Deep Space 845se55      July 2008

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 un-amplified music which has always been my “gold standard” for audio quality.

The amps also have excellent technical performance.                                                                                                                           

Photo 2.

The first prototype monobloc had all the power supply components and audio amp components on the one chassis. It was found that the chassis weight for one channel went over 42Kg, and it became extremely difficult to move easily, and all the parts had to be crammed together which brought a lot of problems in getting rid of the heat generated. So I adopted the same principles I used in my 300W monobloc amps and have two chassis per mono channel with power supply on one and audio circuit on the other, and with each chassis connected with very heavy duty umbilical cabling. The two chassis construction reduces the weight of any chassis to manageable levels without too much risk of spinal injury. It also gave me room to place all hot running resistors inside a heatsink on one end of the chassis top instead of underneath, and this helps the amp stay cool even on hot summer days.

Each audio amp chassis weighs approximately 30Kg and is 520mm deep x 230mm wide x 280mm high. Each power supply chassis weighs 16Kg and are 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 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 which measured well and sounded well during tests.

The amps are made to work with any brand of 845. The KR have cathodes needing 10V at 1A, while Chinese 845 need 10V at 3.3A, so the KR tubes run cooler than the Chinese types, if they both have the same anode dissipations. The KR845 may have a slightly more detailed sound than the cheapest variety of the Shuguang Chinese 845. I leave the final judgment on sound quality of KR tubes to other people. The KR tubes certainly look better made than their Chinese cousins, although the Shuguang 845 is a closer copy of the original RCA 845 and other old ancient brands. Both measured very similarly low distortion levels, had the same biasing voltages and bias currents and power outputs in the same circuit. The power supply has 3 taps on the Vac cathode heater ac winding, and the cathode heating voltage can be finely adjusted for the correct level by choice of taps. Chinese 845 require a higher current, therefore the highest tap is used to obtain 10Vdc from the choke input supply, and KR can 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 have 20 watts less heat in their cathodes, they could be idled at a full 100watts of anode power. But the KR 845 are 3 times the price of the Chinese types so I have set up the KR to run at about 75 watts of anode dissipation. I hope they last well. The Chinese 845 might be purchased for usd $150 and this is good value. The KR845 are listed at over 3 times this price, so I don't wish to take the risk of idling the KR tubes with too much anode dissipation. 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 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 swearing 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 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. bottom left is the active protection board to shut down the amp if an output tube mal-functions, then towards the right there is 60,000 uF in four caps for 845 heater filtering, then rail discharge resistances. All is genuine point to point wiring with mainly hardwood terminal strips well sealed with varnish. Hookup wire is 1mm thick pvc insulated 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 are over + or – 500Vdc.

Right in the middle are two rows of terminals which allow a tech to reconfigure the output transformer secondary to suit either 3-6 ohms or 6-or-more-ohms.

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 to the 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....

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 1950 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. Most amp makers who make larger amp numbers in a factory all are frightened to show the secrets behind their creations. 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, or paid the huge sums to advertise in that magazine, and 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 acts as a passive component which supplies V1 anodes with a non changing dc current or what is called a constant current source, CCS. It is not a perfect current source, but close to one and with a real

source impedance of several megohms at least. Therefore the MJE350 cannot include any sonic signature in the signal path. But with the MJE350, V1 anode load is effectively only the following cap coupled biasing resistance R11, 180k. The Ra of V1 is about 5k, and the RL is 36 times greater, and when triodes are loaded with RL many times the Ra, 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 of the relative same phase of the 2H, so to minimize THD, the CCS transistor helps to lower 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, and 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.

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 and did not use separate grid R. 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 but with its two triode elements fitted into a nine pin tube, so the technical character is identical and it ensures the audio signal is initially amplified very linearly, while maintaining excellent musicality, micro detail and warmth, transparency etc that one enjoys with the best tubes when set up the way I do.

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 7k to supply Ia = 36mAdc total 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.

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, RL per tube = 12k, and so for both the load is 6k.

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 rated at 1,000V 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 CLRCRC set up to give a damped LF pole, and many tests were done to ensure the continual mains voltage level changes and LF noise 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 if mains noise is very  bad.  The noise performance despite the twin rail use is extremely good. Despite so little global NFB these amps are the quietest tube amps I have built yet.

