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
of a pair of 55 watt class A monoblocs each using a pair of
triodes for single ended operation. These amps were a very
handcrafting challenge but I think my discerning customer who
months will never find any better sounding amplifiers with such
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
impact similar to well performed live unamplified music which has
been my “gold standard” for audio quality.
The amps also have excellent technical performance with wide
low distortion, low noise, and excellent damping factor.
The first prototype monobloc had all the power supply components
audio amp components on the one chassis. But total chassis weight
reached 42Kg, and it became extremely difficult to move easily,
had to be crammed together too tightly to allow low chassis
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
heavy duty umbilical cabling.
This reduced weight problems and I could have all hot running
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
hot summer days. Access to all encased resistors is possible by
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
Rectifiers are all silicon, and the power supplies always run cool
placed on the floor. The audio amp chassis can be placed on a
equipment stand above the power supplies so the front on/off
can easily be reached, and you can keep an eye on the tubes.
This shows a close up view with 2 x KR Audio 845 output
3 x Sovtek EL84 triode drivers, and 1 x NOS AWV 6CG7 input.
Initial tests were done using "expendable" Chinese made 845
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
same anode voltage and anode current at idle, ie, same anode
The KR845 may have a very slightly more detailed sound than the
type B Shuguang Chinese 845, which I think is the best Chinese
I leave the final judgment on sound quality of KR tubes to other
The KR tubes certainly look better made than the Chinese 845 which
a very close copy of the original RCA 845 and other old ancient
Both measured very similarly low distortion levels, and both
in my auto biasing circuit with the same biasing voltages and bias
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
two 13Vac power transformer cathode heater windings so the cathode
heating voltage can be finely adjusted for the correct level by
Chinese 845 require 3.3Adc, the highest current ever likely for
therefore the highest Vac tap is used to obtain 10Vdc. The KR may
the lowest voltage tap for the lower heater current.
RCA or other brands of NOS are so rare now that it’s pointless
to find any.
Because the KR tubes use 20 watts less to heat their cathodes,
could be idled at a full 100 Watts of anode dissipation. But the
are more than 3 times the price of the Chinese types so I have set
KR to run at about 75 Watts of anode dissipation to ensure long
See the notes about the operating conditions below.
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
cathode heating in each 845. In the center back is a large potted
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
wearing and cursing because they cannot read tiny lettering in the
of a listening room.
"Beneath the bonnet"of each audio amp chassis, top left, you can
entry and terminations of the incoming umbilical cables. Towards
right is a heatsink for three diode bridges for dc heater
then rail discharge resistances, then underside of Johnson tube
and far right you see the compact wiring of the 4 tube sockets for
and driver stages. At bottom left is the active protection board
down the amp if an output tube draws excessive anode current, ie,
malfunctions. Towards the right there is 60,000 uF in four caps
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
PVC insulation and is multi strand copper chosen for very long
reliability and with very generous current ratings and much with
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
reconfigure the output transformer secondary to suit loudspeakers
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
reduced power ceiling. 95% of listeners would find the 3-6 load
to be excellent for any kind of speaker ever made, as long as 25
was enough power required.
Here is the under chassis view of each power supply with its
cover removed. There is a small 5VA auxiliary mains transformer,
This supplies 12Vdc power to the active protection and dc turn
The power transformer has a board on the RHS with 48 terminals for
ends of windings. The range of voltages available allow for the
different output tubes and configurations in future if 845 become
unavailable. Suitable alternative tubes are 4 x KT90 in parallel
the same output transformer but with a different winding
the primary. Or two x 13E1 can be used, or various push pull
So if ever there are no 845, the amps can be altered by a skilled
and sonic purity and great sound can be maintained without
Instead of recycling the amps at the scrap metal dealer, if all
remain intact the amps can re-built into something else, and even
of EL34, 6L6GC, 5881 could be used.
But in 20 years, I may not be around, and the kind of willing and
skilled tradesmen like myself may not be available.
This workshop picture shows the rear of an audio chassis (left)
its power supply (right). It would be difficult to make a mistake
the umbilical cable plugs because one is painted bright red, and
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
Binding posts that rely on a wire poked through a hole in the post
then with a knob turned tight are an unreliable connection and
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
a single ended input only, shown recessed on the left.
On the amp rear side to the right, the umbilical cables emerge and
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
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
relays within the power supply. Thus 240V mains wiring is not
into the audio amp chassis and so there no diode switching noise
or hums from where the on-off switch is so closely situated to the
audio input circuits.
