Amp techs! Just completed a plexi build, with one issue...

TheMagicEight · Jul 30, 2012

I've just completed a '69 plexi build that sounds great, save for a weird distortion on high notes when cranked. When I play high notes, there's an overtone that sounds like something scratching, akin maybe to a record needle. It's faint and dies off pretty quickly, letting the note sustain naturally, but of course I'd like to get rid of it. I wonder if this is a ground loop? I do have the output jacks grounded to the chassis and not part of the star ground, so maybe that could cause this? Or perhaps it's parasitic oscillation? Would that make sense, seeing as how it only happens when cranked?

Any insight is very much appreciated.

PlinytheWelder · Jul 30, 2012

This is a good read on grounding...
http://aikenamps.com/StarGround.html

steve_k · Jul 31, 2012

Unless I am reading your wrong, it sounds more like ghost noting to me. I always thought that came down to filtering. A bad ground is bad ground and would be audible at all levels.

mboogman · Jul 31, 2012

I'd say either filtering or parasitic oscillation, leaning more towards the oscillation. Make sure all your signal wires are as short as possible, especially the input into the first tube stage.

FourT6and2 · Jul 31, 2012

It does not seem like parasitic oscillation to me. P.O. sounds like uncontrollable, high-pitched feedback. Not sure what the issue is, though. Could be a symptom of a lot of things. I would head over to the Metro Amp Forum and ask there. My first guess, though, is ghosting. Which means you need to address the filtering. Could also be tied to your bias section or how you have the amp biased. Could be a lot of things, actually...

TheMagicEight · Jul 31, 2012

FourT6and2":336v7lf3 said:
It does not seem like parasitic oscillation to me. P.O. sounds like uncontrollable, high-pitched feedback. Not sure what the issue is, though. Could be a symptom of a lot of things. I would head over to the Metro Amp Forum and ask there. My first guess, though, is ghosting. Which means you need to address the filtering. Could also be tied to your bias section or how you have the amp biased. Could be a lot of things, actually...

Interesting that you hit on the bias section. I've got it biased properly - about 37mA at 470V or so - but the bias section design is from a Mesa Rectifier (using a separate PT tap specifically for bias). I'm going to try redoing that to Marshall spec and see what happens.

There are two things I can think of when it comes to filtering that I've done wrong. One is I've never used can caps before, and when tightening them down I might have overtightened one of them at first. I read somewhere else that a cracked cap might cause this type of issue. Mine isn't cracked on the outside, but maybe internally? The other is that I'm using 1/2 watt 10k resistors in the preamp while I'm waiting for 1 watt resistors to ship. Perhaps that could cause this? What else might you suggest checking for?

I've isolated the output jacks, so I seriously doubt it has to do with grounding at this point. I also swapped all tubes and went through the whole amp with a wooden spoon to see if there was a bad solder joint.

Thanks!

TheMagicEight · Jul 31, 2012

Well, I just looked at the schematic again and saw that I've reversed the white and red leads off the transformer. I assumed that the longer of the two (white) would correspond with the long gray wire on the secondary on the same side, but I guess not! Hopefully that fixes this, though I'm not sure if it should even matter. I suppose this will reverse the phase? Transformer operation is a little out of my scope at this point...

Edit: Took one more look and I do have it correct.

FourT6and2 · Jul 31, 2012

good luck! let us know what happens.

And seriously, head over the Metro forum.

TheMagicEight · Jul 31, 2012

FourT6and2":37mv1o4h said:
good luck! let us know what happens.

And seriously, head over the Metro forum.

Will let you know!

I actually posted to the Metro forum yesterday, but I guess I need a mod's approval before my post goes live (I'm a new member there).

TheMagicEight · Jul 31, 2012

Well, I've been playing around and have gotten nowhere fast. It's not the bias circuit (completely redone to Marshall spec), it's not the filter caps (swapped them all out), not the tubes. Going to try a couple more things and then if nothing works, record a clip to see if anyone's heard this before.

FourT6and2 · Jul 31, 2012

TheMagicEight":344mnabm said:
Well, I've been playing around and have gotten nowhere fast. It's not the bias circuit (completely redone to Marshall spec), it's not the filter caps (swapped them all out), not the tubes. Going to try a couple more things and then if nothing works, record a clip to see if anyone's heard this before.

Yeah, clips would help.