Ampeg Gemini I hum/buzz

[MonkHouse93]

http://www.unofficialampeg.com/schematics/g12.gif

I have an ampeg Gemini I that has very audible hum. I was wiring up the filter cap when I noticed that when every cap is wired in, there is audible hum, the ripple measures 4.5VAC on the 70uF node. When I remove the wire connecting that cap, the ripple jumps up to 34VAC-ish, BUT the hum becomes much less audible. Then I replaced the original can-cap (70/40/40uF) with one that has 40/40/20/20uF. No matter what configuration I use on the new cap - 80/20/20, 40/40/20, 40/40/40 - The hum is the same except when I take out the filter cap at the +node of the bridge rectifier. The original 70/40/40 does the same thing, so I'm not sure that the cap is the issue.

The pentode portion of the phase inverter tube, 7199, failed, so I replaced that. I do notice that when I pull the phase inverter out, the hum goes away. I happen to have two, and both yield hum, so I'm not sure that the tube is causing the hum. I pulled preamp tubes and the hum was still there. I only have one set of power tubes to try

I'd like to know what the voltages are supposed to be too, but I can't seem to find a schematic that shows that.

I'm starting to run out of ideas, and I don't necessarily want to leave that first filter out of the circuit with 35V ripple, although the amp is pretty quiet like that. . .

[Usable Thought]

http://www.unofficialampeg.com/schematics/g12.gifThen I replaced the original can-cap (70/40/40uF) with one that has 40/40/20/20uF

Since we are talking about filter caps, what is the relative condition of all the various caps in these two cans? Specifically, how old are they? The fact the ripple goes down when they're all in would seem to suggest this is not an issue - but electrolytics do get noisier as they age, so maybe it could be a factor anyway?

Also, since hum has many causes, do you know whether this particular hum is 60Hz or 120Hz? If you're not sure, perhaps you can listen with a spectrograph to find out? Inexpensive spectrograph apps are available for computers, iPhones, iPads, etc. To me 120Hz sounds more like buzz but often they are both called "hum."

And lastly, when did the hum problem begin and under what circumstances? I.e. "amp has been sitting unused in a garage for several years"; "was worked on by a tech"; "has always hummed"; "just started humming last week"; etc.

[Enzo]

what brand 7199? Sovtek? Every one of those I try has been hummy, and believe me I am not a tube snob.

[Usable Thought]

what brand 7199? Sovtek? Every one of those I try has been hummy.

What do you think is the connection between a possibly hummy tube, and the OP's observation that pulling a filter cap strangely seems to lessen the hum? I ask out of ignorance, not skepticism. (I'm a relative newcomer to tube amps & also a newcomer here - I mostly have posted on The Gear Page before finding this forum.)

[MonkHouse93]

It's an amp that someone gave to me to work on, he got it from someone with all the can caps' wires disconnected. He bought it that way. All filters are new, from tubedepot. One broke literally so I put a couple sprague atoms I had in there, the can cap I specifically bought. So I'm assuming the caps are okay.

I'd think it's 120Hz, I have a scope I don't know why I didn't think to measure that, I'll do that tonight.

The 7199 is Hammond; the first one I tried wasn't. The first one worked, then the pentode side have out and there was no buzz. New 7199 is a Hammond, it passes signal, but there's buzz. I think it's somewhat related to that stage?

And as for tubes, some just hum. It doesn't sound like tube hum but I didn't rule it out yet

[52 Bill]

It's an amp that someone gave to me to work on, he got it from someone with all the can caps' wires disconnected. He bought it that way. All filters are new, from tubedepot. One broke literally so I put a couple sprague atoms I had in there, the can cap I specifically bought. So I'm assuming the caps are okay.

All of the Ampegs that I have worked on have a very specific power supply grounding scheme to avoid ground loop hum. For example the can caps are mounted by a phenolic wafer that keeps the can isolated from the metal chassis and the ground connection is made by a wire to a specific grounding point.

When replacing the caps you need to try and reground them back to the original ground points or at least as close as possible. I think that removing a ground loop would cause the hum to go down while removing a cap from the circuit.

As for the tube, it too can be a source of hum as there can be multiple sources all contributing to the overall problem.

[MonkHouse93]

That might be it… I grounded the cap right to the chassis… perhaps I should ground it to the ground bus the other filters grounded to? I read about that in a couple other threads

Fwiw the original can cap didn't have any ground connections

[52 Bill]

That might be it… I grounded the cap right to the chassis… perhaps I should ground it to the ground bus the other filters grounded to? I read about that in a couple other threads

Fwiw the original can cap didn't have any ground connections

I would probably try and ground it to where the transformer secondaries are grounded.

[MonkHouse93]

I'm thinking that's the HT (pink/black) centertap. Its grounded when the power switch is off, but it floats when the switch is on. I can't find a filament centertap


Those two caps I put in. The rectifier's minus node was hanging open so I stuck it there, that's the ground bus I referred to previously. The OT has its own ground wire that's connected there too.


That's the can, the yellow brown and pink wires go to it

[MonkHouse93]

Wow, that was it. I grounded the can cap to the same location as the spragues. It was so quiet I wasn't sure it was working

Thanks a lot everyone!

[52 Bill]

I'm thinking that's the HT (pink/black) centertap. Its grounded when the power switch is off, but it floats when the switch is on. I can't find a filament centertap

If the center tap for the HT is lifted the amp will go into standby. When it is grounded the HT will be there.

I'm glad that you got the hum sorted out.

[MonkHouse93]

Correct me if I'm wrong - I'm pretty sure being that it's bridge rectified it doesn't need a center tap, since the ground return occurs through the bridge; as opposed to a full wave rectifier with a rectifier tube, or two diodes, without the center tap the current can't go backwards through a diode. HTCT isn't even shown on this schematic, not that it's terribly accurate.

[52 Bill]

Correct me if I'm wrong - I'm pretty sure being that it's bridge rectified it doesn't need a center tap, since the ground return occurs through the bridge; as opposed to a full wave rectifier with a rectifier tube, or two diodes, without the center tap the current can't go backwards through a diode. HTCT isn't even shown on this schematic, not that it's terribly accurate.

I'm sorry, I didn't even look at the schematic. If it is a full wave bridge made up of 4 diodes then no, the HT center tap is not used. If there are only two diodes, then the center tap is used.

[g1]

If that is the power switch, then I'm not sure what they are doing with that. I'd suggest you find out whether there is really a CT on the HT winding, and what all that switch is wired up to. Does the amp also have a stand-by and polarity switch? Does the stand-by work?
Here's and actual ampeg schematic for the version with 7199:
http://tubeamplifierparts.com/schema..._schematic.gif