I don't mind publishing schematics for free on line. People think
But nobody is going to copy what I have done and make a profit
these amps will have a cost of production far in excess of most
toy like amplifiers which one can inspect around the Internet. The
other manufacturers who supply similar power levels of Single
triodes dare not publish the full details of their amps lest the
their shortcomings be displayed to everyone. They don’t want you
how they managed to get the cost of production to down to a very
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
to remove the quality to cheapen the cost of production, thus
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
low volume productions. So copy cat makers are never going to copy
because they cannot offer a fake amp at a low price because the
has become world famous and priced way above most peoples' ability
Warning. There are high
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.
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
This keeps out dc in sources and extremely low frequency signals.
The MJE350 transistor might seem to be quite out of place in a
but it acts as a passive component which supplies V1 anodes with a
changing current or what is called a constant current source, CCS.
The CCS acts in a manner identical to an equivalent resistance of
megohms to an imaginary supply voltage of thousands of volts. Such
resistance and supply are utterly impractical, and not needed. It
have been possible to use a pentode to perform the same function
MJE350, but in this case the MJE performs better than any tube.
Because the effective real dynamic collector resistance is so
impart any sonic signature in the signal path, apart from allowing
operate with less THD than if it had a "normal" resistance between
So with the MJE350, V1 anode load is effectively only the
coupled biasing resistance R11, 180k. The Ra of V1 is about 5k0,
RL is 36 times greater, and when triodes are loaded with RL >
they give the best sound, and the lowest possible distortion
If you were to replace the CCS with a simple resistance of about
THD/IMD would maybe increase 3 fold. The THD of V1 is mainly all
but it will add to that of the output stage because both V1 and
have the same phase of the 2H, so to minimize overall THD, the CCS
transistor helps to lower overall distortion, and maximize the
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
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,
those who detect the German tubes are better might be drawn to
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
6DJ8/6922 does tend to oscillate at around 200MHz if you parallel
halves without using two separate series grid R "stoppers".
The cathode biasing resistor, R5, would need to be reduced in
until Ea measured about +120Vdc.
The 6CG7 is an evolution of the famous octal based 6SN7. Oz made
often used exactly the same anode, grid and cathode structures but
slightly closer, and a slightly lower anode Pda rating given for
smaller 9 pin size.
There was often a screen also fitted between each anode taken to
The 6CG7 technical character is identical to 6SN7 and it ensures
is amplified very linearly, while maintaining excellent
micro detail and
warmth, transparency etc that one enjoys with the best tubes when
as I show.
German 6CG7 and some Japanese 6CG7 were made with smaller anodes
mades or 6SN7, and had slightly high µ than Oz versions. But
6CG7 will function well in a circuit designed for them.
8dB of global NFB is applied from the output transformer through
to the top of R6.
C6, C7, R8, and R11 form a LF gain stepping network to optimize
V2,3&4 are triode connected EL84, each with individual cathode
Any brand of EL84 may be used, and you could have 3 different
together if need be because they each have their own cathode
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
what becomes ONE super triode with the ability to produce a
of 164Vrms of signal output with less than 2% THD, and with good
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
provides a high ac impedance anode supply load which dissipates an
small amount of ac power, so hence the excellent linearity,
RL is many times Ra, and RL approaches a CCS.
The load seen by the 3 x EL84 is approximately 20k, and mainly due
the grid biasing R28&R29 of 23.5k of the two following 845
Again, RL = 28 x Ra, thus ensuring minimal THD&IMD and maximum
There is zener diode shunt regulated Vdc for V1 supply to assist
Any noise in the zeners is filtered by R21 and C9, and the CCS
prevents any other noise entering V1 anode circuit.
Each 845 is set up in conditions as follows :- Ea =
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
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
is at 9Hz.
The 845 anode current is supplied from two rails, one at +600Vdc,
the other at -624Vdc.
This unusual arrangement reduces the likelihood of arcing within
OPT between anode
windings and earth potential secondary windings. The filtering of
two rails is by
C-LR-C-R-C set up to give a damped LF pole. Many tests were done
continual mains voltage level changes and LF switching noise
connected to the mains supply does not create LF resonance signal
between grid and cathode of the output tubes, and therefore does
appear in the output
in excess of 0.5mV, even if mains noise level change is very
The overall noise performance despite the twin rail use is
Despite so little global NFB these amps were the quietest tube
have ever built.
