This schematic is a straight forward amplifier which takes advantage of the nice triode characteristics of the 6CM5 or EL36 to make a PP triode class AB amp with around 25 watts output. Another tube that could be used is the very little known
6FW5 which is an octal tube and exactly like 6CM5 electronically. The 6FW5 HAS NO TOP CAP and this is a
blessing because top cap connections are dangerous because people leave them exposed  and some victim will reach over and touch the +375V and maybe its your grand child.
Maybe there are some stocks of 6FW5 lurking somewhere. There would not be many on Oz but would be a lot in the US.
Meanwhile, the 6CM5/EL36 would have to do. There used to be a 6CM5 in every second TV set until HV transistors were
finally able to be made with a reliable outcome. As solid state took over from tubes the 6CM5 was one of the
last tubes to be retired in favour of silod state.

Because there is not a really high B+ for the driver stage, V3/V4 to work from and because the driver has to produce
up to about 47Vrms at lowest possible distortion I took the "dead" grid of the V3/V4 LTP to -25V because
it was available from the zener string which regulates the bias voltage and which appears in the power supply schematic.

Note the zobel networks used to stabilise the amp even with only 12dB of global NFB. Such zobels tame the gain
and reduce phase shift at HF where otherwise the gain and phase shift would allow oscillations at HF.
Depending on the OPT chosen and its shunt capacitance and leakage inductance, the values of R and C shown
will not necessarily be used. Unless you know how to build an amp with NFB so it is critically damped,
you will find your amplifier may oscillate, especially with a 0.22uF cap on the output without any R load.
So values of  C6 & R12,  C11 & R28, C12 & R29, C15 & R31, and  C14 & R30 all have to be trimmed to
values which ensure stability at HF!!!

There is also a LF gain / phase shift correction network with C5 & R11. This should always be used
regardless of how much inductance the OPT primary has.



This is very close to the original power supply.  If your chosen power transformer does not have a bias winding then
you may use a separate bias transformer using one taken from a canobolised solid state amp. I have a few old
small transformers with a 240 primary and a few windings which can be used to make a suitable voltage for doubling
and then RC filtering. The filtering of the bias circuit above is a bit excessive.
The bias voltage is also shunt regulated but really need not be regulated because when the mains voltage rises say 10%, The B+ will tend to rise since the B+ is not regulated. However the combined parallel Ra of all four output tubes is 150 ohms only, so the B+ rise with a change in mains voltage will not be much, since the source resistance of the B+ is probably much more than the Ra or the 4 tubes in parallel. To counter the rise in B+, if the grid bias is also allowed to increase with a mains rise
then the tubes tend to be biased to conduct less current by increased grid bias. I used to be fanatical about
regulating and smoothing bias circuits but it isn't always necessary in a triode amp.
But I also wanted shunt regulation of the voltage applied to control the constant current source  Q1 MJE340 for the
cathode current to V3, V4.  Again, this isn't strictly necessary if a voltage divider is used to replace the zener string
and large value caps of 1,000 uF are used as bypass caps. Such effective bypassing with humungous cap sizes is legitimate, does not spoil the music and the caps for the voltage wanted are small and cheap.

