Ampeg Gemini VI Clone Parts

[douglastreason]

Hi everyone, I'm new to this forum and any forum for that matter. I have started my first build from scratch. I'm trying to clone my 1965 Gemini VI. I have obtained the original Ampeg schematic and the Piazza schem. The pot values seem outrageous as in multiple 4m ohm, and some switching pots too. Not sure if these are correct or not. The pots inside the original chassis have no indication as to their values. Any ideas as to whether these are in fact the correct values and where i could obtain such an item, or even thing i could do to substitute a more readily available pot. Thanks so much in advance and sorry if these concerns seem dumb. I'm learning and have a strong desire to continue learning.

[Leo_Gnardo]

Welcome aboard, and one of my faves, the Gem VI!

Yes 4M vibrato depth control. Nearest thing these days, you can get a 3M reverse taper meant for Fender vibrato speed. No switch on back. Maybe you can put a toggle on your dashboard for that chop chop effect if you like it.

Antique/Tubesandmore.com have some switch-pots, other outfits too. If you can't find what's necessary (like those treble pots with click at CW end of travel for super bright) then consider adding toggles for them too.

Fliptops have transformers, including a re issue 151A OT if that floats your boat. When I encounter these amps needing repair, I save a lot of $$ by using either a replacement Fender Deluxe or Vibrolux/Bandmaster OT. And for the PT, one intended for Vibrolux also but have to add/swap bells so it will fit standing up; the Vlux PT ordinarily sits in a big hole in the chassis, Ampeg does not have this.

Good luck with your project, keep us posted!

[douglastreason]

hey thanks for the quick response leo. Yeah, i ordered most of my parts from antique ce. including the fender deluxe OT. I got a hammond a100 pt on eBay for 50 bucks because i guess those are a good fit as well. So, 3m reverse taper...does that mean the speed would start fast then slow up as the pot is turned? Like resistance is opposite a normal pot? I was also debating on swapping out the 7591's for 6l6's with some alterations because of the high cost for 7591's. And if i was to add a switch to the dash for the bright setting would that be spst or dpdt? Thanks again

[Leo_Gnardo]

hey thanks for the quick response leo. Yeah, i ordered most of my parts from antique ce. including the fender deluxe OT. I got a hammond a100 pt on eBay for 50 bucks because i guess those are a good fit as well. So, 3m reverse taper...does that mean the speed would start fast then slow up as the pot is turned? Like resistance is opposite a normal pot? I was also debating on swapping out the 7591's for 6l6's with some alterations because of the high cost for 7591's. And if i was to add a switch to the dash for the bright setting would that be spst or dpdt? Thanks again

3M reverse, if you wire it up right, as usual slow pulse when CCW and speeding up as you dial CW. The reverse taper just spreads the choice across the dial so it doesn't "do nothing" then all your speed change is in the last eighth of a turn. "Reverse" of a normal audio or log taper pot. If you reverse the other "normal" linear pot, you still have a linear pot, right?

Fender uses a 50K reverse for depth, but Ampeg 4 Meg. So a 3M reverse will work as a sub. The circuits work differently, so if you want to emulate the Ampeg, that's the choice. Same reason, it spreads the effect across the dial instead of "cramming it all into a corner."

Ampeg uses a 500K speed/rate pot. If you use a 3M reverse here, you'll need to reduce the value of the three 0.05 uF caps in the oscillator bridge or you'll find the rate very slow. 0.02 or 0.01 should work, you may need to mix n match them to find a set that will be comfortable with 3 Megs. Reverse 500K, I don't know where to find easily. We have to deal with what's offered on the market.

SPST I'm pretty sure, that's what's on the back of Ampeg's intensity/depth pot.

I've used 6L6 and 6V6 both as subs for 7591 in Ampegs. These days, if you want you can get good new 7591, not terribly expensive, from JJ and ElectroHarmonix. About 25 years ago EH brought out a dodgy 7591XYZ which was just a Sovtek 5881 wafer-base with pins reassigned to match 7591.

I see the treble pots are 4 Meg also. Max available ez to find pots 1 Meg. You may need to adjust treble caps to maintain a frequency response similar to Ampegs. In this application i don't think you'll find reverse taper pots work all that well.

A warning about Piazza's schemos - they often have misteaks. He tried hard, but didn't proof read. Best off sticking with the factory schemos.

[douglastreason]

thanks a bunch Leo,
I'll keep you posted!

