first time builder champ problem

[cmill24]

I just finished my first amp build a fender champ kit from tube depot with a vintage output transformer from mojo musical supply. So i start her up for the first time and the thing sounds great.....until I get the volume to 10 from 10-12 there is a high pitched squealing. if I start turning the volume up without a guitar plugged in it does the same thing at this volume but not as loud. anything below 10 still sounds good. Any ideas what this could be? as above the output transformer is a VOS one that the labeling with it says is reversed and the read lead goes to the output tube plate and blue to the B+ could it be something with this or something else? also there is a small pot that says feedback under it that i put on the circuit board. is this a bias pot? or something to do with negative feedback?
any help is greatly appreciated as I think I am hooked on the whole building aspect.

[mitch]

Hey cmill - I'm pretty much a novice at this myself so you may want to wait for a more informed poster but try swapping your OT primary's. Red to B+/blue to pin 3 of your 6V6.
The trim pot sounds like it's to adjust the neg. feedback and not a bias pot. This feeds and inverted signal back through the amp to help reduce hum. You could also try just disconnecting the NFB and if the squealing goes away it probably means your OT primarys are reversed.

[Bruce / Mission Amps]

Yeah I agree, probably a variable negative feedback pot and that pot wiring is a very likely thing to be creating this issue.
See if moving around and tucking it in the wiring of the pot changes the way the squeal happens or the frequency of the squeal.

[cmill24]

so i tried reversing the red and blue from the OT to the power tube plate but that did not fix it. the small pot is definitely a feedback control, if i turn it all the way up I get a squeal that is on all the time which goes away with turning it down. Still have the problem with as soon as the vol gets to 11-12 it is very loud scratchy, squealy, and poppy. just below this it is fine. could it be a problem with the volume/power switch potentiometer that the contacts are bad when it slides into this range? I have switched tubes in the amp and that didn't change anything. I would rather not start pulling everything and rewiring everything to figure it out. Tried squirting some deoxit into the potentiometer but that didn't fix it either.

[Bruce / Mission Amps]

...Still have the problem with as soon as the vol gets to 11-12 it is very loud scratchy, squealy, and poppy. just below this it is fine. ...I would rather not start pulling everything and rewiring everything to figure it out. Tried squirting some deoxit into the potentiometer but that didn't fix it either.

Well, old man, we can't help you if you won't follow a good lead.
You need to at least take the NFB wire off the speaker jack to see if it is the NFB circuit.

Which kind of Champ is this?
A 1meg pot is too high for this circuit. I'd switch that to a 25K-50K pot myself.

[cmill24]

I also tried checking the resistance on the potentiometer to see if that was bad. It is supposed to be a 1M ohm pot and is reading out 0.95 Mohms. is that enough off spec that it would cause this prob.?

[cmill24]

just tried taking off the NFB wire from the speaker jack. Didnt change anything. still sounds great until 11-12 on the knob then sound gets super distorted and scratchy noises as you twist the pot.

[cmill24]

the model of champ is a champ 5F1 on the schematics. It has a 1 M pot on the schem. as well.

[mitch]

You're talking about the vol pot being 1M right? Not the NFB pot being 1M I assume.
There are a few things that I know that can cause this (and probably more I don't know) as I just had some of the same problems myself on a 5f1. My problems turned out to be a combination of grounding and lead dress.
Maybe post a gut shot for someone to look at.

[cmill24]

yep its the power/vol pot that is 1M not the NFB. How do you fix the lead dress?

[mitch]

Well I don't think the vol pot reading .95M is a problem. Your amp kit didn't come with any guide how to route wiring? Keep your hi volt wiring away from your signal wiring. Keep your heater wiring down tight against the chassis. Keep your ground wiring tight against the other side of the chassis etc,etc.
Use a chopstick and with the amp on, push wires around to try to locate the issue. DON'T STICK YOUR HANDS IN THERE WHILE THE AMP IS ON. Make sure you discharge the caps before working on it even while unplugged.

[Bruce / Mission Amps]

the model of champ is a champ 5F1 on the schematics. It has a 1 M pot on the schem. as well.

Ah.. OK I thought you meant the NFB pot was a 1 meg pot.
A 1meg audio volume pot is always somewhere around 800K to +1Meg so you are fine.
I does sound like it could be a localized "sloppy wiring" lead dress induced feedback issue though causing an oscillation.
Yes, a bad pot can do that too.
If you feel up to it, take a couple digital shots of the wiring and maybe something will jump at at someone here.

[JHow]

Are you using shielded wire on the leads to the 12ax7 grids? I have built several 5F1 and this has helped eliminate squeal. I typically only ground one end of the shielded wire. Also, as mentioned above, chopsticking will help. Also, I usually run the 68K resistors off the board.

[cmill24]

heres some pics. I ordered a new potentiometer for the vol/power to see if that is the problem.

[cmill24]

ps I have the NFB wire from the speaker jack disconnected incase you see a dangling wire.