Two supplies of 10Vdc are applied to 845 cathodes XX and YY so
of 24mV of residual 100Hz noise is balanced by R30&31,
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
potted and do not cause any magnetic interference in the potted
the same chassis.
L2 is a solenoid type of choke in a CLC filter, so the Vac across
choke is tiny,
and thus its change in magnetic field is negligible, so potting
The main power supply chassis have all the above within to
positive and negative Vdc rails for the 845 and other tubes, and
for relay switch on and protection circuits. All the ac cathode
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
chassis, there is no resulting diode noise in the output. See
below sheet 6 about power transformer and iron core component
A large number of rail voltage arrangements are possible.
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,
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
All fuses below may only be replaced by a technically trained
F2, F3, F4. Three fuse wire
rated for 10A soldered into the underside
of the psu chassis and covered with black polyester sleeving for 2
ac cathode heater windings and one other cathode heater winding
3 x EL84, and 1 x 6CG7.
F5, F6. Two x
slow blow, 3AG, soldered into place under psu chassis
for the two main HT rails of +660Vdc and -640Vdc, derived from
R58 Provides some protection for the auxiliary small power
under the psu chassis. This 1 watt resistor will burn out if the
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.
Most tube power amps don't have any kind of active protection
against the unavoidable and inevitable eventual failure of one or
output tubes during what is called a "bias failure" event.
I have had to repair very many "good" hi-end brand amplifiers that
a lot of trouble due to poor design, or through mishap caused by
or malfunctioning speakers etc, etc, etc.
To avoid smoke in the listening lounge room, and collateral damage
other parts within the amp, most amp makers do fit a couple of
But fuses provide only partial protection, and they don't always
when one wants them to, and owners are notorious for replacing
something convenient like aluminium foil from the kitchen or using
2A fuse instead of 0.2A which encourages a faulty amp to burn the
Active protection is needed to stop the smoke and damage and to
tell an owner when something is wrong, and if possible to shut
the amp and prevent fuses blowing.
Nevertheless, there are a fair number of fuses fitted in these
and apart from the mains fuse they are all soldered into place
fuse holders are notorious for not holding a fuse firmly and
loose over time, and thus becoming intermittent especially with dc
Fitting new soldered-in fuses is a painful exercise requiring a
a soldering iron. The most likely problem in any tube power amp is
the sudden or gradual unwanted increase in the idling dc current
in each output tube. This current is usually called the anode bias
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
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
with dire results if not dealt with ASAP, within seconds.
The above simple circuitry will shut down the amp in 90% of bias
or tube failure cases, and if the failure was caused by some
it may be easily reset to go again simply by switching off, then
explained in the text in the schematic above. Under normal
anode current is around 70mAdc at idle. This generates 150V across
R34&R35. Should the anode current ever rise to about 102mAdc,
be a cathode bias voltage of 225Vdc, which means the Pda will have
about 100W, and although this is the maximum rated Pda for 845,
wrong in these amps and due to a fault condition. So to give the
it only takes an Ia increase of 32mAdc, or 45%, and you cannot
to blow with such a small amount of current change, so active
the only reliable way to prevent Ia rising to perhaps 400mAdc in
400mAdc would damage the cathode bias resistors, and perhaps the
primary winding if the condition lingered for too long, and I have
happen in many amps brought to me for repair.
The protection circuits have utterly no effect on the sound.
The power transformer does not run very hot because the turns per
gives a B-max of less than 0.9 Tesla, and wire sizes are generous
for no more than 3 Amps per square mm. of copper wire section
The transformer is neatly layer wound with layered construction
drawn section through the winding bobbin.
I give a two year warranty on amp transformers. But if one were to
and I was not around in future then there are no standard easily
replacements for any iron cored wound components in these
any known commercial transformer winder.
All are custom wound and may have to be ordered as a special order
perhaps Sowter Transformers located in the UK. Sowter would be the
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
include the the many taps for alternative tube usage. Most other
transformer winders HATE TAPS anywhere; they just cannot cope with
routine levels of complexity and thought I put into all my work.
OR one might rebuild the two power supplies on ONE new chassis
larger single PT rated at 1.2KVA. This sounds like a lot, but it
single transformer would have the same core lamination tongue size
but have a GOSS stack height of 100mm instead of the 72mm now
Wire sizes would be thicker, but fewer turns per volt are used,
no more difficult to wind than winding a pair of trannies with
rated at 650W for cool running.
