Strange Champ Phenomenon

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by tfunster, Feb 1, 2006.


  1. tfunster

    tfunster Silver Supporting Member

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    I just got a Fender Champ Replica (just a chassis) from this guy who makes them using vintage parts like old military transformers and NOS tubes.
    The tube layout is 6x5 Rectifier, 6V6 and 6201 Preamp tube.
    The amp is fine and sounds great with the stock tubes but when I swamp in a 6L6 and 12AX7 (which the guy told me is fine), I get a weird high pitched feedback when the volume is set from 2:00 to 5:00. Turned all the way up or about halfway it's fine but the tone doesn't give up the goods until about 75%. I've tried just a 12AX7 with the original 6V6 and it's the same problem. There's also a strange ring mod sounding going on when I'm playing the guitar with the volume set in that trouble zone.

    1) Anybody knows what's going on??? I've e-mailed the guy and sent pics, but I thought I'd try my luck here too.
    2) Also, I had a spare JJ 6V6 laying around that I tried to put in, but it won't go in. The JJ has a different pin layout than the old RCA 6V6. Is this normal??
    3) The RCA 6V6 is one of those metal tubes ones which I hear are less desirable than the glass ones. Are the glass ones much better?? What's the tonal difference??
    thanx guys for any help
     
  2. Matt H

    Matt H Guest

    sounds like the amp has oscillation problems (perhaps triggered/worsened by microphonic tube)

    a 6021 is an 8 pin submini tube, if i'm not mistaken- nothing remotely (physically) like a 12ax7 (9 pin mini... a monster compared to a 6021). I'm guessing (hoping) that you're just giving the wrong number for that.

    there's ZERO reason that a JJ 6v6 would have "a different pin layout" than an old RCA 6v6... if anything, the JJ just might not have pin 1- however, same tube socket, and pin 1 SHOULD BE GROUNDED. (not used as a tie point for a resistor or anything if metal shelled tubes are being used).

    "but it won't go in"- that makes NO sense. both are octal bases... if it's really a 6v6 in there, then any octal tube will physically it in the slot (regardless of whether it's electrically compatible or not).

    there should be a plastic guide pin in the middle of the ring of pins... it'll have a protrusion (a key, as it were) that'll line up with the octal socket... th is prrevents the tube from being put in incorrectly.
     
  3. tfunster

    tfunster Silver Supporting Member

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    Matt
    thanx for the reply
    The preamp is 6201, not 6021. My Bad.

    And yep, the JJ 6V6 does not go in. I allign it perfectly but it's a no go. The RCA fits right in.

    Could you give a little more info about oscillation problems?? Plus I tried four different 12AX7's in there, and I know all tubes are microphonic to a degree, but these sound just fine, when they're not oscillating that is.
     
  4. Matt H

    Matt H Guest

    oscillation is likely a lead dress issue- in other words, the wiring job was sloppy. (or a soldering job was sloppy, poor ground)

    ic an't think of a reason the JJ won't go in if it's aligned properly. do the pins not line up with the holes? or is it just tight?
     
  5. tfunster

    tfunster Silver Supporting Member

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    the pins line up just fine, and will go in for about 3mm, then it won't go any further.

    I noticed that this amp does produce a lot of hiss, especially with the volume cranked, maybe this is also has to do with a poor soldering job.

    But why would the amp run okay with the 6201 and 6V6 but not the 12AX7 or 6L6???
     
  6. Matt H

    Matt H Guest

    a 6201 is basically a 12at7- it's lower voltage gain on tap than a 12ax7.

    simply put, a 12ax7 amplifies more- so more noise in the amp (anything from resistor noise from using carbon comps, to squeal from lead dress issues and oscillation) will be "more".

    sounds like the tube socket is just really tight, don't force the tube, but you do know about gently rocking tubes in and out of sockets, right? (do it in a circular motion) you shouldn't have to force it though.


    but yeah, it sounds like it might not be the best constructed amp- then again, it's not exactly a "Great" design to begin with in terms of noise and technique.

    things like redoing the heater wiring to a more modern scheme (where you dc reference the heater circuit via balance resistors or a center tapped winding) cna reduce hum.

    carbon comp resistors are noisier than metal film or carbon film... the champ was pretty underfiltered to begin with... etc.
     
  7. Shea

    Shea Member

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    Maybe the pins on the JJ aren't perfectly straight. I've had a hard time getting brand new JJ 6V6s into the socket before.

    Shea
     
  8. tiptone

    tiptone Silver Supporting Member

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    Aren't the JJ 6V6s known to have larger-than-normal pins? Or am I inventing a memory again?

    tfunster: do you have another 6v6 that you could try?
     
  9. tfunster

    tfunster Silver Supporting Member

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    The pins of the JJ look VERY SLIGHTLY larger than the RCA.

    I e-mailed the guy who made the amp and he says that the yellow wire that goes from the middle of the volume control to the 12AX7 is too close to the speaker wire. Here's some pics. What do you guys think???




    [​IMG]





    [​IMG]


    [​IMG]





    [​IMG]
     
  10. TheAmpNerd

    TheAmpNerd Member

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    Yuuch!

    I can't see the date codes but I don't think anyone wants an amp that employs used caps, especially electrolytics! Some look damaged and some
    look swollen (going to physically leak shortly).

    Talk to the guy who built it, he should have trouble shot it before
    he sold it. Follow his recommendations or send it back and have
    him fix it.

    At least he used heat shrink on some of the wiring splices.

    The wiring to the rectifier tube looks like one of the pins
    is bent down and almost touching the chassis.

    How does it sound?

    How much did you pay for it?

    Did a cab and speaker come with it too?

    Try to enjoy as much as you can and let this be a learning
    experience.
     
  11. tfunster

    tfunster Silver Supporting Member

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    I paid $120 for it. I think it sounds very good, maybe as good as a victoria champ that I played a few years ago. It's just a chassis, and I am playing it through a 1x12 earcandy closed back cab with a Red Fang. It has only one tone, but that's what the champ does.

    I'm not sure what to do, the guy offered to take it back and refund the money. That might be the best option because I'm only going to run into more trouble as time goes on. Unfortunately, repair isn't really an option as the guy lives in Quebec and it's not worth the money or time it's going to take.
     
  12. Shea

    Shea Member

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    I've modified a couple old, PTP PA amps for guitar. They have no circuit boards - just some mounting lugs here and there, and stuff mounted directly onto tube sockets, pots, and jacks. Both of them had oscillation problems that I had to sort out by rerouting some things. So, based on my limited experience, you gotta deal with oscillations when you go this route.

    I've gotten some good tones out of them, though, once everything was working right.

    Shea
     
  13. tfunster

    tfunster Silver Supporting Member

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    Well here's something strange. I haven't done anything at all to the amp, but all of a sudden it doesn't do that weird oscillating thing anymore with 6L6's or 12AX7's. It's pretty noisy at full volume with my strat, but I blame that on the electricity in my apartment more than anything else.

    Anyone have any ideas as to why the problem has (so far) gone away??? I'm still hesitant to keep the amp because for all I know the problem can start again anyday.
     
  14. sixstringslut

    sixstringslut Member

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    Wiring looks a little wild. Maybe they could be redressed if you plan to keep it. This is a single ended amp, they are not known for noiseless operation. They do weird things sometimes thats part of their character. Though they look simple, these are one of the harder amps to build. I would start with twisting the ac lines and keeping them as close to chassis as posible. Maybe moving off/on switch somewhere else to get ac's away from everything.
     

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