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Guild Thunder I farts out when being pushed... PS issues?

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  • Guild Thunder I farts out when being pushed... PS issues?

    Hey All,

    I recently purchased this old Guild amp (schematic below) from a local store. Being myself, I took the hood off to see what they did to make it a functional beast again. They said they changed a couple resistors and some caps. To my surprise, the original can caps are still connected, but they replaced the first (separate) cap that comes off the rectifier. They've also added a cathode bypass (100uF, I think?) to the power stage, perhaps to tighten the bass, but this seems like treating the symptom, not the cause... While I do think the power section of a pair of 6GW8's could stand to gain from a little support there, I think the real problem lies elsewhere.

    Here's the real issue: When I start playing hard at higher volumes, especially with low notes and chords, the amp spits and sputters like it can't get enough juice.

    I have three suspicions as to the cause:

    1) bad filter caps -- Bad caps may lead to some of the crackling and hum I had at one point, or that could just be lights (etc.) bleeding to the bench when it's out.

    2) weak rectifier tube -- A weak rectifier could be either worn out due to age and use, or simply can't provide for the power-sucking bottom end due to electro-mechanical limitations of the 6CA4.

    3) heavily saturated OPT -- The only reason I suspect the OPT is because it seems to be getting a bit warm... warmer than I'd like or even expect. (as a comparison, even my Silvertone 1484 with stock (wimpy) OPT doesnt get that warm after hours of gigging...)

    Any ideas on how to really peg the cause without a spray and pray? Thanks,



  • #2
    To a moderator, I believe this should either be in the repair and restoration sub-forum, or Maintenance, troubleshooting... sub-forum. Sorry for the inconvenience...


    • #3
      I would do a recap, return it to the factory design, and swap in new tubes. Once you have a base line go from there. I know it isn't what you want to hear, but it's pretty much standard proceedure.


      • #4
        Push comes to shove I would lower the first stage cathode bypass capacitor.

        That 30uf is probably passing way too much bass frequencies.


        • #5
          I had one of these, dead stock. I sold it to a shop in PA last Spring. Anyway, only because this fixed mine: the Trek switch was a bit wonky, and if I didn't adjust it "just so," the amp sounded as you describe. I thought I was just overdriving the amp and its crummy CTS 10" speaker, but my friend pointed out it sounded fine til I engaged the trem.

          I'm not saying it IS, but I'm tossing it out as a "not in the usual order of suspicion." It's easy to check, and easy to fix. I remember that switch being hard to engage.

          Also, getting good new power tubes could be... fun? No testing available on those... mine came with Mullards all around, so needless to say I yanked the rectifier & 12AX7s... and I loved the way they rigged the toggle switch inside forthe "Bright" option. This amp was a work of art, and it really did sound good once I got rid of all the crackle and anemia.

          Yours isn't black and have one long horizontal scratch on the face, does it?

          "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
          "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
          "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -


          • #6
            Just to be sure, this amp is the Thunder 1-12. It has a 12" driver, which I've swapped to preserve the original. I have a a 70's alnico Eminence driver in atm.

            Power tubes are new GE 6GW8'so. And yea,the trem switch is dodgey, but wiggle it back and forth a couple times and nothing is weird anymore. The usual issue with that is getting the trem to engage fully.

            Jazz, I'll look at the cathode bypass caps. I think the shop guy may have swapped them all with 47uF caps... to clarify, I don't at all mind the spit and sputter of a slightly starved supply from overbearing bass. In fact, it's the super-trash factor I love! But when it comes to taking out nearly all sound from output entirely..? I'm not quite convinced that those little things can be responsible for that much "ducking"...

            FYI, I have the caps on hand to replace those of the can. I just wanted to see if folks had ideas of other problems. When I bought this, I didn't know that 2/3 or the coupling caps had cracked membranes and were likely compromised and leaky due to humidity...


            • #7
              Turns out this may in fact be a huge part of it... I double checked the guts today to find that the shop tech replaced all of them with 47uF caps...

              RANT WARNING:
              I'm really pissed about this - they had this "fixed up" to put out on the shop floor. I payed $400 for this. Granted it WORKS, but for that price it should be FULLY functional... The manager is willing to compose me for parts and time to fix it myself, which in itself is commendable, but I shouldn't have to do this... END RANT.


              • #8
                The output stage is fixed bias, but you refer to cathode bypass cap?

                The 0.15uf coupling caps to output stage are hi-fi/large, so are likely to exacerbate blocking distortion when overdriving the output stage.


                • #9
                  The cathode bypass caps I referred to are in the preamp stages.