An alternative is to rebuild the power supply on a new chassis to
use of multiple power transformers which may be available as trade
from Hammond Engineering, through the Australian dealers, EVATCO
Queensland, and thus avoid custom winding any transformers.
Other toroidal transformer alternatives are possibly available
in Sydney but I cannot say that the Hammond wound in Canada or
wound in Australia will be silent running like the existing
because their de-fault Bac = 1.2Tesla, and these products are NOT
In the event that a transformer is damaged and cannot be used, it
for a suitably trained tech to disconnect it from the circuit,
external screws, and remove it off the psu chassis.
The terminal board and top sealing layer of resin and sand
be removed with chisel and hammer and remaining loose dry sand
the transformer can be drained out. The transformer should be able
removed from its pot and thus the pot can be re-used.
To dismantle the wound transformer all bolted angles and bolts are
The grain oriented silicon steel core can be salvaged by heating
in a wood fire to a dull red to vaporize all plastics in the
then allowing it to cool down for a few hours. The burnt wire is
for re-cycling, and laminations should all fall easily apart and
for re-use. The heating will NOT affect the core magnetic
A new transformer is then wound using a new plastic bobbin to suit
70mm stack of 51mm tongue laminations so that it will fit inside
The newly wound transformer must be varnished while being wound or
after with a soak and bake method. A new terminal board is made
After testing the new transformer it is re-assembled back into its
dry sand used to fill the pot except for the last 15mm. The sand
thoroughly vibrated and settled. Spray-can varnish is applied to
and next day the top 15mm of fill can be done using a 50-50 mix of
(fibreglassing) resin and sand. This seals the pot and prevents
out. The spray varnish prevents liquid resin soaking into dry sand
The completed tranny is re-installed into the chassis and
At present, nobody I know in Australia has the competence
to do such work on transformers
the gentleman who
made the amps.
Once outside the warranty period, transformer replacement is
but I think you may find I am cheaper to employ than the hi-end
when a power transformer fails.
The power transformers and other wound components such as chokes
OPT have been designed to run cool, and all windings have fuses,
protection is used against output tube failure.
After winding so many power and output transformers and chokes
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
The output transformer is layer wound and varnished with Wattyl
polyurethane two part varnish generously applied to windings as
transformer was wound. It is potted in a zinc plated sheet steel
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
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
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
occurs at 4Hz at loud levels normally used.
At the 50 Watt output level the onset of core saturation occurs
At 50W, the HF -3dB point is above 30kHz, all with zero global
Some of the slight sag in HF is due to the stability network of
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
capacitance, and the resonance between leakage L and Shunt C is
above 30kHz. Primary winding resistance = 95 ohms for 2,760 P
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
0.9mm Cu dia wire. Sec winding resistance referred to primary =
so total Rw = 95 + 182 = 277 ohms at the primary input.
Winding loss percentage = 100% x 277 / 6,277 = 4.44% with 6,000
anode load with the OPT secondaries set up for 4.1 ohms or 6.4
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
the first two arrangements of secondaries but slightly higher for
9.2 ohm set up.
With such a high maximum power of 55 watts into 4 ohms, there is
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
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
2805 should be tried with the 4 ohm setting before committing to
changing the setting to a higher one.
Many older high impedance speakers of say 16 ohms are much more
than more modern types, and therefore require a tiny amount of
and power to play very loud, so there is no need to change the
matching to the 9.2 ohm setting, and the 4 ohm setting will be
Most people will never use more peak power over 10 watts with
power well below this figure, often only 0.5W per channel. Because
measures so well at 55 watts, at an average power of 2 watts the
is well below audibility, see the notes below.
Six chokes per channel are used for filtering and to prevent the
losses using alternative methods of filtering or active
No solid state chip regulators are used because they become
unreliable when used in circuits with such high voltages lurking
There are a few simple zener diodes for basic shunt regulation,
so they are under no heat stress. The main positive and negative
rails of over +600V and below -600V have a C-LR-C-R-C type of
Each C is formed with 2 x 470uF in series to make 235uF, and total
per rail = 705uF. There is a resonance between the 4H choke and
235uF at 5.2Hz, but the added 100 ohms in series plus the
additional 100 ohms plus 235uF act to damp the peak in the
thus making each voltage rail less liable to vary at LF below 10Hz
due to the unavoidable mains voltage level changes that occur
and typically of +/- 20mV up to +/-200mV.
