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Topic: Pretty Guitars; u-g-l-y amps |
Jim Sliff
From:
Lawndale California, USA
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Posted 9 May 2009 9:40 am |
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Ah, the joys of working on Gibsons. I've had this cool old GA50T sitting around for 10 years or so and FINALLY dug into it this week. I assume rats propagated in the place - lord knows they would not have needed to build a nest with all these wires pointlessly running in haphazard directions, and "look at me" cap placement; with a few hidden in dark corners. It had two replacement cps when I got it; the long yellow dual-section 20/20 and the can (40/20) The can is fine...good, because I can't find a simple 40/20 sectioned one and would have had to result to finding room for a couple single ones. The yellow one was really leaky...and so was EVERY other cap on the board!
Gibson had this interesting habit of using polarized, high-voltage caps where they're not needed - but polyester and mylar non-polarized ones are cheap, so I stuck mostly) with their original voltage requirement; bypass caps changed from 25uf/600V to 25uf/25V, mainly because I have dozens of them!
It's a 2x6L6 cathode-biased amp with 6SJ7 preamp tunes, a 6SN7 driver and and 6SL7 trem tube. a P12P 12" AND (unidentified) 8" Jensen alnico (a friend offered me a ridiculously expensive price just to get the speakers). Speakers date to late '51, which matches everything else - finally have an amp older than me!
I don't screw around with using a Variac to "form" caps (manufacturers just turn amps on with new caps) So when I was done ( everything triple-checked before soldering and double-checked afterwards, I turned it one...
...and where I hard a buzzy, hissy amp with ghost notes and oscillations, now it's smooth and very quiet considering the construction (bizarre lead dress, filament wires simply run next to each other with no twisting, ancient chokes (TWO of them!) etc. Sounds a lot like my wide-panel Deluxe except it clearly has more power (probably 25 watts or so)
And I haven't even checked resistors yet (I replaced one cooked plate resistor a long time ago) - I can also do a bunch of tube swaps as I have a large stash of glass AND metal octal preamp tubes.
The only thing NOT working is the trem - it has a few really bizarre rectangular caps (haven't quite figured out if they're OK or not), a couple of metal electrolytics (that tested OK), a working footswitch - but a two-stage pot and a THREE stage one (that has an internal adjustment for he third section - but the first two, which are the speed control, are nearly frozen - it take vise grips to move it.) Not sure what I'll do about it - might temporarily wire up duplicate s externally and see if I can get it working
If anyone (Ken, you wanna get involved with a tech's nightmare?) has ideas on the trem, let me know...but as a straight amp, it's stunning!
"After" pics will be posted a little later...
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No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional |
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Marc Jenkins
From:
Victoria, British Columbia, Canada
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Posted 9 May 2009 10:04 am
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| Wow. Messy. I'll bet it sounds great though! |
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Len Amaral
From:
Rehoboth,MA 02769
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Posted 9 May 2009 2:13 pm
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Jim, I admire your enthusiasm when it comes to amps and gear. Way to go!
Lenny |
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Ben Jones
From:
Seattle, Washington, USA
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Posted 9 May 2009 9:16 pm
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| very cool indeed. |
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Jim Sliff
From:
Lawndale California, USA
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Posted 13 May 2009 6:31 am
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It's cooler now that the trem works...
Every solder joint except the filter caps looked original - but the trem wires were in the wrong place (including one obviously originally-soldered resistor!) and the odd backwards-facing pot (part of the trem circuit), which is supposed to be 1 meg, was a 3.5k pot. Yep, not 35k - 3.5k. Oddest thing I've seen in a long while. The trem speed pots were frozen - and that's probably why....there's no way it could have EVER worked!
I didn't have a 1 meg handy so I tried a 500k - works great. Seems like it is some kind of secondary intensity control, or a waveform-shaping pot - so it's cranked all the way and the trem sounds like the old Fender brown amps - smooth and buttery instead of choppy like the BF/SF circuits.
Man, was this thing a pain! Just ned to get a new handle and it'll be almost-new. Sounds incredible; around 25 watts, warm, and a nice, smooth breakup like a wide-panel Super or Deluxe.
Keeper.
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional |
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Bill Leff
From:
Santa Cruz, CA, USA
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Posted 13 May 2009 8:36 am
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| Wow, if it sounds like an old Super that must be an incredible sounding amp. I played one for awhile many years ago and it was the best sounding overdrive tone I've ever heard. |
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Jim Sliff
From:
Lawndale California, USA
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Posted 13 May 2009 9:01 pm
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Yeah, I was a bit surprised at the overdriven sound, as Gibsons have a reputation of staying clean ( and some of the newer ones I've had have lived up to that rep) but it appears to slap the preamp tubes with some fairly significant voltage, and with fresh caps tightening up the bottom end it's really smooth and "singing". Definitely NOT a "sparkly" tone, although there is plenty of treble for my tastes (icepicky "bright switch on" treble is not my cup of tea...) - it's a big, round, meaty sound. The combination of 12" and 8" Jensens really adds to the tonal range as well.
More on the trem - the switch wasn't working either. I really didn't want to replace it if I didn't have to, so two days of Deoxit treatments and it's working fine, as are the pots. I really do think this amp was wired flat wrong, with an original owner that never used trem and simply didn't notice...so he/she never had it fixed and the switch and one set of dual-gang pots just got encrusted with gunk. That 3.5k pot is still a mystery. No date code, no manufacturer, nothing....just what appeared to be original solder joints including the ones that had it attached to the dual-gang "frequency" (speed) control.
It also has a vary slight "ramping' effect; when you hit the switch, it takes a few seconds for it to get up to (or lose) speed. It's an interesting trem circuit, to be sure.
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional |
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