kHz.

In the case of the SE845, the OPT has a turn ratio of 2,760 primary turns to
5 x 72 turn parallel secondary windings to 72 turns give TR = 38.33 : 1.
This gives an impedance ratio, ZR, = 1,469.4 : 1 hence the load ratio
of 6k0 : 4.1 ohms. Now the Ra of the 2 x 845 in parallel = 1k1, and the
winding resistance at the primary = 264 ohms, so total Rout = 1,360 ohms.
This is transformed by the OPT to appear as 1,360 / 1469 = 0.93 ohms.
The damping factor without the global NFB = 4.1 / 0.93 = 4.4 which is
barely enough to give a flat voltage and frequency response to nominal 4
ohm speakers which vary in Z between 3 and 30 ohms. However, this is quite
good compared to many SE triode amps I have measured with OPT winding
losses which are sometimes 3 times greater, and where Ra is also much higher
relative to the anode load, as may be the case with 211 triodes, or even 300B,
thus giving Rout of over 2.5 ohms at the sec meant for a 4 ohm load.
No wonder so many triode amps with no global NFB are not so wonderful
sounding and judged to give bloated bass. Quite a number of Pentode/Tetrode
and Ultralinear amps also end up having Rout way too high because the makers
have forgotten how to apply global negative feedback on which such amps
almost entirely to have a low Rout.

A low DF number is one that is like having a sound system with a very badly
adjusted graphic equalizer or tone controls. Some speakers might sound passable,
while others would sound dreadful!  Most sound would be very colored.
So audiophiles then argue night and day about the sound quality of their amps
and the musical timbre of the chosen brands of triodes, but they should be
arguing about the integrity of the designer and the design. Alas, many audiophiles
are utterly incapable of the slightest rational thought, especially anything involving
the simplest electronic ideas such as Ohm's Law.

In the SE845, the small amount of 8dB of Global Negative Feed Back, GNFB,
reduces the Rout from 0.93 ohms to 0.5 ohms, so giving DF = 6 with a 3 ohm
load, and 10 with a 5 ohm load.

There is much said about damping factors needing to be high, but anything over
4 is acceptable to most people, especially with triode amplifiers and as long as
the speaker has been designed competently so that large impedance variations
do not occur in a way that may cause response problems.
A badly designed nominally 4 ohm speaker might have a drop in impedance to 2
ohms, and although that may not cause a large perceived response problem it may
cause much more distortion to occur at frequencies where the impedance is low,
and this will damage the music, and maybe damage the amp.

Many tube amps may have Rout = 1 ohm, and might be used with ESL speakers
with high impedance at LF and quite low impedance at HF; Quad ESL57 ranged
from 33 ohms at 50Hz to 8 ohms at 1 kHz to 1.8 ohms at 18kHz, and one
might expect boomy bass and missing highs with amp Rout = 1 ohm.
But no, the DF at bass is 50 / 1 = 50, and excellent, and its 8 at 1 kHz, and in
the critical region of music there is very little difference in response levels.
The ESL 57 was designed to operate from amps with Rout = 1 ohm. So if you
had an amp with very high DF at all F and with Rout = say 0.1 ohm, the HF
might become too prominent with ESL57. One might always add a 1 ohm
series resistor to make the sound more bearable.

So one has to be flexible about damping factors, and not be too obsessive
and that isn't easy for many audiophiles, especially the hard to please
majority who just cannot understand anything I have said above.

And for explanations for more things that are inexplicable such as negative
feedback, please travel to elsewhere in my website to become truly confused
by science.

My 845 amps depend on a large amount of applied science and calculations
to give the best sound possible from tubes. I doubt very much that there
would be any change to the sound if I were to use exotic materials such
as 50% nickel in the output transformer cores, or pure silver wire.
Audio Note in Japan made Ongaku amps with 211 and they had 50%
nickel and 50% grain oriented silicon steel E&I core laminations,
and they used silver enameled wire. All sorts of fancy claims were made
about the superior sound because of the exotic materials but there has never
been any reliable blind AB test between the Ongaku and the same circuit
with mere copper wire and all Fe-Si GOSS core, and made by the same man,
and honestly tested and reviewed. Many claimants in the hi-fi industry would
desperately avoid any exposure to tests which challenge the validity of the
claims made.

Audiophiles make outrageous claims about sound quality. Some maintain it
is the part quality or brand that matters most, with tubes, capacitors, resistors,
cables, solder type, all of which is not the full story which should include
the choices of tube type, circuit used, and measured distortions and the
amount of negative feedback and how it is applied. Then there is the
choice of input and driver tubes and their technical set up. I've never
witnessed an audiophile changing his output transformers.
There are miles of plain old copper wire in there. They'll wax
lyrical about pure choke loading to gain stages, but this often achieves
much higher distortion than when using a simple resistor only, depending
on who has made the choke and the tube type chosen.
At low typical listening levels there can be considerable iron caused
distortion which resembles the dreaded crossover distortion in SS amps at
low levels. Chokes are only fabulous as I use them with a series resistance,
and where the signal is high as in the above EL84 driver circuit and where
the Ra of the tube is very low. Solid state CCS load on the input tube works
better than any choke plus resistance, but is limited to where signals are
low because of the fragility of solid state where high voltages lurk.
Chokes without the series R as well reduce the bandwidth and increase
phase shift at extremes of F and may cause instability oscillations where
even a small amount of global NFB is used, which is a reason why
global FB is not used; the makers don't know how to use it with
corrective phase shift networks.

Without the numbers being naturally very good, no quantity of exotic minor
parts or materials will make a lemon taste sweet. But if the numbers are really
good, then some slight sonic gain might be made with choice of tube and
capacitor brands.

I find that capacitor brands used in tube amps make little difference,
but do make a difference in speaker crossovers, and my tip today is that you
should only use polypropylene capacitors in speakers with high current
capacity such as "motor start" polypropylene caps, and its best to never
use any bi-polar electrolytics. Most makers don't do this because of cost
and size of poly caps. I only use polypropylene coupling caps in amps,
and I cannot tell any improvement has occurred if an alternative brand of
polypropylene cap is used.

But feel free to experiment, it won't do any harm, if you know what you are doing.

More pictures.....


Photo 8.


Photo 9.

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