I have found the steady average value of mains voltage can be from
on a cold winter evening to +255Vac on days with little mains
247Vac being most common here in Canberra.
Many high end brand amps have been designed to run in the USA with
nominal 110Vrms mains or 220Vrms. Many such amps have fixed bias
and tubes run unnecessarily hot even with the correct mains
But here the mains voltage is often 250Vac, and with fixed bias
often overheat badly when tube Pda rises close to or above the
Pda. Jolida and ARC amps suffer in this manner badly, and need to
alterations made to their PSU to prevent the B+ from exceeding the
voltage rating and to stop the tubes exceeding their Pda levels.
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
The SE55 work just fine where mains = 250Vac, but even if mains =
will still work fine even though Ea and Ia in tubes is reduced and
thus giving 10% less maximum power.
This shows the arrangement of resistors used within the aluminium
heatsink at the rear end of the amp chassis. Cheap ceramic bodied
wound types are used with their dissipated power being well below
the rating for the resistance chosen. These are readily and
available from many suppliers. A total of about 50 Watts is
in the resistors shown, and to keep them all cool and thus more
the heatsink was built up to enclose the resistors behind a
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
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
will fail, unless the current flow is 10 times the value in them
Resistors usually fail by fusing open, and they may be prized off
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
While it may have been wiser to use much more expensive aluminium
bodied resistors each screwed to a heatsink, there was no need,
method used is entirely adequate, and any fused resistors are
source and easy easy to replace.
Here there is the layout for rugged cables used to get power from
supplies to the amp chassis. When the amp chassis are examined
a copy of the above, just exactly how everything is set up becomes
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
If a pin is broken off a plug, the whole plug is made useless. The
is to cut the plug off, and rewire a new octal plug onto the lead
A cheap type of octal plug from RS components could be used to
The original plugs were made using only the bottom base from an 8
This had the sides ground off so the base fits neatly inside a
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
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 power supply, and therefore promoting many bad tempered
trying to make the amplifiers work. Both plugs MUST be plugged in
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
any normal 240V wall socket plug used for many other household
It is impossible to turn on the amps with only one octal plug
or with plugs reversed, i.e, and red into black socket, black into
However, if somebody were to wrench one of the power supply cables
the power supply while the amps are turned on, the amps will turn
If any person were to immediately grab the pins of the plugs after
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
from these capacitors, see D1, D2, sheet 2, thus preventing a
The 845 cathode heater supplies are biased at the cathode voltage
but at turn off there is a relay to reduce this voltage to less
less than 0.5 seconds, so it would be very unlikely to experience
accidental shock unless one tried desperately to do so.
It would be unwise to allow a pet dog to chew on cables. Most
will get a
stern message from a wire they chew, and learn to leave them
The cabling used is particularly rugged industrial grade cabling
PVC insulation than used for high power 240Vac rated cables.
cabling is normally used for 415Vac 3 phase high power supplied to
electric motors. The highest voltages are carried in the two thick
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
Little hands will reach to touch anything. However, the tube heat
pain before a burn is sustained, and children soon learn the
anything hot. If there is any doubt, supervise children, if not,
the amps are secured so a bench, and additional mesh screens made
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
things in our lounge-room. When I did eventually become curious
hot vacuum tubes in open-backed radio sets, I was about 15 years
not 15 months old.
Place the power supplies on the floor behind the amps and speakers
well away from any likelihood of being tripped over by passing
Although these amps have been made fairly ruggedly, they don't
being dropped, or pulled off a bench. Metalwork can easily bend
dent in falls because of the weight. When moving the amps, turn
off and wait until they have cooled down for 10 minutes.
The umbilical cables can be unplugged, and coiled up and tied up
the rear carry handle on the amps.
Don't let trailing cables get under your feet, or catch on
So how clean is the signal from these amplifiers?
I'd like to say it is much cleaner than most other tube
Do you ever wonder why the nitty-gritty technical aspects of tube
are hardly ever mentioned as you surf around the Internet?
