Precision Electronics Amplifier question

[Garydean]

Precision Electronics Amplifier question

1950's audio amp.

This amplifier was converted to a guitar amp. It's 10w and uses a 6X5GT for the rec, 12ax7 pre and a 6l6gb for the output tube. The sound is ok but it's a bit bright and lacks headroom. I'd like to do some tone cap changes to darken it up a bit. There is a bit too much sag as well. It is a class A amp. Correct me if I'm wrong, a SS rectifier in this situation will not help the sag as the output tube is running close to max all the time. A SS rect will also put out more VDC, where the 6L6 may not be able to handle it. So what part of the circuitry of class A amps determines more or less sag? I've yet to fire it up to test voltages, just got it in. I do see the filter caps are original. Perhaps raising the the values would give me more headroom as well?


Thank you,

Gary

[Enzo]

What voltage on a 10 watt amp will increase so much as to endanger a 6L6?

What B+ voltage is there now? I'd bet it isn't 500 volts or something. I'd bet your solid state rectifier would not harm anything.

But are you sure "sag" is what is happening? Monitor the B+ voltage while the sag happens. Is there a substantial drop there? WE want to be sure what we are trying to solve before taking the cure.

Raising the cap values? They are 50 years old. How about just fresh new ones. If there is an 8uf or 10uf filter, I might make it a 20uf. But if caps are the cause of your sag, I'd bet it was from being dried out rather than too small.

Is the 6L6 cathode bypassed? is that cap dried out? What does the cathode voltage do when the amp sags?

And how about the coupling caps? ANy of them get leaky - pretty likely - and DC levels through the amp suffer. I could see where the signal makes the DC charge across some cap change in a way that resembles sag.

[J Martin]

I have one of those amps. It just has a weak power supply. The rectifier is only good for 70 Ma. I thought about putting in a 6AX5 rectifier that is pin compatable with the 6X5, but will pass more current. Its hard to tell if that little transformer will carry the extra heater current, though. So it will sag no matter what due to the entire supply weakness. I decided not to mod mine as it would lead to changeing too many parts. You might as well build from scratch then. I think it is cool as it is with a few fresh caps.

[Garydean]

I finally got to changing the cap can and the amp sounded better but still kind of brittle and lacking bottom. The sag is not as apparent.

I then decided to replace the 12ax7's 100k plate load resistors to 150k. This browned it up quite a bit. I now have 200v on the 12ax7 plates instead of 238. Seems to make a difference and breaks up sooner. I'm gonna try a few different speakers as well.

Here's the question.

I did notice that the 12ax7 socket is wired in reverse. Why? Would switching that make any difference?

thank you,

Gary

[Enzo]

What do you mean "in reverse?"

[Garydean]

Hi Enzo,

For example the input signal goes to pin 7 on the 12ax7. Looks like they wired the sides the opposite way. Or maybe this is normal? Most of the schematics I've read have the input connected to pin 2. I guess because the input jack is on the back panel, wiring the other way would result in crossed signal wires?