Page 2 of 2 FirstFirst 12
Results 36 to 52 of 52

Thread: Weak tremolo in Gibson GA-40

  1. #36
    New Member
    Join Date
    Mar 2007
    Posts
    4
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    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....

    0 Not allowed! Not allowed!

  2. #37
    Senior Member
    Join Date
    Dec 2007
    Posts
    932
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    12
    Quote Originally Posted by jackleg View Post
    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....
    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.).

    0 Not allowed! Not allowed!

  3. #38
    Senior Member capnjuan's Avatar
    Join Date
    Apr 2007
    Posts
    120
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    13
    Quote Originally Posted by jackleg View Post
    ... do you think the trem caps are the culprit??? ...
    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.

    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

    0 Not allowed! Not allowed!

  4. #39
    New Member
    Join Date
    Mar 2007
    Posts
    4
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    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...

    0 Not allowed! Not allowed!

  5. #40
    Member roughcut studios's Avatar
    Join Date
    Apr 2009
    Location
    boston
    Posts
    50
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    11
    "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 ...Jon

    0 Not allowed! Not allowed!

  6. #41
    Senior Member capnjuan's Avatar
    Join Date
    Apr 2007
    Posts
    120
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    13
    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.

    CJ

    0 Not allowed! Not allowed!

  7. #42
    New Member
    Join Date
    Mar 2007
    Posts
    4
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    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!!

    0 Not allowed! Not allowed!

  8. #43
    Member roughcut studios's Avatar
    Join Date
    Apr 2009
    Location
    boston
    Posts
    50
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    11

    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........... jon

    0 Not allowed! Not allowed!

  9. #44
    New Member
    Join Date
    Mar 2007
    Posts
    4
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    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?

    0 Not allowed! Not allowed!

  10. #45
    Member roughcut studios's Avatar
    Join Date
    Apr 2009
    Location
    boston
    Posts
    50
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    11

    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.

    0 Not allowed! Not allowed!

  11. #46
    Member
    Join Date
    Aug 2012
    Posts
    49
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    8
    capnjuan,

    New member, first post, I have a GA-20T I am rebuilding, I layed out the preamp it's using a 12AY7, I am having trouble with understanding grid-leak biasing. How is it getting voltage? There is no wire going to it.. pin 3 and 8 are jumpered then going to ground. Just trying to understand the circuit, it's just not making sense.
    I would appreciate any help.

    Thanks
    Thnder

    Quote Originally Posted by capnjuan View Post
    Hi Mike: finishing my thought here. This is the GA20T;s channel 1 preamp:



    Each input jack of channel 1 feeds one half of the 12AY7, the tube relies on grid-leak biasing, and the cathodes of the 12AY7 are jumpered together. If you want to experiment with the amp, it might make more sense to un-jumper the cathodes, add cathode bypass resistors and caps, get rid of the 10meg resistors and input caps, and use a more reasonable value like 470K-1meg.

    The front end of channel 1 bogs down pretty badly; if you wanted something out of the ordinary, the trick would be to create separately biased tube halves and take the output of the 1st half of V1 and feed it to the other half.

    Anyway, good luck with your amp! John

    0 Not allowed! Not allowed!

  12. #47
    Member
    Join Date
    Aug 2012
    Posts
    49
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    8
    capnjuan,

    New member, first post, I have a GA-20T I am rebuilding, I layed out the preamp it's using a 12AY7, I am having trouble with understanding grid-leak biasing. How is it getting voltage? There is no wire going to it.. pin 3 and 8 are jumpered then going to ground. Just trying to understand the circuit, it's just not making sense.
    I would appreciate any help.

    Thanks
    Thndr

    Quote Originally Posted by capnjuan View Post
    Hi Mike: finishing my thought here. This is the GA20T;s channel 1 preamp:



    Each input jack of channel 1 feeds one half of the 12AY7, the tube relies on grid-leak biasing, and the cathodes of the 12AY7 are jumpered together. If you want to experiment with the amp, it might make more sense to un-jumper the cathodes, add cathode bypass resistors and caps, get rid of the 10meg resistors and input caps, and use a more reasonable value like 470K-1meg.

    The front end of channel 1 bogs down pretty badly; if you wanted something out of the ordinary, the trick would be to create separately biased tube halves and take the output of the 1st half of V1 and feed it to the other half.

