Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: LazarusLong on December 22, 2019, 10:39:13 am
-
Hello, Sorry if this topic has been beat to death.. I recently acquired a Gibson GA-79 amp. It appears to be all original. The original tri-cap is labeled, 20/10/10-450v. Of course I cannot find a new cap that is an exact match. What would you guys use, and why?
Also, I am interested in learning as much as I can about tube amp electronics. Any suggestions as to books, websites, etc would be much appreciated.
-
I'd go with the JJ 40/20/20/20 that Doug sells, very well liked n used, why; think of it as the "modern upgrade" :icon_biggrin:
start here for some light reading :)
http://www.valvewizard.co.uk/
a little heavier reading;
http://www.aikenamps.com/index.php/tech-info
don't worry, if you're a fast reader, more recommendations WILL come :laugh:
-
If it's a can maybe this will fit...
https://www.tubesandmore.com/products/capacitor-ce-mfg-450v-20101010-f
Of course you can always use three individual caps.
-
I would note that modern caps are very much smaller than old ones. I'd disconnect the old can, neatly lay out a terminal strip, and use 22u, 10u, 10u 450V separate caps.
Yes, 40u (20+20u) 20u 20u would be an "upgrade". But would change the stiffness and tone of the amp.
Ideally try it both ways. But life is short. Working as original is a safe bet.
EDIT-- Sluckey found "a right part" while my fingers were funbling. Ignore the extra section (we may find something to do with it later, but keep it simple to bring it back to life). But forty bucks bothers me. Also the idea that the next care-taker will probably have to do it again, at what cost? And the fit may be different. Neat mounts for separate caps may be easier and cheaper down the years. Then the original can-cap looks "STOCK" from the player's side. (Techs won't be surprised by an old amp that plays OK having 2019 caps hidden inside.)
-
If it's a can maybe this will fit...
https://www.tubesandmore.com/products/capacitor-ce-mfg-450v-20101010-f
Of course you can always use three individual caps.
I do think it will fit. Heres a dumb question. Since that one has 4 caps, I simply ignore the extra 10uf?
-
And thanks for the quick replies!
-
Since that one has 4 caps, I simply ignore the extra 10uf?
Yes. Could you post a close-up pic of your cap? And a pic of the inside of the chassis?
-
Couple pictures.
-
The two 10 uf capacitors are connected parallel in the circuit for a combined capacitance of 20 uf. You could replace the 20-10-10 multi-section cap with two 20 uf capacitors. Per the schematic, there are also two 20 uf capacitors in the power supply chassis.
https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga79.pdf
-
You could replace the 20-10-10 multi-section cap with two 20 uf capacitors.
Agree, you could also use this axial two section F&T 32/32. https://www.tubesandmore.com/products/capacitor-ft-450v-3333-f-electrolytic (https://www.tubesandmore.com/products/capacitor-ft-450v-3333-f-electrolytic) which could go right in the same location.
-
One of the cleaner Gibson builds I've seen, Gibson wasn't known for neat :icon_biggrin:
If you're really bored, think about keeping the paper tube, hollow it out, add small sized e-caps inside the tube. A couple members have done this, I'm not one :laugh:
might be able to surf up web examples :dontknow:
-
Since your multi-caps are the firecracker style I'm gonna suggest forget about my metal can suggestion and replace with two 22µF caps. Do the same in the power supply chassis.
-
Next stupid question, I replace that old can with 2 22uf caps. How do I wire those in? Up until this point I've only ever replaced caps on speakers, and a couple older solid state amps. Real straight forward. Just remove the old and solder a new one in, paying attention to pos. and neg. Here, I am going to need a little more understanding to make sure I am not making an ignorant mistake. I should mention, that yes, I have read a little bit about working on old tube amps, and yes, I totally understand the danger!
-
Up until this point
do you have the schematic in reply#8
find the caps, trace the wires, make notes, verify here.
reading schematics is a REAL handy thing, there are helps online, worth a surf