[AdrianH]

Well that is the plan anyway, as if I do not have enough projects on the go, but after playing with an old aircraft transceiver I have a valve to play with, it was the reason I picked the TR1998 radio.

The valve is a twin beam Tetrode meant for VHF RF power output, but within the data sheet for the TT15 it has an interesting detail for using the valve as a push-pull class AB1 audio output. Well I hope I am reading it correctly. So this is where I will be looking to your assistance in checking things out for me?
The basic specs given for what seems to be best operating points are:

Va = 300 V
Vg2 = 175 V
Vg1 = -21 V
RL (a to a') 7K Ohm
Max input 41 V p-p
Max power out 16.8 Watts and
Max distortion 2.25%

I have attached images of part of the spec sheet showing this and the curves for the valve. Now I think this is with fixed g2 voltage, which would simplify making a transformer for it. I doubt very much I would end up running the valve to full output so would like around 10 watts maximum out of the amp when finished and if I manage to make one work then I will add another amp to make stereo!

The idea is to base it on the circuits of the Radford Valve amps. I have N.O.S. ECF82 valves to drive the output valve, thinking that if these can drive EL84's and EL34's then this should not be an issue, although I note that the TT15 requires a 100K grid to cathode resistor, this may be a bit lower than normal.

Driving the ECF82 could well be the EF86 if I can find a couple cheap enough, although I may also try the EF91, just because I have a handful of them out of this transceiver as well as other common 7 pin devices, (EC90, EL91).

Will it be Hi-Fi, I doubt it, would it be OK for my hearing, probably if I can get it to work without self oscillation.

I have picked up some imported valve holders to start with, but the valve will not fit! It looks as though the central locating fin is too wide for the holder, so now wondering if I try to file out the holder or file down the fin.

Anyway thoughts always appreciated, apart from derogatory of course.

Adrian

p.s. Just noted that as the g2's for both Tetrodes are common on the same pin that the g2 voltage must be fixed.