Ampeg Gemini 1 Guitar Amp

[CJ]

were gonna tear into this cool tube amp, circuit # G12

its exactly like the Gemini 2  only different, 

 

[CJ]

this amp has a smaller cab and a 12 instead of a 15,

has the pair of 7591s, 
somebody wired it for 6V6GT tubes which is a Felony in many states,

there is also a green "mystery box"  which you all can see clearly in the picture below,
the speaker line plugged into it, output goes to the speak, 

this will obviously require further investigation as it does not belong here,

this is another Felony , one more strike and that's it.

 

[CJ]

we will have this handwriting analyzed so we can track down the perpetrator of these heinous crimes,  8)

i bet that this tube changer guy is also responsible for the mystery box,

 

[CJ]

mystery box exposed,

looks like a pair of chip amps, has a volume knob on it, so they were taking the speaker signal and re-amping it with transistor, this is Strike 3,  but unfortunately this was done in 1995 so the statute of limitations has run out on these crimes against amplification,

 

[CJ]

this chassis is very similar to the Gemini 2, differences are lower plate voltage, about 400, which is why the 6V6 tubes survived,

and the 7591's are cathode biased with a resistor instead of fixed bias which uses a neg voltage from the pwr supply,

this keeps the 7591's from developing max power so they do not pop the smaller 12 inch speaker, and also sounds real nice as there is more compression going on,

there is a 250 ohm cathode resistor  in there right now, Ampeg wants a 140 ohm, so this resistor was changed when the amp was switched to 6V6 power, 140 ohms develops a bias of about -14 volts, we could probably use a 200 ohm resistor and add a little tube life to the JJ 7591s ordered from Antique Electronic Supply for 16 bucks each,

there is also a pot on the back panel that appears to be an Ampeg rev to get the reverb to work with both channels,

this amp also uses the smaller output transformer, the OT-151A - 682 709

somebody replaced the orange  cardboard multi-cap with a couple of lytics, which is a smart move, the main can has broken it's permali mounting washer, so we will solder the can to the chassis which is the way old Leo used to do it, this provides some solid ground to stand on,

we will also change the coupling caps that feed the power tubes as sometimes AC signal voltages can exceed the value of the supply voltage thru some strange process that Owsley discovered on Garcia's Twin Reverb amps,

notice the Vibrato roach kludged in at the top right, probably replaced by the ex felon tech,

 

[CJ]

here is a revised transformer print,

looks like  got the wire pri wire colors got swapped on the first  print, 

so this transformer plays out of phase, this is so the feedback polarity will correct to provide NFB to the inverter circuit,

here is the corrected print>

 

[CJ]

test drove this amp today with new JJ 7591S tubes, sounds great!

heavy reverb, there is a reverb tone/level pot in the back, installed by Ampeg but not on any schematics i have seen,

had a bunch of hum, so we soldered the multi-cap can to the chassis and soldered in some more ground points and now it is quiet,

changed some resistors in the pwr supply to get the pwr tube screens and preamp plate supply voltages down to schematic levels, 


here is a pic showing the old series ground-green line, and the new grounds circled in yellow, put in some higher voltage inverter  caps also,