Ampeg Gemini I v's Gemini II

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Ampeg Gemini I v's Gemini II

Postby jimboyogi » Tue Mar 08, 2011 10:32 pm

Has anyone here had experience with both? I'm particularly interested in impressions of the output sections, ie headroom, breakup, compression, and how these fit into a band for various music styles.
The output circuits on these two are amost the same (2 * 7591's). However the Gemini I is cathode bias with lower voltages, the Gem II is fixed bias with higher voltages. But which sounds better :P
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Re: Ampeg Gemini I v's Gemini II

Postby al30 » Wed Mar 09, 2011 6:09 am

The Gemini I is approx 20 watts with a 12" speaker. The Gemini II is approx 30 watts with a 15" speaker. I have a Gemini I. It's a great amp. But it's not very loud to my ears. No experience with a II.

http://www.ggjaguar.com/gem1.htm

http://www.ggjaguar.com/gem2.htm
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Re: Ampeg Gemini I v's Gemini II

Postby Jonesie » Wed Mar 09, 2011 3:16 pm

I on the other hand have a Gemini II. I've played it with a fairly loud band and it's stayed clean. It doesn't really start breaking up til about 3'oclock on the volume knob, and up til then, it's a really linear volume increase. It rattles the windows in my room.

The gain on these things sound great too. You can get a cool tight Marshallish sound out of it pretty easily. Plus, 15" speakers rule.

The tech I brought mine to said that if I EVER wanted to sell it, please contact him first, because it's the best amp he's ever heard.
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Re: Ampeg Gemini I v's Gemini II

Postby Soochdaddy » Wed Mar 09, 2011 8:16 pm

I've got a '68 G-12 (Gemini I).

It's been in the shop for a couple of months and should be done any day now... lead times... and I can't wait to get it back.

I took it in because I wanted to get a clean bill of health, a 3 prong cord, and a cap job.

It's a fantastic amp. The reverb and trem are great, the cleans are heavenly, takes pedals extremely well, and at 22 or so watts with the 12" Jensen hangs at rehearsal volumes just fine. I dig the ultra bright and echo/repeat functionalities as well. It's got a shock mounted chassis as well.

Both of my other amps are class AB, where this one is class A but I have yet to push it. As far as I know it is pretty much original internally and I didn't want to crank it until I got the clean bill of health and the necessary maintenance that should be performed on a 43 year old piece of equipment.

Can't compare it to a G-15 but if I had to choose one of my three amps to keep forever, the Ampeg would be my choice...
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Re: Ampeg Gemini I v's Gemini II

Postby jimboyogi » Thu Mar 10, 2011 2:42 am

Thanks all for the feedback and insights :)

I've just tonight put together a prototype amp with 2 * 7591's, cathode bias ATM. Sounds great at low volume, but it's late here and I can't turn it up until tomorrow. I plan to try different biases (cathode, fixed, and in-between) and get some figures for output. I'll share when I've got more information.
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Re: Ampeg Gemini I v's Gemini II

Postby al30 » Thu Mar 10, 2011 9:26 am

What are you using for transformers?

AL
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Re: Ampeg Gemini I v's Gemini II

Postby jimboyogi » Thu Mar 10, 2011 12:27 pm

ATM i'm using a locally made, very beefy 8K primary OT, designed for an AC15 or Marshall 18W.

Down the track I'll probably get one of these, they have been recommended - http://store.triodestore.com/tf65wscta48o.html
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Re: Ampeg Gemini I v's Gemini II

Postby al30 » Fri Mar 11, 2011 7:37 am

Thanks for the link. That's the first I've seen those. Good luck with the build and keep us updated.

