This is a REALLY bizarre one

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by dazco, Mar 3, 2012.

  1. dazco

    dazco Member

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    I was using those russian made 12AX7 mullard RI's for a long while in V1 of my 2204 style home built amp. Then i switched back to the old 80's chinese i've always used. Now months later i tried the mullards again in V1 and it acted like what happens when a grid resistor is missing....hit the strings and you get sound but it's nasty, then it fades to nothing and no more sound till you unplug the cable then plug it back in or turn the amp off them on again, etc. Thought it was a bad tube but i have 3 and all did this and all worked fine last time i used them. I also found it does this with a EH tube, but those 2 tubes aside, all others i tried worked fine.

    Today i found if i remove the 68k grid stopper at the input it works with those tubes ! of course i suspected the resistor so i tried a couple others and same thing, Ended up changing the tube socket, the shielded cable from the jack to the grid stopper, and even the jack and 1 meg resistor. WTF could this be?!!! This is just too bizarre. :eek:
     
  2. VaughnC

    VaughnC Supporting Member

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    I had something similar happen a while back. Mine was due to the fact that some tube pins were too thin to make contact with the tube socket contacts. An NOS tube would work fine but a new EH would not...and, by eye, you could even see the EH's pins were thinner. And, due to their design, the socket contacts would only accept NOS size pins. Just a thought.
     
  3. dazco

    dazco Member

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    Thanks. Just checked them with a digital caliper and thats not it. Also, they worked fine before, plus as i said i even tried a new socket. But thats anyways 4 the reply.
     
  4. Cyg_1963

    Cyg_1963 Member

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    I'd check the biasing cap(s).
     
  5. dazco

    dazco Member

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    Good call, i'll check em. I know i heated them a bit too much thanks to the garbage turrets tubesandmore sent used on the turretboard i bought. Even used flux and it took a lot to get it to flow. Only thing is, wouldn't a bias cap issue leave it buzzing even if you kill the preamp signal?
     
  6. guiltless

    guiltless Member

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    I had the same problem with their turrets. I think the turrets they use are plated and the plating comes off.
     
  7. dazco

    dazco Member

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    They didn't look like that, but whatever it was they suck....well, actually they DON'T suck which is the problem. :D

    Anyways, update: swapped caps for bias circuit and no joy.
     

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