The B+ rail is not regulated. The transistors MJE340 and BU208A make it look like it is regulated but what is there is an  "electronic ripple reducer", otherwise called a capacitance amplifier but really what the bjts actually do is act
as a giant emitter follower with the two bjts connected as a darlington pair, with very low output resistance and very high
input resistance at signal frequencies. When I first built the amp it was regulated with a string of zeners to hold the
MJE340 base at about +377 V.
Some years later a friend asked me to lend the amp to him to try with a pair
of horn speakers he'd built which were about 105dB/W/M efficient. Unfortunately there was hum to be heard and I altered
the regulator to what you see above and the hum disappeared. In fact there is only 10mV of hum
at the bjt output emitter, although ripple voltage at the input of the collectors is about 11Vrms.
So the above active hum reducer acts with an attenuation factor of about 0.001.
To get the same amount of attenuation with a CLC filter with C1 = 50uF as shown, ( C2 & C3 in series, )
and C2 = 100uF, I would have to use a choke of 25H which would have been 10 times more costly than a couple
of R&C and two cheap and common bjts mounted on some scrap aluminium for a heat sink.
The mechanism of the hum persisting in the above emitter follower is because there is some ac collector hum current
current since the ripple of 11V works into the collector resistance which is perhaps about 10k. HFE of the
pair of transistors is about 200, so about 0.005 mA flows at the base and since the base resistance is 1k, about
5mV of hum must appear at the base. The passive filtering of R2 & C6 and R5 & C8 attenuate the 11Vrms of ripple
from the top of C2 by 0.00011 and in any case the resulting 1.25mV of hum at top of C8 is phase shifted by nearly 180 degrees and probably counters the hum from the collector to base resistance path.
So about a mA of ac hum current flows in the collector and so some base current must flow and since there is a 1k series base resistor, R7, there is a tiny hum voltage at the base. The 1k is needed to allow the base voltage to be quickly pulled down by the current in the four 1N4007 diodes in series between base and output if the output is ever shunted to ground in a fault.
The action of R8, 2.7 ohms acts to allow a voltage of 1.4V to be generated across itself when I out = 0.5A approx.
If this occurs the threshold voltage of 2.5V across the 4 diodes is exceeded and the base is pulled towards ground
and the excessive current should cause the mains fuse to blow.
The resistors R 2 and R5 which total 300k have a dc base input current flow so that when 230mA flows in the
collector-emitter path, base current is about 0.16mA, thus base voltage is about 50V below the voltage at the top of C2.
The transistors don't seem to be able to be killed in the circuit even if a short circuit occurs. Complicated? sure is, and much has to be considered and included in a high voltage regulator or ripple reducer or else the use of solid state as slaves to the tubes lets the smoke out of the devices, and once it has come out, it won't go back in!
If the input to the collectors is shunted to ground then current stored in C10 could flow backwards through the bjts.
This means instant death to a bjt if the reverse voltage from emitter to collector exceeds about 6V!! So hence the diode between base and collector allows any back flow to harmlessly bypass the bjts.
There is a 100 ohm series R4 rated at 10 watts on the heat sink. there is normally about 23v across this R
and about 5 watts is dissipated. About  25v is across the bjt,  so 5 watts is dissipated in the bjt.
With an increase in current, the voltage across the 100 ohms increases, and voltage across the bjt reduces, so
short circuits kill the resistor, not the bjts. The maximum current flow possible from
the 425V at top of C2 is limited to 4.25 amps. This is less than the BU208A maximum collector emitter current rating.
Series pass element regulators should always have this feature.

Maybe you are not impressed with the idea of enslaving power transistors to the whimsical current desires
from the tubes. OK, use a choke filtered B+ instead. See my pages on power supplies and follow your nose
to where I discuss the benefits of using large capacitances and smallish chokes to get a CLC input circuit with very low ripple with 470uF, 2.5H choke and 470uF with diode rectifiers. It means though that the B+ winding with a full wave bridge needs to be about 284Vrms. About 142Vrms is needed for a doubler circuit with the two input caps at 470uF each which need to
be rated for 250V each, but the same 2.5H and 470uF still follow; the 470uF can be 450V rated where there is approx
375V across the caps. Selecting a power tranny with slightly too high HT winding is better than selecting one which gives
a disappointing low B+ voltage. Having say +400V B+ at 230mA for the 2323 can easily be reduced by placing
series R between the diodes and caps to be charged to reduce the peak charging currents in high value electros
with silicon diodes. Using a B+ = +350V will give much less power.
When I wind transformers for such amplifiers I usually have about 4 taps at about 15Vrms steps in from the ends of the HT winding to accommodate different output tubes and be able to adjust the B+ voltage without using too many series
trimming resistances.

There is shunt regulation of the anode supply to the preamp input stage using a string of zener diodes at the first stage of the power amp.
This prevents any possibility of LF instability.

There is no active protection on the amp because it is a Junk Box Special and I have a lot of spare 6CM5, and if I fuse an OPT
then I won't cry too much; I'll just have to wreck another few Sundays to make replacement OPTs which would withstand
a short circuited or saturated tube for longer than the existing trannies would. The wire used in 1950/60s OPT was
usually very thin and prone to easy fusing. The use of thin transformer wire was an unscrupulous capitalist plot to maximize meagre company profits and have you come back to buy again later. ( But when transistors displaced tubes in 1960, no wonder people happily threw out their parked and defunct tube amps. And the strange thing was that although the costs
of early SS amps was much cheaper they initially sold for very little less than the tube amps they replaced ).
 
If I was keen I would place 200mA fuses into each cathode circuit on each output tube.
These should blow if a tube saturates and thus save the OPT but maybe not. I am wary of fuses because they don't
always blow when you want them to.
See my other pages on active protection.

The amp in 2006. It gave no troubles after 11 years, and in 2009 it still goes fine.