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Weak tremolo in Gibson GA-40

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  • #16
    I thought that might be what it was, but the only original cap left in that area is the orange Astron 20mf/25vdc with the resistor piggybacked. The others have been changed with (2) 20uf/500vdc Sprague atoms and (1) 10uf/450vdc from V5 to the 10k resistor (far left on Circuit board) with the yellow wire in your picture. It seemed strange to me that whomever did the cap job wouldn't clean that up! Any harm in leaving it pooled in the bottom of the chassis with wiring stuck in it? Does it conduct?


    • #17
      Hi Mike; I can only tell you about a GA8 I have. When I got it, alot of the transformer potting had melted out, pooled, and maybe with some DC leakage, contributed to some bad rust at the rim of the 5Y3 tube socket and chassis. It was easier to take the old socket out and replace it. When I did, I did the scrub-a-dub; picking at the crud chunk by chunk, mopping with Q-tips in alcohol in between. It took a while but it got done. I can't say I'm sure it doesn't conduct.

      It won't bother an insulated wire but I think if it's in contact with a socket connection with B+ and the chassis steel, I think that's a problem. I'm maybe a little more obsessive-compulsive than others but I often sell my rehab projects. Because of that, I try to get them as clean as I can ... even if it means crud-picking. The only thing that would hold me back would be the certainty of damaging wiring insulation. Your call. John


      • #18
        Bump: Hi Mike - this is my '59 parallel-6V6 GA8. Consequences of melted potting and maybe a little B+. See old socket at left of pic; intervention was necessary. I cleaned to bare metal and applied something like phosphoric acid to damp down further corrosion.

        I'd want to know what was under the crud ... J


        • #19
          John, Fired it up! Played through it for about an hour. It's quiet and sounds great! Tremelo is subtle, but nice. Do you know if the channels are interactive, like a 5E3 Deluxe? It sounds like it even if it's not jumpered. Plugged into channel 2, if you turn up the volume on channel 1, the sound changes. Am I hearin' things? If jumpered, will it make a big difference or can this even be done?

          Thanks again, Mike


          • #20
            Hi Mike; no, you're not hearing things ... the channels are joined at the 'Voicing'/tone control hip:

            The red circle at the bottom is the channel 2 volume control. The red dot to it's left shows the 'Y' where the signal splits to the volume control and to the tone control. The blue circle is the channel 1 volume control and the green circle is the 'Voicing' / tone control common to both channels.

            When the channel 2 signal reaches the line at the top, it backfeeds the channel 1 volume control and loops through the tone control. If the channel 1 volume control is full off - presents the highest resistance to ground - it shunts the signal back though the tone control but doesn't effect volume. If the channel 1 volume control is turned up, it can vary the apparent volume or tone.

            I wasn't there back in the day but I'd bet a set of new guitar strings this is just the accidental outcome of saving the cost of a second tone control for channel 2; with respect, Gibson never met a cost it couldn't avoid.

            When you say jumpering, if you're referring to channels, I wouldn't recommend it. Channel 1 was designed as a mic channel refer to Owner's Manual ( scroll down. It's a 'high' gain channel using grid-leak bias.

            If it's gain you seek, unleash the channel 2 5879 this way:

            Bypassing the filter section. Gotta go now, will finish this later. John
            Last edited by capnjuan; 02-08-2009, 06:48 PM.


            • #21
              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


              • #22
                Thanks again John! It's not my amp, so I won't be doing to much experimenting. It's my guitar players amp. I just like poking around with these circuits and trying to learn something. I am currently learning how to read schematics and am reading books on tube amps maintenance and repair. I am actually a Bass player who likes to plink around on guitar and fool with tube guitar amps. Other than the nine Basses and a couple of Bass rigs, I have a Fender Blues Junior, Hot Rod Deluxe, and John Mayer Sig Strat. I have done some modding to both the Blues Junior and Hot Rod. Another friend of mine brought over an original '51 5B3 tweed deluxe that was his dads. All original except for 2 coupling caps. It is making some crackling and popping. I recomended he have all the electrolytics replaced.
                Anyway, thanks again! You have a lot of knowledge with regard to these amps and I am going to keep these pics and schematics handy to refer to.



