Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pompeiisneaks on April 05, 2016, 05:57:07 pm
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I've got a new problem with my Vox AC100/2. The thing had no hum at all, and still only has hum when I put the reverb tank INSIDE the chassis. This makes me think its getting the hum from the transformer below it. (Does this come from the PT? Makes sense, but want to confirm that too). Is there anything I can do to try and block the hum in it? Something I can put between them? Or do I need to move it somehow? (There's not much room in a head cabinet for a reverb pan)
Edit: Oh this is a standard 4AB3C1B reverb pan from Doug. It's in a bag too, but not sure if there's something else I can do to shield it? The opening is to the top side not down towards the tubes/transformers
~Phil
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C type tank is input isolated. try grounding input shield to tank body.
--pete
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The output end of the tank is hi-z and very low signal. It's very susceptible to hum sources. So it's important to have the output end of the tank far away form the PT. So, without knowing how your tank is oriented, try physically flipping the tank end for end. Get better?
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I don't know that I can flip it, but the white end is the shorter one and closest to the PT, the red end is farther away, I think if I read right, the output is the white which would also be the more susceptible side. I guess I could just swap the colors on both ends and mismatch the inside part and then spin the reverb tank around? Or do I maybe have it backwards? If the reverb is there but fairly mild at max volume does that mean I have it reversed, or does it just mean I could tweak the resistors to make it have a bit more depth to the reverb? I've heard that mod before too, just increase the pot from 1M to 2M or increase the output resistance right there by it?
I'll see if I can find a way to swap it around.
I may just try to ground it to the chassis, do I just get a short wire and solder it to the outer ring of the rca connector and then connect that to the chassis of the tank?
Thanks,
~Phil
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I don't know that I can flip it, but the white end is the shorter one and closest to the PT, the red end is farther away, I think if I read right, the output is the white which would also be the more susceptible side. I guess I could just swap the colors on both ends and mismatch the inside part and then spin the reverb tank around? Or do I maybe have it backwards? If the reverb is there but fairly mild at max volume does that mean I have it reversed, or does it just mean I could tweak the resistors to make it have a bit more depth to the reverb? I've heard that mod before too, just increase the pot from 1M to 2M or increase the output resistance right there by it?
I'll see if I can find a way to swap it around.
I may just try to ground it to the chassis, do I just get a short wire and solder it to the outer ring of the rca connector and then connect that to the chassis of the tank?
Thanks,
~Phil
just try with a test clip before doing anything permanent. is the return cable shielded and grounded?
--pete
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It should be, no? RCA CABLES are that way by design no? Also, there was zero him until I put it above the PT.
This cable came with the original vox
~Phil
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It should be, no? RCA CABLES are that way by design no? Also, there was zero him until I put it above the PT.
This cable came with the original vox
~Phil
yes, it would be shielded in this case.
--pete
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Also, there was zero him until I put it above the PT.
You know what the problem is. You'll probably have to move it away from the PT. But first, just take a gator jumper clip and connect the reverb shell to amp chassis. If that doesn't solve it you'll need to move the tank.
Why can't you flip the unit end for end? Just pick it up and turn it around. You don't have to swap anything inside the tank. If your cables are custom length, then just use a stereo patch cable for this test.
The red connector on the tank (not talking about cable) is the sensitive output. Sounds like it is already oriented correctly.
Post a pic that clearly shows the tank orientation and connection to the amp. Take it out of the bag so we can really see.
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Got it down to a very nominal level. Grounding didn't help, but rotating 180deg did. The OT is what is inducing the hum, and its where the sensitive end was, so when I spun it around the hum almost disappeared. (There's a very light hum at max volume, but its the best I can do inside this chassis(.
Thanks!
~Phil