Mystery Valve Gear - Air Ministry?

[Excessive Leakage]

Good afternoon!

I have had this little amp in storage for some time, not least as I am struggling to identify what valves should be fitted to it   . Datewise, the large electrolytics on top are dated Oct 50.

Valve pin wiring (all IO bases) from left:

V1 Pin 2 Black wire to V2 Pin 2
V1 Pin 3 Red/Black
V1 Pin 4 Red/Black
V1 Pin 5 connected to Pin 8
V1 Pin 7 Red wire to V2 Pin 7

V2 Pin 2 Black to V3 Pin 7
V2 Pin 3 Red/Black wire and also connected to Pin 4
V2 Pin 5 connected to Pin 8
V2 Pin 7 Red wire to V3 Pin 2

V3 Pin 2 Red Wire to V4 Pin 6
V3 Pin 3 wire
V3 Pin 4 Red wire to V4 Pin 4
V3 Pin 7 to V4 Pin 1

V4 Pin 1 to V3 Pin 7
V4 Pin 1 to Pin 8 via 56R resistor
V4 Pin 4 to V3 Pin 4
V4 Pin 6 to V3 Pin 2
V4 Pin 6 to Pin 8 via 56R resistor

V5 Pin 2 to mains tx yellow wire
V5 Pin 3 to mains tx red wire
V5 Pin 5 to mains ts red wire
V5 Pin 7 to mains tx red and yellow wires
V5 Pin 8 looks like it was connected to something but there is no obvious broken connection or hanging wire. FW Rectifiers I checked on would likely have Pin 8 connected?

A label on one of the pots says simply 'Rx adj', another 'Bias 180n' and the front label '8n F. Bias' (I take 'n' to be an ohm symbol). It is not clear whether it is an ex-Air Ministry item or possibly a, well constructed, homebrew.

Input tx is a Radio Spares and the Gardners Radio Ltd output tx is rated at 10W, wired to 8000ohm, with a centre tap. No markings on the choke.

Any assistance in identifying it, especially a suggested valve arrangement, would be greatly appreciated.

Many thanks

Adam

[Ed_Dinning]

Hi Adam, looks like a fairly standard audio push pull amp of the 50's
Valves connected to the op trans will almost certainly be 6V6's as they need an 8K load
Rectifier will probably be a 5Z4, rectifier of choice in the 50's (V5)
Other valves probably EF37 or similar, top cap audio pentodes
Should all be easily obtainable

Ed

[beamcurrent]

I agree with Ed's suggested valve line up.  

To me it has the look of a well constructed home project. suggest you look through old wireless worlds or other 1950s magazines to see if any published circuits look likely.  

The Caps being dated 1950 means it is not older than that, if it is home constructed the caps could have been on the shelf for a while.

Bit worried about the pot labeling ???  does not make much sense.  remote possibility it has some association with tape recording??

Brian

[Refugee]

There is a chance it could be a film sound amplifier.
They had photo cell sound sensors and lamps that were driven by a regulated oscillator.
The oscillator would be mounted close to the lamp off the chassis and the cell bias would be on the amp chassis.

[Excessive Leakage]

Thanks for your helpful comments. It is a bit of a head scratcher  

I only mentioned AM due to chassis colour and number of AM marked components. However, these would have been plentiful as surplus after WWII, so the 1950s home built project sounds the most likely.

6V6 did cross my mind when I saw the 8k loading but I was wary of experimenting before checking with the forum!

I'll test the caps before applying any power. Some of the electrolytics look rather 'tired' after 60-odd years.

Best regards

Adam

[pwdrive]

You should be able to reverse engineer a schematic for that if none comes up.

Good luck.

Lawrence.