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Welcome to the forum Jon
(This thread is a few months old now, and I think he fixed it a while back)
(This thread is a few months old now, and I think he fixed it a while back)
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Building a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo -
Nothing like a timely response. I guess I should have looked at the date!
Just got so juiced by seeing a GA-40 post, I jumped before looking.
I have recently built a lite version of the GA-40, without the tremolo channel,
with true point to point construction,(no board). Nice and simple.
Sounds great.
...JonComment
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Say Jon, can you post up the board layout drawings of the GA40?Originally posted by roughcut studios View Post
I want to build a clone and it would make it a lot easier on me if I didn't have to draw up a layout.
Thanks.....Comment
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GA-40 board layout
I dug out my files on my GA-40 build. I don't have a scanner, so here's a
quick photo of my drawing.
Great amp. I have recently built a prototype of a lite version with no trem channel. Simple and sweet! Have fun with your build!
.......JonAttached FilesComment
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Hi Jon; I see you replicated the R/C filter in channel 2; did you ever consider just leaving it out or did you just start with the idea of straight reproduction? I have 2 GA20Ts; except for a 12AY7 in the channel 1 preamp, the location of the tone control PPI, and a smaller OT, it's the same amp. I have bypassed that filter in channel 2 in both GA20Ts to duplicate - except as noted - channel 1 in a GA40. One bypass is a cap tapping the plate of the 5879 and going to the ch 2 volume control; the other is the same thing except switched via a set of contacts on the back of 1 meg volume control on channel 1. I don't have anyway to compare the results to a GA40 but what comes out makes a 12AX7 seem tame. The gents on the Hoffman BB ID'd the filter as knocking back low frequency oscillation but nobody had a good answer to how come so many. Congratuations on your GA40 work.! CJOriginally posted by roughcut studios View PostComment
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gentlemen, very informative post.... wish i had found it earlier.... i have a 50's ga20t. very similar to the pics posted in this thread. sounds great, however, the trem does not work.. the footswitch is not the problem. i have replaced the 6SQ7.. the 2nd channel definitely is not as loud as channel 1. do you think the trem caps are the culprit??? thanks for any support....Comment
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You could just replace the three tremolo caps and three resistors, + the 100K plate resistor and two caps and two resistors that make the cathode bias for the 2nd channel. Since the tremolo caps and resistors are not in the signal path, just use some modern metal film resistors and any good coupling caps (600 volt orange drop etc.).Originally posted by jackleg View PostComment
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Hi JL; replacing the trem caps in one of my GA20Ts fixed it. if those caps are original, then other than having the switch wired in wrong, that just about has to be it. If you have a 2-sided circuit board, you'll have to take the controls out of the panel to flip it ... it can't be helped but you, or anybody in their right mind, only wants to do it once.Originally posted by jackleg View Post
At some point there is a trade-off between authenticity and reliable performance. If they work correctly, you can leave the coupling caps alone but as for the rest of the 'worker bees' there including the trem caps ... well, that's your choice. Good luck with your project.
CJ
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diablo and cap, thanks for the speedy replies. its wonderful to be able to communicate with those that know. it could take some time before i complete your suggestions, but, i will be sure to tell you how i made out... thanks again...Comment
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"At some point there is a trade-off between authenticity and reliable performance"
Personally, I like to replace enough parts to make them reliable enough to play and enjoy, and leave everything else unchanged. Just save all parts that were removed, and document the work done. That should keep future collectors happy.
I don't see the point of owning a guitar or amp that's too perfect to play!(or unsafe to play!)
I'll leave that to folks with a lot more money than me.
Good luck with your repair ...JonComment
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Hi Jon; except for the GA20T and maybe the GA40LPs, I'd agree with you. The GA20T has the coupling caps, channel two R/C filter, and the trem caps on the underside of the board. If you - or the person who winds up with your amp - has to flip it to do maintenance, it's about a 2 hour job to spend 5 minutes replacing a .25 resistor.
If I bought your GA20T and the trem crapped out on me because you justified not doing further work when you already had physical access because you wanted preserve the essence of the amp for posterity, I'd think your sensibilities were misplaced.
I don't think every Saturday morning driveway mechanic working on his vintage ride is doing so just because he enjoys turning a wrench ! Sure, keep it as authentic as possible but, at some point, you have to consider the next guy too ... they might not want to be humping an amp on/off the bench or down to/back from the amp shop because someone thought a 50 year-old resistor gave the amp 'character'.
I try and leave every thing as authentic as I can but I turn my amps over and I don't want them coming back because I didn't take the extra five minutes to update something or the owner stuck with repairing something that I could have tightened up when I had the amp on the bench. Reliability counts just as much as tone ... at least I think it does. As ever, YMMV.
CJComment
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thanks so much,,, i have just flipped the board and am wondering how many of the caps i should replace and what type of cap should i buy??? i checked all of the resistors and they were of the proper values... should i replace the huge 20 mf electrolytic, too? cheers!!Comment
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gibson double sided board
Now that you have gotten inside, go ahead and replace all the coupling and trem caps,as well as all electrolytic bypass caps. You do not want to have to go back in next week or next year to replace a part that you skipped over to save time or keep originality. Just save all the bits that you remove for any future collector types, and document the work done for the benefit of other service folks or future new owners. Safety and reliable operation should be at the top of anyone's list. You should also install a grounded line cord if you haven't done so already.
I love vintage, but I hate smelly blue smoke and electrical shocks.
Good luck........... jonComment
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roughcut, are you saying that i should change every available part while i am there??? and, if so, what type of caps should i use... the local electronic stores in my area may not have the high ends. maybe, i will order them from a reliable mail-order company. any suggestions as to whom?Comment
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gibson double sided board
I don't like to pull things apart more than I have to, so once I dig in, I would replace all plate resistors, coupling caps, and bypass electrolytics for peace of mind. Those parts are probably cheaper than your time, and are often sources of trouble in older amps.
Almost any modern polyester or polypropylene film cap will do the job, such as the ever popular "orange drops". You might try the SBE 715's (although you might get 10 different answers from 10 different people about which cap is the "best" cap for your application)
Antique Electronics Supply (tubesandmore.com) , and Mouser Electronics (mouser.co) are two excellent sources.Comment
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