Hot Rod Deluxe Hum...

[BStringThumper]

Hi, Everyone-

I'm looking for some suggestions please on a Hot Rod Deluxe, not sure which version, that has a hum. The odd thing to me is the hum is there with all the controls at zero.. but.. if I turn the volume knob to the #2 position its quiet. If I go passed #2 it gets louder and gets louder as turning the knob up higher. I have done the usual hum elimination tests and filter caps are new F&Ts. The hum is less evident with V1 removed. Since the hum is controlled by the volume it has to be something upstream, which basically is the input jacks. They have Switchcraft metal jacks with flying wires. They might be wired wrong?? But why does the hum go away in the #2 volume position??? Could it be hitting a oscillation frequency and cancelling each other out?? This hum is a new one to me.

Thank you for any thoughts on this problem.

[52 Bill]

Ground loop? Were the new inputs installed with insulators? Do they have a ground wire that connects the board to the chassis via the new jacks?

[glebert]

If hum is there with all controls at zero to me that says that it is in the PI or power tubes. If you plug a guitar straight into the power amp in jack is the hum still there? That pretty much takes out the preamp completely.

If you pull the PI tube is the hum still there? Have you tried a different set of power tubes? Power tube mismatch can be a source of hum.

Don't know why you can null out the hum at a certain volume, it sounds like cancellation to me. I don't know that it should be part of diagnostics at this point.

[Delta362]

Did this hum show up after the filter caps were replaced? I suspect the ribbon cables between the main and tube boards may not be in their original bend orientation after the main board was updated.

[BStringThumper]

Thank you, everyone, for your input. I did try different tubes. I did plug into the Power Amp In jack and the hum is quieter. It seems to be coming from V1 because it's quieter with that tube removed. I unscrewed the metal input jacks from the chassis and it did not make any difference. From what I can tell it looks like the input jack wires are oriented correctly. I could install the plastic type. The ribbon cables have been replaced and spaced. Moving the wires around make no difference to the hum. Pictures are worth a thousand words. Any more thoughts on why it would be quiet with volume on 2??

[BStringThumper]

The pictures didn't look that blurry when I took them. sorry.

[Helmholtz]

A bad filter cap/connection can cause power amp and preamp ripple hum at the same time.
As gain stages invert the hum phase, cancellation effects are possible.

[Enzo]

Your knob on 2 thing tells me you have two sources of hum that are out of phase. One is after the control, the other before. At zero you hear only the after hum. When you start turning up the control, the out of phase earlier hum comes up and cancels the later hum. At 2 they happen to be the same level so they exactly cancel. As you turn it up farther you are amplifying the early hum more and more. This is not at all unusual in amps.

[BStringThumper]

Hi Enzo, hope you are doing well. Thanks for your input. I guess I've been lucky I've never encountered this problem. I'll check everything around V1 again. I did find that the hum is quieter with the chassis out of the box and away from the speaker, but it still acts the same way. This amp was in pieces. I don't know how it sounded before I got it. Thanks, everyone, for the help.

[Enzo]

Turn volume to zero. Fix the hum you hear. THEN turn the volume up and chase down any hum that is there.

[BStringThumper]

Well.... I'm a little embarrassed. New V1 and V2 preamp tubes quieted it down to an acceptable level. It's not perfect. I had a collection of used tubes that I tried in that V1 position. The amp was quieter with V1 out so I concentrated there. Thanks for the input and sorry about the goose chase. Until next time. Cheers!!!

[g1]

Like Delta362 mentioned, the routing/dress of the ribbon cables is critical for hum in these amps. Also the rear panel with it's shielding must be in place and making electrical connection from shielding to chassis.

[Pedro Vecino]

Doesn't it have a ground connection to the chassis?

[Enzo]

Yes it should. ALso the rear cover has shielding too, so never hurts to put in place to see if that is part of the issue.

[BStringThumper]

--------------------------- FIXED--------------------------

I tried to get it a little quieter after my last post. I did install the factory jacks and green grounding wire. The rear shielding panel made quite the difference. The old preamp tubes were the biggest problem. It has new preamp tubes now.

Thanks for all the input and suggestions.