Ampeg Gemini I Hum

[mikeboone]

Hi Guys,

A friend brought me an old Gemini I amp that was not working. The OT had an open winding so I got a replacement OT and all new electrolytics from FlipTop, it looks as though the amp still had the originals.

So I got the new parts installed and there is a hum present. I am measuring about 3VAC at 120Hz on the 70uF tab of the 70/40/40 cap can. The others have less than 100mVAC on them.

So, I lifted the leads from the 70 tab and subbed in a brand new 47uF filter cap to see if perhaps the 70uF was flaky, no joy.

I made sure all 4 tabs on the cap can are grounded to a lug on the chassis.

[Enzo]

The negative return of your bridge rectifier through the standby switch, where is THAT grounded?

[mikeboone]

The standby switch is actually out of the circuit, not sure why but it is. Anyway, the neg return of the rectifier is attached via a wire that is grounded to the same chassis lug.

[mikeboone]

I removed the wire from where it was grounded on the board and ran it directly to the ground lug on the chassis. The AC voltage and 120Hz are still present but audibly, the hum is reduced quite a bit. I also cleaned out the eyelet at the rectifier and re soldered that wire/connection point. Perhaps that was it. Still has a hum but just not as bad.

I am going to check another one of my amps to see if there is a similar ACV/freq present there.

[mikeboone]

So, that's that. I checked on old super reverb and an Orange Rocker 30 at the first filter node. Both had 2-3 VAC and 120 cycles present.

The ampeg was a probably a faulty solder connection.

Thanks for the nudge, Enzo.

[luthierwnc]

I'm getting in a bit late but those amps are really touchy about the ground points. The "V" amps are worse. Reflow all the ground solder joints you can reach with a touch of good quality solder. I usually will ground the meter at the main star and check every ground to see how close they are to continuity. They often vary by a couple of ohms. One handy (and free) tool is to put a mini-grabber test lead on one of a piece of wire and solder a .001 630v cap on the other. Shrink-tube it several times. Ground the clip and touch points on the amp with the barely exposed other end of the cap. It will kill the hum and everything else by shorting a lot of the signal to ground. Poking around with that can really help find trouble in a hurry. FWIW, sh