    Anyway, good luck with your amp! John

    0 Not allowed! Not allowed!

  13. #48
    Supporting Member tubeswell's Avatar
    Join Date
    Feb 2007
    Location
    Wellington NZ
    Posts
    4,200
    Blog Entries
    1
    Thumbs Up/Down
    Received: 130/0
    Given: 43/0
    Rep Power
    18
    Quote Originally Posted by Thndr View Post
    capnjuan,

    New member, first post, I have a GA-20T I am rebuilding, I layed out the preamp it's using a 12AY7, I am having trouble with understanding grid-leak biasing. How is it getting voltage? There is no wire going to it.. pin 3 and 8 are jumpered then going to ground. Just trying to understand the circuit, it's just not making sense.
    I would appreciate any help.

    Thanks
    Thnder
    Grid leak biasing relies on a large grid leak resistance to create a bias voltage - (You'll note those grid leak resistors are 10M). Its not a great way of biasing a tube.

    0 Not allowed! Not allowed!
    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

  14. #49
    Member
    Join Date
    Aug 2012
    Posts
    49
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    8
    Tubeswell,

    Thank you for the reply, I should have made a new thread, I have questions.. on this amp there is no physical wire going from the grid leak resistors (this version they are 1megs) to the cathode. So is this leakage occurring within the tube its self? It's probably a silly question for those who know the answer, I am just frustrated enough to ask.

    When I get home today I going to look again.

    Thanks again,
    Thndr

    0 Not allowed! Not allowed!

  15. #50
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    31,873
    Thumbs Up/Down
    Received: 1,726/7
    Given: 0/0
    Rep Power
    54
    You might look up "grid leak bias" in a tube manual like the RCA or even a book like the RHD4 - Radiotron designers handbook. You can get a more detailed explanation.

    The grid leak resistor connects to ground. The cathode connects to ground. They are thus connected together, whether there is a wire or not.

    The hot cathode in a tube boils off electrons forming what they call the space cloud. With the very high resistance to ground from the grid, it sits there more or less in empty space. The space cloud electrons, well some of them will land on that grid and accumulate there. This causes the grid to build up a small negative charge from all those electrons. That negative sharge, even if it is only a volt or less, is still bias for the tube. SO I guess yes, the leakage does ocur inside the tube.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  16. #51
    Member
    Join Date
    Aug 2012
    Posts
    49
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    8
    Quote Originally Posted by Enzo View Post
    You might look up "grid leak bias" in a tube manual like the RCA or even a book like the RHD4 - Radiotron designers handbook. You can get a more detailed explanation.

    The grid leak resistor connects to ground. The cathode connects to ground. They are thus connected together, whether there is a wire or not.

    The hot cathode in a tube boils off electrons forming what they call the space cloud. With the very high resistance to ground from the grid, it sits there more or less in empty space. The space cloud electrons, well some of them will land on that grid and accumulate there. This causes the grid to build up a small negative charge from all those electrons. That negative sharge, even if it is only a volt or less, is still bias for the tube. SO I guess yes, the leakage does ocur inside the tube.
    Thank you for answering, I made a new thread for this awhile ago. The drawing is deceiving to me (newbe), I'll take your advice and do somemore reading. My worry among otherthings is the person who worked on this before me has wired the tubes to pc board incorrectly by accident. I made a partical layout diagram of what I am seeing in the new thread 1959 GA-20T.

    Back to the books
    Thanks again
    Thndr

    0 Not allowed! Not allowed!

  17. #52
    New Member
    Join Date
    Apr 2019
    Posts
    1
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    I know that this is an old post. Could you please tell me or show me on the pictures were the .0047 and .047 caps are in this amp? I am new at the rebuild thing just try to find these caps in the amp. GA 20 T. Thanks

    0 Not allowed! Not allowed!

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Gibson Tremolo circuit
    By EETStudent in forum Theory & Design
    Replies: 31
    Last Post: 07-23-2008, 08:50 AM
  2. Weak tremolo
    By drewl in forum Maintenance, Troubleshooting & Repair
    Replies: 4
    Last Post: 08-08-2007, 03:23 PM
  3. pickups too weak
    By neil in forum Pickup Makers
    Replies: 2
    Last Post: 10-10-2006, 06:21 AM
  4. Gibson Invader Tremolo Woes
    By Pickupmaker in forum Repair and Restoration
    Replies: 1
    Last Post: 09-27-2006, 09:00 AM
  5. Hiwatt output weak
    By Guitarist in forum Maintenance, Troubleshooting & Repair
    Replies: 2
    Last Post: 07-28-2006, 11:51 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •