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View Full Version : Ampeg Gemini II Bias Question

mad dog
11-28-2007, 09:50 AM
How is fixed bias adjusted on these amps? Is there a screw adjustable pot coming through the chassis somewhere, or do you have to pull it? I'm hoping there is an adjustment pot on these ...

Thanks.
TopBooster
11-28-2007, 10:00 AM
What year is yours? Mine is a '66 and it has the bias pot soldered to the eyelet board...

Be careful when pulling out the chassis. The front catches onto the internal aluminum ceiling panel VERY easily. If you feel it catching, use a wide slotted screwdriver through the front to carefully get it past.

If they get bound together like they were on mine, you have to go as far as removing the chassis shelves with a pry bar, so the chassis will drop down.
mad dog
11-28-2007, 11:09 AM
Top: I think it's a 66 or 67. Don't have it in hand yet, didn't get the year from the buyer. But it is a Gem II. So if the bias pot is soldered to the eyelet board, I'd have to pull the chassis to adjust it, yes? Thanks for the tip on how to do so. Though I've played many ampegs, I haven't looked inside one yet.
LowWatt
11-28-2007, 11:24 AM
Top: I think it's a 66 or 67. Don't have it in hand yet, didn't get the year from the buyer. But it is a Gem II. So if the bias pot is soldered to the eyelet board, I'd have to pull the chassis to adjust it, yes? Thanks for the tip on how to do so. Though I've played many ampegs, I haven't looked inside one yet.

This page has the serial number codes : http://members.aol.com/bluemuse/dating.html

I just checked my new one and its a 1966. Thanks for askign this question. I'm going to be getting a second set of 7591 tubes as well and was curious about this one.
AL30
11-28-2007, 11:40 AM
I thought those were cathode biased?

AL
TopBooster
11-28-2007, 11:46 AM
I'm working on a friend's Gemini I right now, and it is definitely cathode biased. The II's were always fixed, AFAIK.
TopBooster
11-28-2007, 11:48 AM
So if the bias pot is soldered to the eyelet board, I'd have to pull the chassis to adjust it, yes?

Yes, but check the rear of the chassis first before pulling it. It is possible that Ampeg moved the bias pot at one point.
AL30
11-28-2007, 11:49 AM
The II's were always fixed, AFIAK.

With 7591's?

AL
TopBooster
11-28-2007, 11:56 AM
7591's for the early models, and 6L6's for the later ones, I believe.

Good point...mad dog, do you know if your Gemini II has 7591's, or does it have 6L6's?
AL30
11-28-2007, 12:27 PM
7591's for the early models, and 6L6's for the later ones, I believe.

Good point...mad dog, do you know if your Gemini II has 7591's, or does it have 6L6's?


mad dog - you said you were going to get another set of 7591's correct?

The schem is here - it's kinda tough to read. Looks like it may be out of the Aspen Pittman, Groove Tubes book.

http://www.freeinfosociety.com/electronics/schemview.php?id=736

AL
LowWatt
11-28-2007, 01:14 PM
mad dog - you said you were going to get another set of 7591's correct?

The schem is here - it's kinda tough to read. Looks like it may be out of the Aspen Pittman, Groove Tubes book.

http://www.freeinfosociety.com/electronics/schemview.php?id=736

AL

Nope that was me. I see 3 12AX7s in the circuit, but I don't have enough experience reading schematics. What is the function for V4, V5, and V7 (the 12ax7s)?
mad dog
11-28-2007, 03:33 PM
This time, no 7591s for me. The amp was stock with those tubes, but was modded later with a replacement OT (allegedly from a Fender super), so it could be used with 6L6s. 6L6s in it now. That was actually a big reason I went for it. 6L6s are my favorite tubes ... plus I want to hear what those tubes and a somewhat bigger OT do for volume and headroom. This whole adventure is one big experiment to me, so of course I plan on trying out different 6L6s when the amp arrives.

Very helpful info here ... thanks everybody!