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Hi, all. After years of procrastinating, I'm finally working on my Gemini. I've had this amp since the late '70s. It's the 7591 / 7199 version.
The amp has always disappointed me in terms of sound. It always sounded dull, but since there were some obvious issues, I never got rid of it. The first issue is the reverb--the tank was missing when I bought it.
Yes, the power tubes are old. And yeah, they need to be replaced. It should be said that there was some arcing, loss of volume and a high pitch squeal when the treble controls were turned up (more about the treble controls later.)
Youtube of the 7591s when the treble is cranked (you cannot hear the hi-pitched squeal on the camera's mic, and it's fairly muted anyway--not an out-of-control oscillation.)
I bought a new tank a year or so back (yep, it's an appropriate impedance for the ampeg reverb schema), and replaced the C6G7 reverb driver tube, but no dice.
So three days ago I pulled the chassis. The resistor for the screen supply has some heat damage, I'm sure it has something to do with the arcing (you can see it between tags 62 and 69 on the board.) It needs replacing, but is still near it's speced value.
http://music-electronics-forum.com/a...1&d=1234377823
And the bias supply cap looks bad (Gold medium size cap in the chassis pic), so that's on the to-do list, but the supply still reads -22V.
RE: the reverb-- first thing I noticed was a missing ground on one of the reverb lines. OK, this is a ground-loop thing--connect the grounds inside the reverb, not at the chassis. Success! The reverb is functional for the first time in 30 years.
Here's where the story gets weird.... Both reverb and tremolo are available on both channels. And that's not supposed to be the case. There's a strange interaction between the controls--the effect on each channel can be eliminated by turning down the tone on the other channel. And the volume controls on the other channel act more like a tone mixer.
Once the reverb was working, the power tubes stopped squealing and arcing--treble settings now had no detrimental effect.
So there are two slotted POTs on the back of the chassis. One is a 100 ohm rheostat to balance the 7591 cathodes, the other one I thought was the bias voltage (would make sense, right?) But no, it seems to connect the signal path from the tag board to the Echo (reverb depth) POT.
You can see the routing for the pot in this photo (prominent grey shielded wire to and from the pot at the bottom.) You can also see the real bias voltage adjustment--the little blue trim pot on the tag board.
http://music-electronics-forum.com/a...1&d=1234377949
Did some evil genius modify this years ago? Should I leave it? I sort of like a lot of the sounds that result from knob twiddling...despite their non-intuitive interactions.
Edit: correct schematic below
The amp has always disappointed me in terms of sound. It always sounded dull, but since there were some obvious issues, I never got rid of it. The first issue is the reverb--the tank was missing when I bought it.
Yes, the power tubes are old. And yeah, they need to be replaced. It should be said that there was some arcing, loss of volume and a high pitch squeal when the treble controls were turned up (more about the treble controls later.)
Youtube of the 7591s when the treble is cranked (you cannot hear the hi-pitched squeal on the camera's mic, and it's fairly muted anyway--not an out-of-control oscillation.)
I bought a new tank a year or so back (yep, it's an appropriate impedance for the ampeg reverb schema), and replaced the C6G7 reverb driver tube, but no dice.
So three days ago I pulled the chassis. The resistor for the screen supply has some heat damage, I'm sure it has something to do with the arcing (you can see it between tags 62 and 69 on the board.) It needs replacing, but is still near it's speced value.
http://music-electronics-forum.com/a...1&d=1234377823
And the bias supply cap looks bad (Gold medium size cap in the chassis pic), so that's on the to-do list, but the supply still reads -22V.
RE: the reverb-- first thing I noticed was a missing ground on one of the reverb lines. OK, this is a ground-loop thing--connect the grounds inside the reverb, not at the chassis. Success! The reverb is functional for the first time in 30 years.
Here's where the story gets weird.... Both reverb and tremolo are available on both channels. And that's not supposed to be the case. There's a strange interaction between the controls--the effect on each channel can be eliminated by turning down the tone on the other channel. And the volume controls on the other channel act more like a tone mixer.
Once the reverb was working, the power tubes stopped squealing and arcing--treble settings now had no detrimental effect.
So there are two slotted POTs on the back of the chassis. One is a 100 ohm rheostat to balance the 7591 cathodes, the other one I thought was the bias voltage (would make sense, right?) But no, it seems to connect the signal path from the tag board to the Echo (reverb depth) POT.
You can see the routing for the pot in this photo (prominent grey shielded wire to and from the pot at the bottom.) You can also see the real bias voltage adjustment--the little blue trim pot on the tag board.
http://music-electronics-forum.com/a...1&d=1234377949
Did some evil genius modify this years ago? Should I leave it? I sort of like a lot of the sounds that result from knob twiddling...despite their non-intuitive interactions.
Edit: correct schematic below
Attached Files
Thanks, Bill. I don't have a reference for the "stock sound," so I'll un-mod first... The chassis is in surprising good shape, it's quiet and love the reverb so far. No mods planned if the stock sound is as good as the "evil" mod.
.)
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