AL
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Re: Ampeg Gemini I v's Gemini II

Postby Professor Plum » Fri Mar 11, 2011 6:06 pm

im watching this space intently. i love these amps. :)
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Re: Ampeg Gemini I v's Gemini II

Postby jimboyogi » Sat Mar 12, 2011 1:36 am

OK, here's what I've learned so far today, mucking around with this 7591 amp. I bought a matched pair of each of EH 7591A's and JJ 7591S's from ebay, hoping to compare the 2 brands. Note that all of the information below is only for 1 matched pair of each brand, hardly a statistically valid sample :whistle:

EH 7591A's -
Cathode biased (at ~ 90% max. plate dissipation) - 17.0 WRMS max. output before clipping, but the waveform shows a lot of compression before this output, due to the cathode bias compression effect.
Fixed bias (at ~70% max. plate dissipation) - 19.1WRMS max. output before clipping, but the waveform shows almost no compression up to the point of clipping, due to the fixed bias keeping the bias voltage constant as plate and screen current increase.

The 2 modes sound very much as you would expect them to; more compressed (more like an EL84!) with cathode, cleaner, brighter and more dynamic with fixed, but a "sharper" overdrive sound. But I really liked both modes, these valves have a special clarity to their sound, and the overdrive is the sort that I like best, smooth with some grain, with not too much odd harmonic "buzz". Plenty of bass and treble, mids seem a little less prominent.

Problem was, there was a fair bit of audible hiss/popping sounds coming from the amp when I first put it together with the EH's. After ruling out pretty much everything else, I tried swapping in the JJ's. DEAD QUIET now. Hiss/pop completely gone, now the amp is super quiet. You hardly know that it's on until you hit a note.

But the JJ's definitely sound different (aside from the noise). More mids, less treble, less clarity, and they sound more compressed. They also bias up a bit differently, drawing more plate and screen current (3% more dissipation than the EH's in the same circuit). They also give 5% less max.clean output in the same circuit as the EH's.

Next step for me is to get hold of a 6.6K OT (like Ampeg used) to try these out on. I'm expecting some more output wattage with one of these.
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Re: Ampeg Gemini I v's Gemini II

Postby al30 » Wed Mar 16, 2011 5:24 am

jimboyogi wrote:Next step for me is to get hold of a 6.6K OT (like Ampeg used) to try these out on. I'm expecting some more output wattage with one of these.


Try Fliptops.net. I picked up a Gemini I transformer from the a few years ago. No complaints. If I remember correctly they were being made by Heyboer.

http://www.fliptops.net/catalog/index.php

AL
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Re: Ampeg Gemini I v's Gemini II

Postby jimboyogi » Wed Mar 16, 2011 11:28 am

Thanks for the link AL, I've already ordered the one from Triode, in fact it's in transit across the world. Should be here next week.
By my sums, I should get another 3-4W with the 6.6K OT. We shall see..........
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Re: Ampeg Gemini I v's Gemini II

Postby jimboyogi » Thu Apr 21, 2011 4:08 am

jimboyogi wrote:Thanks for the link AL, I've already ordered the one from Triode, in fact it's in transit across the world. Should be here next week.
By my sums, I should get another 3-4W with the 6.6K OT. We shall see..........


Well it didn't quite work out like that, I've done some comparisons, and here is what I've found -

Moving from an 8K primary OT impedance to a 6.6K gave me almost exactly 1 WRMS extra max. clean output,

All other things being equal, EH7591A's give 0.6 WRMS extra max. clean output than JJ7591S' with partial cathode bias, and an extra 1.4 WRMS with fixed bias. You would be struggling to hear this much difference though.



The seller that I bought my original and noisy EH's from was great and replaced them free of charge, and the new set sound great and quiet. Actually I like both the EH's and the JJ's, but from my VERY small sample, the JJ's might be a better "rock" tube, and the EH's a better "clean" tube.
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Re: Ampeg Gemini I v's Gemini II

Postby Jonesie » Thu Apr 21, 2011 1:47 pm

jimboyogi wrote:Actually I like both the EH's and the JJ's, but from my VERY small sample, the JJ's might be a better "rock" tube, and the EH's a better "clean" tube.


EXACTLY what I found. The EH's were a tad bit warmer, and had quite a bit more headroom, at least in my amp. I'm running the JJ's now, and for what I play, I prefer them.
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