Gretsch Mystery Amp Hum c.1960

[Helmholtz]

Helmholtz, I think I got the scope correct this time!!!

Indeed you did .

BTW, your scope can directly measure and display frequency, check the "measure" menu.

I have two unused halves of 12ax7 available, would it be possible and worthwhile to add another gain stage or would that just make the hum louder? Or is the hum and gain two different issues?

Adding a gain stage is likely to increase hum. Consider a linear booster pedal before the amp instead.

[misterc57]

I will try different tubes in V1 and go with the one least noisy.

Any other suggestions on what could be causing the hum?

Thank you

[Enzo]

Hum is about the most common problem there is, and it can come from a hundred places. 120Hz is fromn power supply ripple currents, which could be lack of adequate filtration, but can easily also be from shared ground paths that should be separate. 60Hz is going to be from ground issues o from radiated hum that is picked up.

Most hum is curable, however, in an amp of this age, consider that back then a certain amount of hum was considered normal. SO to get rid of that may require more than simple "repair".

I will never forget the service bulletin that Fender sent out to all warranty stations about the Champ amps. They told us that hum was inherent in the design and not something to be repaired.

[The Dude]

There are some "oddities" in that schematic. The PI input coupling cap is very small value. Is that what's actually in your amp (.0003)? All of the preamp tubes and PI are basically running off the same node. Probably more if I wanted to study it longer. What are tube plate voltages? It's odd that you have to have the amp on 10 to reach bedroom volume. If this isn't something that's valuable to you from a restoration standpoint, I'd be inclined to do some serious modification using the existing unused triodes and some PS modification to build a better mouse trap- maybe even find another amp with the same tube configuration and start over. IMO, the existing circuit isn't much worth saving. Others would certainly argue that point, but that's my $.02.

[misterc57]

Interesting comments. With the lower gain 12ax7 in V1 the hum would almost be considered normal for that age amp, but having to play at VOL 10 really makes the hum noticeable.

Power tube plates are 314 V

I will check on the PI coupling cap.

I wonder if the odd speakers have something to do with the lack of volume.

This is a hobby for me so I am going to keep plugging away at this one. If I can easily clip in another gain stage I'd like to try that test.

[misterc57]

Good news. After trying some different 12AX7 tubes in V1, I landed on one that made the amp loud and has no hum!

Thank you all for the help. I learn so much from all of you!

[g1]

Good news. After trying some different 12AX7 tubes in V1, I landed on one that made the amp loud and has no hum!

Do you have some 12AX7's that are no good? Or could it be the socket was oxidized or has a bad connection?
I would suggest you go through your 12AX7's in a good working amp and discard the bad ones. If you don't find any, you may have an intermittent problem with this Gretch amp.

[Helmholtz]

With a good tube the input stage should have a gain around 60.
So if you feed a 100mV input signal, the plate signal should be about 6V.

You can use your scope to verify.

That said, even new tubes can vary considerably regarding heater hum.