AWA: Model PA774 8w PA Valve Amplifier

This is an interesting little valve PA amplifier that I picked up today, probably from the late fifties or early sixties.

I don't think it will need too much work, but I'll find out soon enough (famous last words).  It's a little tight on space compared to some others, but not it's not as bad as some other point to point circuits that I've seen.

It has some unusual input and output connectors, and the output transformer has some very high impedance options that will need to be sorted out.  Some of these old amplifiers were designed to run multiple speakers as a 100v line system, which means the output impedance can be extremely high - like hundreds of ohms and as the name suggests, it's putting out 100 volts.  Not very friendly towards your average 8 or 16ohm speaker that is commonly found in guitar cabs. 

AWA MODEL PA774 8 WATT VALVE PA AMPLIFIER     

One microphone, one P.U. channel (for old school record players) with a tone control for the P.U. channel - just a basic low-pass filter.

8 watts, push-pull arrangement using 6BM8 (ECL82) valves.  These are unusual in that each valve contains a single triode and a pentode.  I think these are best known as being used in the Gibson GA-8T.  

A single 12AX7 is used for the mic input and the P.U. inputs.  The mic input has 2 gain stages, the P.U. only has one, as turntables back in the day had a much higher output.

It has an isolated negative feedback stage, taken from a separate winding of the output transformer, going to the cathode of the second 12AX7 gain stage.

Being a PA amplifier, I think this was just meant to be left on for extended periods of time - I say this as it does not have a power switch or even a light to know its on.  They also skipped adding a fuse, so pretty bare-bones on the safety front.

 

AWA MODEL PA774 8 WATT PA AMPLIFIER SCHEMATIC

LEAKY CAPS & OUT OF SPEC RESISTORS

Capacitors
The large Ducon electrolytic capacitors still test well.  I did spend some time reforming them on the variac.  

The wax paper coupling caps were no good - leaky as hell.  Putting 30 volts DC across one and giving it a minute to settle down results in 25 volts of DC on the other end.  As a comparison, I measured some functioning capacitors and these came in at around 10 or 12 millivolts.

Resistors
Quite a few resistors were well out of spec, most had to be replaced.  Many were more than 25% over, there were even a few that were out by 100% 

CONVERSION MODS

At the moment, I'm not trying to convert this into any particular amp, although I am taking a fair bit of inspiration from amps with a similar circuit, such as the Gibson GA-8T the WEM Westminister MK9 and the conversion previously done by Tim Robbins (see further reading below).  There's also a Marshall 1930 that used a similar valve set, but it doesn't look great (volume and tone before the 12AX7).

Power

An on/off switch will be added, with a fuse.  The power lead itself is a bit stiff, so maybe time for a new one.

Output transformer

The output transformer is for a 100v system, with some very large speaker impedances to match.  It just won't work for a normal 16 ohm speaker, so it is being replaced with another vintage transformer.

Negative feedback

Negative feedback will be removed.

P.U. Input 

The phono input will be removed, as it's fairly useless - it completely skips the 12AX7 gain stage.

Gain Stage

The 12AX7 gain stage was designed for high-impedance microphones, so it's a little different to what might be found in a guitar amplifier - this will be made a bit more standard, unsure which way I'll head at the moment.

It's also missing a tone control, so I'll have to think about that as well.  There's only three controls on the front of the amp, and I don't plan on making any changes to the front panel.

Phase Inverter

220k plate resistors dropped to 100k.

Power Amp

A few minor changes and this will be close to the power stage of a Gibson GA-8T.

Lead Dress

I'll rewire and re-route the heater wires, as it's closer to other components than I'd like it to be.  Apart from potential noise, it could be a bit neater.

FURTHER READING - TIM ROBBINS

If you've read this far into the post, you must be at least vaguely interested in Australian amplifiers, check out this site by Tim Robbins.  

Tim has documented a huge number of amps that he has worked on in amazing detail, and unlike me and a lot of other people on the internet offering opinions, the guy is highly qualified and has authored and published numerous technical papers on electronics. 

6BM8 VALVE DATA