                • #23
                  Happy to help; good luck! John


                  • #24
                    Thanks Capnjuan for all the good info. I'll have to incorporate that RC filter lift mod in the tremelo channel. Looking over the GA-40 schematic, I have a couple of questions that maybe you can answer. There's a 1M pot in the post PI position. I assume this is the tone control, although I've never seen one before like that. Should I get a linear or audio taper pot? The two volume pots, I assume audio taper is best? How about for the tremelo frequency and depth - audio or linear taper? I'm thinking of using a PT from Ted Weber's Deluxe Reverb #25130. It's got 540V taps / 150 mA, that I figure should give 378V DC before the 5Y3 rectifer drops it down to around 320-330V. Does this sound reasonable? What kind of voltages are you seeing on your GA20? I'll probably build this as a head to do it on the cheap.


                    • #25
                      Hi Diablo; some time ago, when Victoria Amps rolled out their re-issue of the GA40, Tonequest published an extensive review of the original GA40 and Victoria's version. It's well worth reading ... sometimes the articles can be found on Tonequest's site. Anyway, the article mentioned the post-PI position of the volume control and that's what it is, volume - not tone. The Tonequests had never seen one there either.

                      I would just be guessing but I think it would be audio, not linear taper. In the GA20T schematic; both the trem depth and frequency controls are 500K Reverse Audio ... and the circuit is identical to the GA40 schematic; Gibson was pretty sloppy with those kinds of details. Not completely certain but it's my understanding that if the connections are reversed, a convential audio taper pot can be converted to a reverse audio taper ... there are smarter people than I here who can confirm this but, if it were me, I'd probably try RAs first ... at least there's a drawing out there to hang my hat on.

                      Can't be certain but in the GA40LP schematic, it doesn't look like there's quite that much voltage drop across the recitifier. If not, then you might need a dropping resistor of ? value to get you where you want to go. I don't have voltages on the GA20T to offer ... been so long since I fired it up. CJ


                      • #26
                        Thanks CJ. So does the GA-40 have a tone control? I've seen pictures of the faceplate showing a tone control, but I can't find one on the circuit diagram. Maybe I have bad eyes...


                        • #27
                          Originally posted by Diablo View Post
                          Thanks CJ. So does the GA-40 have a tone control? I've seen pictures of the faceplate showing a tone control, but I can't find one on the circuit diagram. Maybe I have bad eyes...
                          Hmmm ... no ... not your eyes ... maybe my memory; GA40LP schematic from Schematic Heaven ...


                          The Tonequest article specifically talked about a post-PI volume control but in looking at this schematic, it looks like what's post-PI is the tone control ... My apologies ... I wish I had a better answer ... CJ


                          • #28
                            Thanks Capnjuan. I don't have anywhere near the expertise of a lot of you guys on the forum, and I appreciate all the help I get here. So far, I've built a tweed bassman, deluxe reverb, and Tore-Vibe from kits, and they all sound great. Without the internet, this just wouldn't have been possible. I remember the first electric guitar I tried in the mid 1960s, played through a new Gibson combo amp with reverb. Boy, did that thing sound fantastic. I don't remember what the guitar was....I think Supro. But, I remember that Gibson reverb sound. I wish I could remember which model amp that was.


                            • #29
                              Hi Diablo; I've been messing with early 60s Gibson amps for the last few years - I like their vintage tone and feel and Gibsons are cheaper than similar make/model Fenders from the same time frame. Agree that the web has made it a lot easier to learn and build good projects. Back in the day, if you screwed up, you had to pay someone to straighten it out. These days, you can get some pretty knowledgeable people to help you out on-line. Good luck with your project! CJ


                              • #30
                                I have repaired a "55 GA40 with the tremolo circuit using a 6sq7, caps, and resistors, and have built a clone of the same amp. I would suspect the caps are bad, and would go ahead and replace them, and while your'e in there check the resistors in the trem circuit for drift. The board is double sided and a pain in the neck to work on, so you only want to pull it one time, that is, replace all suspect parts at one time.
                                Great sounding amps!
                                Be sure to use a nice vintage alnico Jensen p12p for original tone.
                                I am able to supply board layout drawings if needed, as I documented everything when I did my clone.
                                Good luck, Jon


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