Oscillation in OD mode

[tomo]

I found that my amp oscillates in OD mode.(not with PAB)
It oscillates when I hit 5th or 6th string, not big oscillation, but can hear.
Probably 2Hz or so.
Not oscillate(or very very small oscillation) higher string.

This amp is non-HRM, funk's chassis.
I'm trying to solve it, but can not find the cause.
Please advise.

[Luthierwnc]

Hi Tomo;

Can you take a picture of the guts on the preamp side of the board? It could be lots of things but usually that is a signal wire too close to power wires.

Skip

[tomo]

Thank you Luthierwnc,
These are photos.
3 resistors of OD section are not soldered directly now.

same cuircuit as #124, but OD prenet trimmer is 100KB, Drive pot is 100kB.

[Luthierwnc]

I didn't see what I was looking for. My only other idea is to post a clip and let the guys give it a listen. sh

[ampdork]

Just some rambling thoughts.

PAB relay. I might try moving those wires about since you say it is with PAB that you have a problem or at least it is in that condition that the problem is revealed.
Maybe try moving yellow and green wires a bit? I seem to see them routed usually over the relay and around the other side. Maybe yours is coupling to something around the R/J switch?

I have never seen a dumble with the power tube grid stoppers mounted like in your amp. I personally don't care for that but also I kinda doubt it has any bearing on your problem. Just pointing out that dumble usually does it so that the leads are not routed behind the tube sockets. Instead he would fly the grid stoppers up, tie them with a bit of bussbar, and fly the grid leads up to the bar. Less wire length and no routing behind the tube sockets with this method. Again probably not related to your problem, just pointing it out.

Not seeing where you clean master wires are routed but again may not be anything.

The obvious things to check and eliminate are tubes and speakers. Make sure your issue is not a simple one like this. You could rebuild your amp a dozen times but if you have a bad tube or funky speaker the problem will remain no matter what you do. Best to take the time to eliminate the easy stuff first and work your way deeper.

Shots of the front panel wiring would be helpful too, perhaps there is a clue there.

[ampdork]

double post error...

[tubedogsmith]

Ampdork may be on to something with the grid wires from the power tubes. If you have them tied along the back try moving them to a shorter straighter route.

[tomo]

Thank you, everyone.
I'm sorry for delay, I do not have enough time to check.

1.replaced all tube --> no change
2.changed speaker --> no change
3.changed leaddress around PAB relay and tone pots --> no change
4.if DRIVE is less than 9 o'clock, even if LEVEL and MASTER are 5 o'clock(=full), no oscillation. (VOL=12 o'clock)
5.if DRIVE is more than 9 o'clock, even if LEVEL and MASTER are 8 o'clock, oscillation. (VOL=12 o'clock)
6.changed guitar from 335 to strat with EMG pickups --> oscillation became small.

The grid wire to power tubes of #124(and some other amps) are routed behind the tube sockets.
I did not change this grid wire route yet.

Problem is not solved yet.
Please advise.

[glasman]

Tomo, if you want, sent it to me. I will fix it free of charge.

Love watching you play at NAMM!!!!

Gary

[tomo]

Gary,
Thank you very much for your kindness.
But I live in outside US. (the other side of the earth!)

[dogears]

Gary, this a different Tomo. You were thinking Tomo Fujita??

Tomo, if you want, sent it to me. I will fix it free of charge.

Love watching you play at NAMM!!!!

Gary

[glasman]

Gary, this a different Tomo. You were thinking Tomo Fujita??

Tomo, if you want, sent it to me. I will fix it free of charge.

Love watching you play at NAMM!!!!

Gary

Yes Scott you are correct. I was thinking it was Mr. Fujita.

Tomo, is there any way you can make a recording on the sound? It might help in diagnosing the problem.

Gary

[glasman]

Ampdork may be on to something with the grid wires from the power tubes. If you have them tied along the back try moving them to a shorter straighter route.

I agree, the grid wires are tied together all the way back. Two other things. They are running in parallel with the speaker wire and probably also with the primary side.

I would disconnect them and route on a shorter path. Having them cross other wires at some oblique angle.

The grid stop placement is probably ok, the old fenders were built the same way. Of course don't put EL34's in the amp.

Gary