Does anyone else bother to publish curves like these? Ever wonder
There is usually too much to be ashamed of if makers told you the
and because the technical character of the amps concerned is
Many makers realize that nobody cares about the technical aspects
the sound is good. Of course many makers know you can't tell what
good sound, and that you won't notice if THD is 1% instead of only
But I know good sound is only possible if the amps are technically
The ability of the general public to discern the best sound has
poor. The human ear is a frightfully deceiving instrument.
many of my clients never assume anything, and operate by comparing
everything and then they soon realize what good sound should be
We all can see the difference between seeing a real object, an
photograph of it, and an excellent photograph. If we can see
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
hi-end brand amplifiers made by brainless designers who are
optimistic and in constant denial about the woeful aspects of
NOISE. As supplied, the
amps have extremely low noise level that can
only just be heard if an ear is held tight against a midrange
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,
DISTORTION. The graphs
sheet 11 show THD levels between
half a Watt and clipping and for various load values. The vertical
horizontal axies are both logarithmic so it is easier to see low
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
over 3 ohms including ESL. THD is about 2% at an "illegal" power
of up to 60 watts at just past clipping.
THD is 0.5% just under clipping, but the first 10 watts of power
load over 3 ohms produces less than 0.15%. The least THD at all
occurs with a 7 ohm load, due to the natural second harmonic
cancellation that occurs between the driver and output triode
THD artifacts generated during normal loud listening never rise
IMD artifacts consist of those harmonics related to the second
of fundamental tones. Such "2H" related IMD harmonic products are
subjectively "least worst" type of distortions.
If you have 1.414Vrms at the output with 4 ohms, the power level
If speakers with average sensitivity produce 89dB SPL for 1 Watt,
of 1 Watt from both channels combined gives 89dB SPL. Probably
will tell you to turn it down! She will find average levels of
be just right.
But at 1Watt into 4 ohms, THD < 0.04%, and THD is 0.0008 Vrms
If we could listen to that 0.8mV of distortion played through your
speakers and without the wanted undistorted music, we would find
of the distortion would not be audible, or about as loud as
sneaking across the floor to get past a sleeping cat.
Way back in about 1953, exhaustive tests by the BBC and others
that trained listeners were unable to discern any distortion
measured below 0.5% and if the system response had hi-fi
It must be remembered that in 1953, transducers such as
speaker drivers, record cutter head amps and record playing and
recording and radio transmission and reception introduced 10 times
0.5% on a routine basis, and with noise levels that might wake the
Nowadays we have the artifacts created by solid state and digital
and the music still battles to get through all that recording
From the above graphs for various loads, notice that the
where we listen to music, but it increases with power. If one were
to accurately record a gunshot, and replay this recording through
amps to try to reproduce the sound as it was heard without ear
the amps would be forced into clipping, ( maximum possible power
and into high distortion, but it is of no importance because such
so short lived, and only noise, and not musical, and the distorted
won't alter the idea that someone may have been shot.
So the high ceiling level of power is good for "transients" and
Nobody is going to listen to deafening levels of anything musical
We like to sit at least 20 metres away from the local orchestra in
the sound among the players can be deafeningly loud, and never how
or Mozart intended out among the audience. And by the way, to get
of a pistol shot in a room properly reproduced without an amp or
a wall full of speakers and 1,000 watts would be needed.
A single grand piano played as loudly as possible sounds
but majestic when some distance away. With 50 watt amps, if we
it is up to us to have sufficient speaker sensitivity or get more
Majestic bundles of cash are required, and perhaps a change of
well though :-).
The use of TWO 845 per channel raises the power ceiling and lowers
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
levels bandwidth is 5Hz to over 50kHz,
which is slightly better than the amp with no global NFB, as
DAMPING FACTOR. Non
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
gets you from A to B, but the more passengers you have the slower
car goes. The perfect car would behave the same with 4 people
with just the driver. Now amplifiers don't actually "slow down"
might when the load increases, but the output voltage from any amp
drop in level as soon as a speaker "load" is connected, and
drop as more speakers are connected. We want a minimum amount of
voltage drop because all speakers are made to operate ideally with
non changing amplifier voltage and regardless of the signal
The lower the number of ohms in the speaker, the greater the
drop will be. This is confusing. The more people in the car, the
car works to drive uphill for a given speed. But in electronics,
ohms there are, the harder the amp works. The God Of Triodes
audio electronics and made the rules so difficult and absurd
to common sense thinking so that fools wouldn't try to muck around
with electrical things. The basics about ohms, current and voltage
learnt by Googling, and experimenting with basic resistors and
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
with no passengers. The passengers are like the Amps of current,
as Amps are increased, like increasing passengers, the voltage is
speed is lower.
