Ampeg V4 60 Cycle Hum - Pronounced When Reverb is Turned Up

[bepone]

the oxide on the reverb tank connectors? or cable broken inside the tank? or grounding wire on the reverb tank inside?
when connected all , power off, test with the multimeter ground continuity from amp chassis to reverb tank chassis=?

Thanks for the response!

It appears that even with the tank removed the buzz persists.

this is normal, because residual cable is acting like antenna and collecting 60 Hz electromagnetic wave from power transformer or some heater wires in the amp.
in that case, tank out, make a short circuit in the cable (on the connector) and hum will disappear probably. in that case, problem is inside the tank. this is my suspicion, grounding inside the tank.
so connectors, oxide, broken wires inside etc etc.

[Stovemananon]

the oxide on the reverb tank connectors? or cable broken inside the tank? or grounding wire on the reverb tank inside?
when connected all , power off, test with the multimeter ground continuity from amp chassis to reverb tank chassis=?

Thanks for the response!

It appears that even with the tank removed the buzz persists.

this is normal, because residual cable is acting like antenna and collecting 60 Hz electromagnetic wave from power transformer or some heater wires in the amp.
in that case, tank out, make a short circuit in the cable (on the connector) and hum will disappear probably. in that case, problem is inside the tank. this is my suspicion, grounding inside the tank.
so connectors, oxide, broken wires inside etc etc.

That makes sense, but if it were a grounding issue in the tank why would the buzz persist without the tank in the amp ?

[Stovemananon]

Update:

Thanks to all that have helped out in this discussion!

I replaced all polarized capacitors and the hum lessened.

I replaced C16 .047 uf capacitor with a .068 uf capacitor and all of the sudden the hum reduced to a very tolerable level.

I think every capacitor in this amp has now been replaced.

Does anyone know the function of C16 in the bias section? (schematic below)

It reduced the buzz most audibly.

I can't get it to go completely, but this might be as good as we're going to get.

Screenshot 2022-08-18 113103.png

[bepone]

That makes sense, but if it were a grounding issue in the tank why would the buzz persist without the tank in the amp ?

because you have open /unterminated cable connected to very sensitive amp grid=?

[bepone]

Does anyone know the function of C16 in the bias section? (schematic below)

this is to create negative bias . if you changed this value, you are also changed bias voltage too. need to recheck all tubes now.

[Stovemananon]

Does anyone know the function of C16 in the bias section? (schematic below)

this is to create negative bias . if you changed this value, you are also changed bias voltage too. need to recheck all tubes now.

I will put a .15 uf in series with the .068 since I do not have any .047's on hand.
Thank you!

[Stovemananon]

That makes sense, but if it were a grounding issue in the tank why would the buzz persist without the tank in the amp ?

because you have open /unterminated cable connected to very sensitive amp grid=?

I follow now. I'll take a look inside the tank. Thank you!

[Charlie Wilson]

A little off topic, but when you replaced the electrolytic capacitors I hope you used insulated cap cans.
CW

[Stovemananon]

The side of the reverb pan labeled output can not be on the same side of the amp as the power transformer.

It may be that the pan was remove once to check it, repair it or replaced it and it was reinstalled wrong on the lid of the amp.

Also confirm that the sheild of the cable on the output side of the pan is at chassis ground and the whole pan itself is at chassis ground.

If a incorrect replacement pan was put in then this may not be the case.

That makes sense, but if it were a grounding issue in the tank why would the buzz persist without the tank in the amp ?

because you have open /unterminated cable connected to very sensitive amp grid=?

@Stevem and @bepone,

I can't thank you guys enough. The above two quotes helped me solve this issue and I feel mighty foolish.

A little history on the amp before I explain what ultimately solved the problem (so that I seem less like a fool, though a fool I am):

The amp was bought in 1971/72 in NJ by my father who was 20 years old at the time. He believes that it was purchased brand new. He has never had it serviced, not once. Never changed one capacitor, never changed one tube.

We were jamming a year or so ago and heard some strange noises coming from the amp. At that time I didn't know even the small amount that I do now about amplifiers.

In 2022 he brought it over to my house where it was opened for the first time since (I believe) it left the factory.

So when the advice about the reverb tank being installed in reverse came up, I didn't pay much mind to it... as I figured it must have been installed correctly for the past 50 years..

This is not the case.. It was installed backward. And the hum that persisted when the tank was removed WAS because of an "open /unterminated cable connected to very sensitive amp grid"

Now I feel like such a fool. So many hours down the drain, but I guess that is how you learn.. the hard way.

Anyhow,

A huge thank you to everyone that posted here. I greatly appreciate your time and advice.

[Stovemananon]

A little off topic, but when you replaced the electrolytic capacitors I hope you used insulated cap cans.
CW

Yes, the cans were insulated!

I got them from Fliptops and paid the extra $. Also I steamed off the cardboard tubes just for a more original look, though I regret that decision now.

[bepone]

Now I feel like such a fool. So many hours down the drain, but I guess that is how you learn.. the hard way.
A huge thank you to everyone that posted here. I greatly appreciate your time and advice.

No problem, enjoy the amp

[Stevem]

First off your not a fool, as fools never learn!

Anyway, All’s well that ends well, especially if some new knowledge comes along with it !

Enjoy!

PS.
Over the passed winter I had one of those that drove me to the edge nuts with crackling in that reverb circuit that got way worse with temperature rise.

The issue was on that 100% flat out pain in Ass to remove switch board on the face of the amp.

I was a combination of a noisy resistor and a bad solder connection on one end of the reverb pot to the board.
So yes to all who work on these amps, there are caps and resistors on that board also so keep that in mind!

By the way, does that amps metal top panel still have its reverb pan lock lever, and does it work?

[Stovemananon]

First off your not a fool, as fools never learn!

Anyway, All’s well that ends well, especially if some new knowledge comes along with it !

Enjoy!

PS.
Over the passed winter I had one of those that drove me to the edge nuts with crackling in that reverb circuit that got way worse with temperature rise.

The issue was on that 100% flat out pain in Ass to remove switch board on the face of the amp.

I was a combination of a noisy resistor and a bad solder connection on one end of the reverb pot to the board.
So yes to all who work on these amps, there are caps and resistors on that board also so keep that in mind!

By the way, does that amps metal top panel still have its reverb pan lock lever, and does it work?

Thanks Stevem! I was about to move to that small board next, but it looks so difficult to remove! Saw some tropical fish on it, and those often go bad.

Yes, the reverb lock and lever still work well! My dad really took care of this thing.

Quick story: when I got it, it had a three prong cable, but it was still shocking the hell out of me. I measured AC volts on the instrument cable and it read 120!

It turned out that the person who installed the three prong knew nothing about electricity and had not changed the cord. They merely spliced a three prong plug on the end of a two wire cord. On top of that, they had the polarity reversed. So In order to turn off the 120 v on the guitar cable, the polarity switch had to remain up at all times.

I guess the death cap had gone bad?

I used this amp about 16 years ago to gig and would always get shocked when my lips touched the microphone. I guess I was getting bit by 120!