"Upping" the can cap values in an Ampeg Gemini II?

[gmoon]

If my Gemini II sits for more than 4-5 days without being played the sound is definitely degraded. After playing for about an hour, it's excellent again. Also, turning up channel one without anything plugged in gets "sqeally." I hadn't noticed the difference, since the amp was played every day or so for quite some time.

But having replaced all the other filter caps in my Gemini, I never did get the largest "upstream" can cap replaced (I thought it was OK, until now .) It's a 40/40/40 450V, with two sections in parallel-- essentially a 80uF cap and a 40uF.

I reckon I'd better replace the can before checking any coupling caps, etc. Weber has good looking 100/100 500V can caps for less than 1/3 the price of most twistlock metal cans.

Should I replace an 80/40 combination with 100/100? While I know that increased capacitance with a SS rectifier should be just fine, but I have also read that too much initial capacitance in a PP amp is too sterile.

Of course, Ampegs were pretty clean to begin with, so I'm tending think it'll be OK... and 100uF isn't much more than 80uF (especially given 1965 tolerances); only the next tap would be more than doubled.

[soundmasterg]

You could see if Weber will do a custom can for you also. If he can't do an 80uf/40uf in one can, then perhaps he could do a 40uf/40uf/20uf/20uf can and you could hook the sections up in parallel. You could also get a 40uf/40uf/20uf/20uf can from AES and hook the sections up in parallel. Another way you could go would be to use discrete caps inside the chassis and disconnect the can. That might be a little more work to fit things in there but would be cheaper than any can. Personally I'd try to get the values correct rather than going with a 100uf/100uf can as it will change the sound, though the hum level will go down slightly.

greg

[gmoon]

Weber does have a 40/40/40 450V metal can, and I expect that's the way to go... 500V would be better, but they claim to be tested at 500V (and the original was 450V--1965 voltages.)

Or I could get a 525V can at AES.

Thanks, Greg. I just needed somebody to say it might have an effect on the sound.

[RWood]

Should I replace an 80/40 combination with 100/100?

I did just that on my Gemini IV, using a JJ can .

The amp also needed a new OT so it is a little hard to evaluate the before/after but I would say that it's a little stiff now, cleaner than what I recall from years ago. It definately cured the noise problems I was having with the old caps!

I like Soundmasterg's ideas; another one to consider (that I will probably do since I've already committed to the JJ can) is to add a 100uF in series with the second 100uF, inside the chassis, to get closer to the schem's 40uF target.

RWood

ps I've restored several small Kay amps and I really like your Instructibles page!

[gmoon]

The amp also needed a new OT so it is a little hard to evaluate the before/after but I would say that it's a little stiff now, cleaner than what I recall from years ago. It definately cured the noise problems I was having with the old caps!

That's always a problem--by that I mean a baseline comparison for vintage equipment. Although I've had this amp for ages, I'll never really know how it sounded new...

I like Soundmasterg's ideas; another one to consider (that I will probably do since I've already committed to the JJ can) is to add a 100uF in series with the second 100uF, inside the chassis, to get closer to the schem's 40uF target.

That's a creative solution, and a good one. Although now I'm leaning toward the least complex, the few components, the better. It's built like a 60's chevy, and the rebuild should follow the same zeitgeist... A close match for the can would be my preference.

ps I've restored several small Kay amps and I really like your Instructibles page!

Thanks, Rwood. I definitely get the "I'm not worthy" feeling here on MEF (based only on my limited knowledge .) But I've learned quite a lot... (and I love those little amps, especially miked.)

[olddawg]

Can you hollow out the old can and mount new caps inside? New electrolytics are much smaller than old ones. This is done a lot on restorations. You keep the old cap housing, do not have to modify the chassis or wiring, and it looks original if done right. Just make sure that you heat shrink the leads and connections and use a generous amount of silicon cauking/glue/sealant inside the old housing.

[gmoon]

Gutting the can and replacing the innards could work if I can reuse (or refab) the base plate. I've seen examples of this on the web, but hadn't considered it seriously. Maybe I should.

Is silicon adhesive really OK? Some advocate hot melt glue--but I wouldn't fill an enclosed can with it, at least not one stuffed with caps. Seems too hot.

[olddawg]

Gutting the can and replacing the innards could work if I can reuse (or refab) the base plate. I've seen examples of this on the web, but hadn't considered it seriously. Maybe I should.

Is silicon adhesive really OK? Some advocate hot melt glue--but I wouldn't fill an enclosed can with it, at least not one stuffed with caps. Seems too hot.

You should...... The new caps will fit easily in most cans. I use a Dremel motor with a small grinding wheel to cut the lip off of the bottom. Then you can usually reuse the card/fiberboard base cover. Some people use a hand file, even a small saw. It just depends on how much effort you want to put into it. Silicon is definitely a better option. Hot glue can re-melt all over the damn place. You also don't need to fill the whole can. You just need to put in enough adheasive to suspend the caps and mount the bottom cover. If it is a valuable vintage amp it is worth the time and effort. If it is a project/conversion though, I would just mount the caps where they fit (unless you want to practice stuffing a can!). Sometimes a bit of perf board helps.

[gmoon]

You should...... The new caps will fit easily in most cans. I use a Dremel motor with a small grinding wheel to cut the lip off of the bottom. Then you can usually reuse the card/fiberboard base cover. Some people use a hand file, even a small saw. It just depends on how much effort you want to put into it. Silicon is definitely a better option. Hot glue can re-melt all over the damn place. You also don't need to fill the whole can. You just need to put in enough adheasive to suspend the caps and mount the bottom cover. If it is a valuable vintage amp it is worth the time and effort. If it is a project/conversion though, I would just mount the caps where they fit (unless you want to practice stuffing a can!). Sometimes a bit of perf board helps.

I'll seriously consider this. For a second I thought I'd have what I need on hand, but I'm short the mid value...

[gmoon]

OK, I picked up a new "CE manufacturing" can cap from AES (during their end-of-the-year sale), 40/40/40 525V.

Now I've got a new quandary-- the existing cap used a metal mounting plate. But all the pics I've seen (not many) of Gemini II chassis use an insulated plate. I forgot to check when I bought it...

My gut says I should go with the insulated mount...

--------------
EDIT: I went with the metal mount. Fliptops carries only the metal version, and it was apparently stock.

Results: Sounds great! Tighter, and very little noise--even with the tremolo on the maximum "click stop." Before the "noise floor" was pretty bad with the trem, at least when not playing.

[Bruce / Mission Amps]

OK, I picked up a new "CE manufacturing" can cap from AES (during their end-of-the-year sale), 40/40/40 525V.

Now I've got a new quandary-- the existing cap used a metal mounting plate. But all the pics I've seen (not many) of Gemini II chassis use an insulated plate. I forgot to check when I bought it...

My gut says I should go with the insulated mount...

--------------
EDIT: I went with the metal mount. Fliptops carries only the metal version, and it was apparently stock.

Results: Sounds great! Tighter, and very little noise--even with the tremolo on the maximum "click stop." Before the "noise floor" was pretty bad with the trem, at least when not playing.

Heads Up...
Of the last dozen or so of these CE caps I bought this last year to install in repairing vintage amps... four of those amps have now come back with worse, bad brand new filter caps then the 35-45 year old ones I replaced!! One had three sections gone completely bad in four or five months with a horrendous roaring buzz.
I'm wondering if the common ground lead from the cap sections are coming off.