The ohms are a calculated quantity, and there is not an easy
quantity in a car which people discuss, hence ppl get muddled by
Suppose an amp makes 6Vrms without any speaker connected so that's
the measured output voltage without a "load". So there is no
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
From this, we know 5Vrms across the 5 ohm speaker load gives
of 1 Amp. This is calculated using Ohm's Law, I = E / R,
where I = current in amps, E = voltage in volts, and R =
Now the voltage drop at the amp terminals was from 6 volts to 5
with current changing from zero amps to 1amp.
The amplifier "output" resistance, Rout behaves as if we had a
unchanging voltage source but with a concealed mystery series
between this perfect source and the output terminal. This
called Output Resistance, Rout.
We need to know what the mystery resistance value is, and we may
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
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
but the amp acts exactly as if there is such an Rout present and
series with a
perfect voltage source with extremely low output resistance. We
concept into our minds by considering model of what is present,
even though we really have a bunch of tubes and an output
There is a ratio between the amplifier Rout, "output resistance",
also known as the Zout, "output impedance" and the speaker ohm
and it is called the Damping Factor, or DF.
The Damping Factor = speaker load ohms / Rout. The higher this
the better, but it should be at least above 4.0, and preferably
But suppose the amp Rout was 1 ohm, then DF = 4 / 1 = 4, and this
barely acceptable. With 6 volts without any load, and variations
ohms, and the speaker voltage will vary from between 4.5 volts to
and the change in acoustic levels is only 1.5dB. Usually a good
designer would have assumed all amplifiers have Rout = 1 ohm or
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
But suppose the amplifier had a DF = 0.5. That would mean that
resistance Rout = twice speaker ohm value. And speakers vary in
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
of 6k0 : 4.1 ohms. Now the Ra of the 2 x 845 in parallel = 1k1,
winding resistance at the primary = 264 ohms, so total Rout =
This is transformed by the OPT to appear as 1,360 / 1469 = 0.93
The damping factor without the global NFB = 4.1 / 0.93 = 4.4 which
barely enough to give a flat voltage and frequency response to
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
losses which are sometimes 3 times greater, and where Ra is also
relative to the anode load, as may be the case with 211 triodes,
thus giving Rout of over 2.5 ohms at the sec meant for a 4 ohm
No wonder so many triode amps with no global NFB are not so
sounding and judged to give bloated bass. Quite a number of
and Ultralinear amps also end up having Rout way too high because
have forgotten how to apply global negative feedback on which such
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
while others would sound dreadful! Most sound would be
So audiophiles then argue night and day about the sound quality of
and the musical timbre of the chosen brands of triodes, but they
arguing about the integrity of the designer and the design. Alas,
are utterly incapable of the slightest rational thought,
the simplest electronic ideas such as Ohm's Law.
In the SE845, the small amount of 8dB of Global Negative Feed
reduces the Rout from 0.93 ohms to 0.5 ohms, so giving DF = 6 with
load, and 10 with a 5
There is much said about damping factors needing to be high, but
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
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
with high impedance at LF and quite low impedance at HF; Quad
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
So one has to be flexible about damping factors, and not be too
and that isn't easy for many audiophiles, especially the hard
majority who just cannot understand anything I have said above.
And for explanations for more things that are inexplicable such as
feedback, please travel to elsewhere in my website to become
My 845 amps depend on a large amount of applied science and
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
used silver enameled wire. All sorts of fancy claims were made
about the superior sound because of the exotic materials but there
been any reliable blind AB test between the Ongaku and the same
mere copper wire and all Fe-Si GOSS core, and made by the same
honestly tested and reviewed. Many claimants in the hi-fi industry
desperately avoid any exposure to tests which challenge the
Audiophiles make outrageous claims about sound quality. Some
is the part quality or brand that matters most, with tubes,
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
witnessed an audiophile changing his output
There are miles of plain old copper wire in there.
lyrical about pure choke loading to gain stages, but this
much higher distortion than when using a simple resistor
on who has made the choke and the tube type chosen.
At low typical listening levels there can be considerable iron
distortion which resembles the dreaded crossover distortion in SS
levels. Chokes are only fabulous as I use them with a series
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
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
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
I find that capacitor brands used in tube amps make little
but do make a
difference in speaker crossovers, and my tip today is that you
only use polypropylene capacitors in speakers with high current
capacity such as "motor start" polypropylene caps, and its best to
use any bi-polar electrolytics. Most makers don't do this because
and size of poly caps. I only use polypropylene coupling caps
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.
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