Gibson G30 solid-state amp hum

[Rhodesplyr]

I've been working on a 1972 Gibson G30 SS amp, and the hum (60Hz) at the speaker is 11mV, not affected by any of the controls, including the volume control. It's just a constant background hum.

Granted I'm more comfortable with tube amps, but nothing in my bag of tricks for reducing hum has had the slightest effect on this amp. Is this level of hum just par for the course for a somewhat primitive SS amp with a single-ended power supply rather than dual-rails?

Attempts to reduce hum included taking the AC off the circuit board/switched volume control. That had no effect. It's not filter caps. I tried shielding the PT with some steel. That had a very slight effect, but not much.

I'm also puzzled by the fact that I'm getting 60Hz hum when the rectifiers are a diode bridge.

[olddawg]

What happens when use a shorting plug in the input? What is your ripple measurement on the rail voltages.

[galaxiex]

There is a thread about the Gibson G30 and hum issues here...

https://music-electronics-forum.com/...ad.php?t=40551

.

[Rhodesplyr]

Sometimes you need to post about something for the solution to come to you.

I realized I hadn't tried moving the power transformer, so I unbolted and moved it. Hum disappears. I should have known since it's 60Hz hum.

Classic original design flaw. Now the question is where there's room to put it. And there's the problem that the reverb recovery stage is on the opposite side of the PCB from the input stage, so you have high-gain stages on both sides. Hmmm.

[Enzo]

I'm also puzzled by the fact that I'm getting 60Hz hum when the rectifiers are a diode bridge.

Don't be puzzled. 120Hz hum is ONLY hum related to the power supply ripple. Hum from grounding or shielding or radiated issues will be 60Hz.

Schematic please? OK, I am assuming the schematic in that other thread matches? And what features do you have? Do your have the reverb do you have the trem?

Trying all your tricks means you are guessing. There are many sources of hum, and each has its cure. In other words increasing the filter caps won't help a poor ground at the input jack.

You need to isolate the problem. Where is the hum coming from.

This schematic is maddening. The part numbers refer to the parts list, they are not part numbers in circuit. In other words EVERY 0.1uf cap is "C1". EVERY 68k resistor is R1. etc.

With that in mind, the power amp input cap, C1, left end, short to ground. Hum still there? If you have trem, you could short across the trem pot to do this.

[Enzo]

Oh, OK, does just turning the transformer 90 degrees make a difference? Radiated him can be directional. In fact, can you determine where in the circuit it is being picked up?

[Rhodesplyr]

Oh, OK, does just turning the transformer 90 degrees make a difference? Radiated him can be directional. In fact, can you determine where in the circuit it is being picked up?

I'm pretty sure it's being picked up in the tone control section. I tried pivoting the transformer on all its axes. No significant difference. What looks like it might work is to hang the PT underneath chassis. As built, it sits on the same physical plane as the PCB. Some drilling and lead extension will be required. That way, you have the steel chassis between the PT and the circuits.

Where I get zero hum is out in mid-air behind the amp, but that's obviously not a practical solution.

[Rhodesplyr]

Conclusion: Flipping the PT around to hang underneath the steel chassis drops the hum from 11mV to 5mV, which is acceptable for an amp like this.

New mounting holes have to be drilled for the power transformer in order to clear the cabinet woodwork that holds the amp chassis.

Also, FWIW, this amp puts out 20 Watts at onset of clipping with a 1kHz sine wave test tone.