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Thread: Vox JMI AC30 (1964) Servicing

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    Vox JMI AC30 (1964) Servicing

    Hi all

    First post so hope I don't irritate anyone by asking such an obvious question.

    I have recently acquired a couple of old AC30 TB amps. I have started servicing my own amps after hours of reading and so far so good. I just replaced all the original electrolytic filter caps on an AC30. It sounds lovely now. However, everything I have read says you should replace all the old caps (older than 10 years) as they dry out. As you can imagine, I am reluctant to yank out anything original that is working fine as I wouldn't wish to risk losing that tone.

    My question is, which caps would you replace? Any of the Wima signals caps? They look reasonably OK, no signs of leakage or cracks. Are these best left alone?

    Thanks in advance.

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    Lifetime Member Enzo's Avatar
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    One thing to learn is that leakage refers to electrical leakage, not goo oozing out. If you do see goo oozing out, then, yes, the part is bad. But you cannot tell an electrically leaky cap by looking. You would use a voltmeter or a cap tester.

    Do they sound great? Maybe stop fixing them then.

    If originalness is important to you, then that is my approach. If you want relatively reliable service for regular use, then maybe more preventative replacement of components is called for.

    When you replaced "all" the electrolytics, do you mean just the main filter caps, or are you including the little ones that bypass cathodes and such too?

    Certainly at 40-50 years old, any electrolytic in there is suspect. Your film caps could be fine, or they could all be living on borrowed time... and not making the payments.


    And original or not, tone is a matter of the whole thing together. PArt of the tone you like mght even be the fact that some parts are leaky. Put new parts in there and they might be "better" parts, but you may not like the sound as well. Shiny new stiff shoes may not be as comfortable as your old running shoes.

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    'However, everything I have read says you should replace all the old caps (older than 10 years) as they dry out'
    No, that advice is for electrolytics only (power supply and cathode bypass). Film signal caps will most always be ok (although any 'black cat' types from that era are suspect).
    Film caps do fail, either with dc leaking through, or by becoming totally open circuit and not letting ac through.
    I would leave the WIMAs in, unless one proves to be faulty. I've only ever had to replace 1 of these, it had gone open circuit in the tone stack of an AC50.
    Ensure that there's some space between the cathode bypass cap and cathode resistor on the EL84s - don't have them touching, as the heat from the resistor will stress the cap.

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    Noodle of Reality Steve Conner's Avatar
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    Yes, the general rule is that electrolytics should be replaced every decade or two, but plastic film caps can be left alone. (paper and paper-in-oil are a bit more temperamental?)

    However, if I found one bad plastic film, paper, or paper-in-oil cap, I'd be tempted to replace all caps of that same type in the amp. One bad one could be a canary for the others.

    But Enzo's comments apply. A serious restoration on a valuable vintage amp would mean investigating and documenting any drifted or leaky parts, and their effect on the tone. Maybe you like that effect, and you don't want the amp sounding "as new" again. You see people with funky, sputtery old amps who actually like that tone, and if you fixed it up, they'd hide behind the couch in terror.

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    Thanks guys for all your useful comments.

    I only came across this forum by accident but it is clearly an excellent resource that I will continue to use.

    All the best

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    Hi again

    What do you think would be the most likely cause of 'ghost notes' (like an octave pedal but off key). I have replaced the electrolytics as mentioned earlier and tried a new set of matched OP tubes in the amp too but it hasn't gone away.

    This only happens when I drive the amp reasonably hard and other web posts have suggested they all do this. Although I have had Valvetronix Voxes before, this is my first old one, so I would welcome your opinion again chaps.

    Thanks in advance.

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    That's generally caused by either the 1st or 2nd power supply cap after the rectifier being bad, or too small. The stock values of 16uF are on the low side, so if you're sure it's good, try increasing it to 32uF (this is within the rectifier's spec).
    Try measuring the ac ripple on the high voltage B+, at no signal and fully cranked, between ground and B+, either side of the choke. Depending on how your meter copes with that waveform, you may be getting above 20Vac if there are audible problems.

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    Thanks for this. I have just replaced the two large filter caps with new ones of the correct spec (according to The Engineers' service manual and what was in there already), so they ought to be good. Interesting about them maybe being too small; larger value would have been easier to get hold of I suspect.

    When I get a moment I'll pull the amp to bits again and check the voltage as you suggest; I assume I should not detect any AC ripple in an ideal world, right?

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    Check also the cathode bypass cap, it drys out easily as its heated up by the cathode resistor. Despite the "value" of a vintage amp I would replace the 50 Ohm cathode resistor with a 68 Ohm. This has no tonal influence but the lifetime of the powertubes is increased significantly.

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