A couple of years ago a friend of a neighbour asked me to have a look at a 5 valve P.A. amplifier he wanted to get going.

He said he had plugged it in and there was flash in one of the valves. When he brought it over I could see why. Someone had got at it and made a complete mess of it with wrong valves in the sockets bar one. It had a 6V6G where the rectifier was meant to be, a 2A5 & 42 pair for the output, a 1F6 for the 79 & a 6J8G for the 6J7G.

The neighbours friend left the amp with me & it is still here. I traced out the circuit as found and deduced that the valve line up should be as follows: 6J7G, 79, 42, 42 & 5Y3G or 5V4G. It also appears to have a line impedance output transformer with 4, 25, 150, 280 & 500 ohm secondary taps which suggests a P.A. amp. Whoever got at it made some serious blunders and for me to rectify is beyond my scope of experience without a proper road map. The chassis is also quite compact without much room to move around.

I've included the circuit as I found it & some of the blunders in it are obvious. The voltages on the power transformer are as with no load & measured with a cheap DMM. Hopefully someone here might be able to give some advice on correcting the errors or some other suggestions to get this thing going again. As the likelihood of locating a matched pair of 42's is virtually zilch, could I use a pair of 807's instead or something else that is still readily available?

PA Amplifier Circuit Diagram

6F6 is electronically the same from what I have read.
But they are an octal base so would have to change the
sockets.

https://www.radiomuseum.org/tubes/tube_6f6.html

I did think of 6F6's, but wouldn't they be just as hard to find as 42's?

The octal version of a 79 is a 6N7.

Not quite as hard.
Strange circuit, 2 x 42s gives 18.5 watts a 79 gives
8 watts on its own.
500 ohm transformer is 20w.
The other option is a full rebuild with 6V6's
heaps of simple circuits out there.

Ugh, the substituted valves are a real dogs breakfast. How that was supposed to work is anyone's guess.


If you do decide to use 807's, just beware they are big and have the anode connection on top waiting to bite the unwary. (I'm sure you know that but still...)

Never seen a 79 or 6N7, so let's see... it's a double triode, so presumably a pre-amp and inverter.

2A5 is same as 42 except for the heater voltage. I've been after a working 2A5 for a long time. Don't know if yours would still be working after the application of incorrect heater volts.


Let's see what your schematic says, when it appears.

Also depends on what your neighbours friend wants
to use it for too

Got a 2A5, when I find out where one of the VCTs two handbooks vaporised to I will test it. I have the tube trapped: Its in the tester. I did find a hidden screw driver getting the valve. Gotta love organised chaos.

Also depends on what your neighbours friend wants
to use it for too

You could probably guess!!
I know your stance on converting PA amps to guitar amps & I am totally with you in that regard.

The 2A5 & 1F6 didn't survive their ordeal & I am sure it was the 1F6 that flashed.

I will shock you! the exception to the rule is home brew
especially if badly made as they present a blank
slate to rewire and redesign for a multitude of guitar tones.
There are so many circuits out there to follow. 😀

Josh

I'd be punting the existing valve types.
Just buy cheap types, 6SJ7, 6SN7, 6V6 and a 5V4.
All are octal rugged single ended and work well.
As Josh says there are a million circuits, the existing circuit except for the odd things in it is so typical.
Look at back issues of RH for typical circuits.
I would have the 6SJ7 as the preamp, 6SN7 as the phase splitter driving the 6V6's and the 5V4 can easily drive the show and is a far better rectifier than a 5Y3.
If you draw a blank I can draw up a circuit for you from your circuit.
Cheers, Fred.

Thank you everyone for the feedback so far.

I would like to restore this amp as close to as found with the two inputs & two volume controls, albeit with the valve line-up suggested by Fred. Except for a matched pair of 6V6's, I have NOS and used 6SJ7, 6SN7 & 5V4's.

Since this amp originally used push-pull 42's, I'm guessing the primary impedance of the OP transformer would be about 10K. I would like to keep the existing OP transformer at this stage. Would using 6V6's still be OK?

Fred, it would be fantastic if you could draw up a circuit based around the one I supplied using the valves you suggested. What about the 100mA filter choke? Would that still be used? Or can it be dispensed with?

I would like to keep the existing OP transformer at this stage

Check that out for continuity, etc, before going much further.

Hi, no problem and you have answered a question I would have asked, the choke rating.
That's big enough to take the whole amp rating, i'll pen out a circuit tomorrow using your valves and email it to you.
We can follow the same outline as the original amp with the same controls, just the details a bit different.
Cheers, Fred.

If the output is rated for PA it will be 100V line out andnot practicle for guitar.

A good reference for simple vavle gutar amp ideas is:

https://www.valveheaven.com/category/the-lamington-amplifier/

They use current 100V line transformers (Altronics) as output transformers and also have a silmple push pull 6V6 amp circuit as well.

Hi, I have sent an email with a mud map circuit and a long rant about some points of design.
Its too long to try and put in this forum, the site crashes out 500 for that length of send.
Just for every body Josh is right about the Altronic trannys, some can be used on class AB PP valve pairs with no problems, I have done this and even used filament and battery charger trannies on non crtical designs like single end 6V6.
"Golden ear paper designers" get too precious about Hi-Fi waffle and should actually try building something and test it than to crap on with theory!
Happy to help with any questions or querys no matter what circuit you use.
Fred.