Amp squeals when tone knob turned all the way down

[sting7777]

I was re-vamping the ground scheme on an amp that TJ built for me (pro jr. to 18 watt conversion).

There has always been a bit of hum, so I started moving the ground buss to a multi star grounding scheme (with an isolated input jack) per the aiken amps guidelines. Basically the pre-amp grounds tied to a star, the power stuff tied to a star, both stars tied back to a common PT bolt chassis connection, and the AC ground has its own very short run to the chassis.

It is a bit quieter now (still a bit of hum so I am going to replace some more signal leads with shielded cable).

When I was going through for a final check making sure the tone and volume worked as expected, I turned the tone knob to 0 and got the LOUDEST noise - sort of a squeal but more like a tone generator/snyth noise. It varies with the volume and causes the dog to shake like a leaf and hide behind the couch. This is without the guitar plugged in.

Any ideas?

[sting7777]

Some more info...

With a guitar plugged in, I don't get a squeal - but I get crazy ground noise. The odd thing is that when I play my strat in position 2 with my hands on the strings - it goes away. Any other position - I can hear the guitar but the ground noise is really bad. Position 2 is just as noisy if my hands aren't on the strings - touching the bridge doesnt help - it has to be my left hand muting the strings... very weird.

BTW - this guitar does not have a RWRP middle pickup...

WTF?!

[sting7777]

OK - so I just went a did a true star ground with every ground having its own home run to a main ground point (PT chassis bolt).

AC cord still has its own connection.

Both the input and speaker jacks are isolated.

The amp is very quiet now - but with no guitar plugged in - still does its squeal thing - maybe this is the case with all amps and I have just never done this before - seems odd though.

With the guitar plugged in - the tone control works as expected now - no extra ground noise or weirdness when turned all the way down.

Any ideas on the squeal thing?

thanks in advance...

[Offshore Angler]

"Doctor, it hurts when I do this..."

[Cygnus X1]

"Doctor, it hurts when I do this..."

Who plays an amp with the tone all the way down anyways?


Glad you found the issue.

[NTBluesGuitar]

I'm a bit late on this one. I would have suggested that you first run the filter cap grounds to the lower right PT bolt, leaving the preamp-buss on the upper right bolt (or somewhere separate from the filter cap grounds).

I think I've had an amp do the same thing with the tone cap...might have been a 5E3-type. Never needed to troubleshoot it, though, but when you have it squealing, try a little probing with a chop stick. Might be as simple as wires needing new routing.

[sting7777]

Well - with the true star ground - the amp is super quiet - just a bit of transformer hum left. I re-did some of the thin teflon signal wiring (sorry about that TJ - didnt know whaqt I was buying when I gave you that stuff) with some larger gauge wire - and now when I chopstick those signal leads - no more noise.

I'll try the tone knob thing again later today when the dog is outside and see if that cleared up as well. Never heard anything like it.

[sting7777]

Who plays an amp with the tone all the way down anyways?


Glad you found the issue.

Yeah - I know - but it makes me nervous that something isn't wired quite right. Imagine if that sound came on while playing - people would think the bar was being invaded by aliens. Actually - it just came to me - you know that sound that Tarantino plays during Kill Bill whenever Kiddo gets angry - the sort of synth siren sound - thats what my amp can do now...

[NTBluesGuitar]

It's an oscillation...a similar thing happens when an OT is wired with polarity the wrong way. It's happened to me before, and there's nothing like that piercing banshee scream hitting you when you didn't expect it.

[sting7777]

Problem solved. I added a 2W 10K grid resistor to V1 (pins 2 and 7)

Noise is 100% gone.

NICE! (and I'm glad OSA isn't my primary care physician)