[douglastreason]

Ok, Ive run into another snag. The original O.T. runs the 16 ohm tap back into the grid of 7199 and then onto the plate of 6cg7. The replacement i have is a fender deluxe O.T.and only has the 8ohm tap that feeds the speaker. What should I do?

[Leo_Gnardo]

Ok, Ive run into another snag. The original O.T. runs the 16 ohm tap back into the grid of 7199 and then onto the plate of 6cg7. The replacement i have is a fender deluxe O.T.and only has the 8ohm tap that feeds the speaker. What should I do?

16 ohm tap, run thru an Rfb of 150K resistor first. Back of the envelope calc 0.707 x 150K about 106K so, 100K close enough for rock n roll. I don't see the fb going to the 6CG7, nor any reason why one would want to do that. It does join up to the cathode of the 7199 pentode. Perhaps the schemo you're viewing is misleading: Piazza shows connected lines with a dot at their intersection, a little skip & no dot for unconnected lines. Ol' fashioned schemos put a little half circle where lines don't connect, it's a bit more obvious.

[douglastreason]

sorry, i'm still confused. I'm pretty green at this. I guess my question is because my new trans has no 16 ohm tap, is it necessary to have one? I will connect the 8 ohm tap to my speaker. Do I have to find a O.T. that has both 8 and 16 ohm taps? Also what is Rfb? or back of envelope? I apologize for asking you to explain these terms, but like i said i'm def a newbie. Thanks again

[J M Fahey]

sorry, i'm still confused. I'm pretty green at this. I guess my question is because my new trans has no 16 ohm tap, is it necessary to have one?

No, you can connect the feedback resistor to the 8 ohm tap BUT you have to reduce its value so effect is the same.
"Back of an envelope/paper napkin calculation" means a quick calculation written on "anything available" because it is not critical at all.
In this case Math says new reduced value should be 106k which is a non existing or hard to find value, happily replaced with very common 100k .
Does it change sound?
Not a bit, or at least nothing you, I , or any other can hear, so don't worry.

I will connect the 8 ohm tap to my speaker. Do I have to find a O.T. that has both 8 and 16 ohm taps?

No.

Also what is Rfb? or back of envelope? I apologize for asking you to explain these terms, but like i said i'm def a newbie. Thanks again

For your own convenience, read as much as you can but not get hooked on Internet Myths, Folklore or nonsense.
Guess that excludes a ton of Forums, except here

Search and download Jack Darr's Guitar Amplifier book .... and read it many times.

Clear, concise (meant for 60's TV repairmen) , straight to the point.

Search Amazon for a (used) copy, out of print for many years now , but invaluable.

[Leo_Gnardo]

No, you can connect the feedback resistor to the 8 ohm tap BUT you have to reduce its value so effect is the same.
"Back of an envelope/paper napkin calculation" means a quick calculation written on "anything available" because it is not critical at all.
In this case Math says new reduced value should be 106k which is a non existing or hard to find value, happily replaced with very common 100k .
Does it change sound?
Not a bit, or at least nothing you, I , or any other can hear, so don't worry.

No.



For your own convenience, read as much as you can but not get hooked on Internet Myths, Folklore or nonsense.
Guess that excludes a ton of Forums, except here

Search and download Jack Darr's Guitar Amplifier book .... and read it many times.

Clear, concise (meant for 60's TV repairmen) , straight to the point.

Search Amazon for a (used) copy, out of print for many years now , but invaluable.

What Juan said ^^^. Rfb = feedback resistor.

Now I'm worried you may have soldered together all those spots Piazza shows - in his skip track way - as non connected. Another good reason to get the factory schematic, and learn to read it so you can wire your prize correctly.

FWIW I find Tom Mitchell's "How To Service Your Own Guitar Amp" to be a valuable guide for the beginner, or even for a gear veteran who wants to understand what's going on inside his amps. About $30, and there's a version with video @ $80. I've only seen the book, and Tom's saved me from having to write one.

[g1]

Is there another model #, like GS15R ?
If so, it may be possible to find the proper factory schematic.

[douglastreason]

So I would connect the 8 ohm tap to my speaker as normal, but solder another wire with a resistor in series off that and back through into the 7199?

[MannGold]

Hi,

I'm new to this forum, so nice to meet you all !

I am looking for an old fuse holder, actually the top, for my sixties Ampeg Gemini - anybody ?

Kind Regards,
Rodrigo