MK 2 "EU version"

Here is another Audio Amplifier which uses the less expensive types of vacuum tubes available.
It is essentially the same design as my 2008 offering see :- Economical Valve AF Amplifier
However; because of some subtle design changes I thought another web page was warranted!
This design uses all octal base valves so is even more "retro" in appearance than before.
In keeping with the Aesthetic requirement I have used golden orange paint with black features.
The most significant difference is the use of PL36 in place of the EL36/6CM5 for the push-pull output stage(s).
As the PL36 was commonly used in European/UK TV receivers (and most likely still plentiful)

I have called this design the "EU version" !

The PL36 really differs only because of its 25 volt, 300mA filament.
As can be seen from the schematic 4 filaments are series'd and fed from the 100v secondary of the transformer.
This relieves a current burden on the 6.3 volt "filament" winding which was seen as a major consideration in my earlier design.
So: it may be possible to implement a re-cycled ex Color TV mains transformer without the need to rewind extra filament windings to supply the several ampere's current needed to heat all these valves!
The 6.3 volt winding now only supplies the voltage-amp (6SJ7) and phase splitter (6SN7) (of both channels).

Valve Current Total
6SN7 0.6 X 2 1.2
6SJ7 0.3 X 2 0.6
ALL 1.8 A

6.3V filament load

The 100v winding is used for 4 valve filaments (PL36), the HT supply and the 50 volt -ve gridbias.
Because of this multiple use of the one HV secondary winding, by referring to the schematic it can be seen that one end of the AC filament string is at DC potential of approx +135 volt.
Clearly the danger of heater to cathode breakdown must be considered.
The PL36 valve data states the following :

The PL36 heater to cathode isolation voltage maximum is:
with positive cathode 250 volt
with negative cathode 200 volt

Therefore we are safe, and in fact my 'build' works fine/sounds fine, with no AC hum and most importantly no "flashovers" !

An Internet valve seller states the following;-
"The PL36 was orignally designed for use in demanding TV timebase circuits,
however - they have been found to be excellent for valve HI-FI audio amplifiers.

A Larger heater voltage allows for lots of emission at power peaks"

I can't substantiate the above but it's an interesting theory.

I found it necessary (upon experimentation) to include a 200 ohm 15 watt dropping resistor in the mains primary so as to reduce the HV secondary down to 100 volt rms (on load).
What I seem to recall as an S.O.T. (select-on-test) component.
Initial voltage was about 15 volts higher and hence would have lessended valve life due to filament overvoltage. So I had to add the series voltage dropping resistance - not good design I admit, but little else that I could do.

This handbuilt amplifier has again taken me many hours, but my efforts were rewarded with that Rich Valve Sound ................ that which many will strive to attain !!!

Frank Hughes VK6FH May 2009