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 auxilliary small power transformer under the psu chassis. This 1 watt resistor will burn out if the auxilliary 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 eventual failure of one or more output tubes.     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 shunting 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 intermittent with dc flow. Fitting new 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 sometimes 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. 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. 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 exactly what is done by the above type, but maybe without so many taps for alternative tube usage. If nobody can be found to replace one power transformer it may become necessary to rebuild the two power supplies on one new chassis using a larger single power transformer rated for about 1.2kW. This sounds like a lot, but it means the single transformer would have the same core lamination tongue size of 51mm, but have a 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 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 toroids wound in Australia will be silent running like the existing transformers

unless they have been designed to run with B < 0.9Tesla. 

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. 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 slightest 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 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 galvanized iron 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 number of 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 anode resistance of the two 845 in parallel is 1,100 ohms and when 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, and 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, 0.45mm wire. Secondary winding resistance = 0.124 ohms for 5 x 72 turn secs in parallel, 0.9mm 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 ESL 63, 989, 2805 should be tried with the 4 ohm setting before 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. 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 through 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. The main positive and negative voltage rails of over +600V and below -600V have a CLRCRC 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 mains voltage changes that occur continuously, and typically of +/- 20mV.

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, it is not likely that they will fail. Resistors usually fail by fusing open, and these can 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.

Sheet 10.

Here we have 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 tube plug. 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 the amplifier chassis. Diodes have been placed to prevent the flow of current from the major amp voltage rails and thus prevent 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 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 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. 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, practiced care is the best insurance.

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 the amps have been made fairly ruggedly, don't drop one, or allow it to be pulled off a bench. 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. But I know good sound is only possible if the amps are technically very good.

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.

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 down.

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 2 volts at the output with 4 ohms, the power level is one watt, thus producing 89dB SPL into most modern European made speakers with average sensitivity. if this was the average level, your wife will tell you to turn it down!          It is loud. If the THD was 0.05%, then the distortion voltage is 0.001 volts or 1 millivolt. If we could listen to that 1mV 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 rat sneaking across the floor to get past a sleeping cat.

Way back in about 1953, exhaustive tests by the BBC and others revealed that listeners were unable to discern any distortion present if it measured below 0.5% and if the system response was 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. 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 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.

Suppose an amp makes 6 volts 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 speaker, and maybe we would measure the amp voltage drops down to 5 volts. Let us suppose the speaker load resistance or impedance measured was 5 ohms. Consider Ohm's Law. I = E / R, where I = current in amps, E = voltage in volts, and R = resistance in ohms. For our speaker, with 5 volts present the current = volts divided by resistance = 5 / 5 = 1 amp in this case. 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. We need to know what the mystery resistance value is.

To easily calculate this R, we must apply Ohm's Law. R in ohms = E in volts divided by I in amps, so R = E / I . So to Rout = change in load voltage divided by change in load current. In this case, Rout = 1V change / 1amp current change = 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 in ohms divided by amplifier output resistance. The higher this number, the better, but it should be above 4.0.
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.

So if the amp has Rout of say 8 ohms, and the speaker R varied from say 3 to 30 ohms, the voltage at the speaker would vary very much, and so would the acoustic response of the speaker, and an amp with 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. With Rout = 8 ohms, and speaker nominal R = 4 ohms, DF = 4 / 8 = 0.5 which is very poor.

The output resistance of the 845 amps = 0.5 ohms, so DF = 10 with a 5 ohm load. The small amount of 8dB of global NFB used in the amp reduces the Rout from 1.1 ohms without any NFB connected.

There is much said about damping factors needing to be high, but anything over 4 is acceptable to 99% of 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 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 claims were

made about the superior sound because of the exotic materials but there has never been an ABX test between the Ongaku and the same circuit with mere copper wire and all GOSS core, and made by the same man, and honestly tested and reviewed.

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. Rarely does one ever see an audiophile change 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 higher distortion than when using a simple resistor only. At low typical listening levels there is considerable iron caused distortion which resembles the 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 no good where even a small amount of loop NFB is used.

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 sonic gain might be made with choice of tube and capacitor brands.

I find that caps used in 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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