Problems - 30watt JTM45 kit

[Jalamaroz]

I have a question and a couple of observations.

In the picture below, what is the wire with the red arrows on it doing and what is it?

It's a nice tidy build. But that's not always the best layout. You really do need to keep any leads attached to grids as short as practical. Even if it means NOT looping them to the back or front into another wire bundle. I can't see where your NFB lead for the presence control is, but I think it's under the tone stack. Not a good place. You want that lead as far from any preamp leads and circuits as practical. Running leads on the face and rear panels and bundling looks tidy, but it shouldn't be done in lieu of good layout practices. Keep output leads away from preamp leads. The multi point ground you're using is ok, but it's best not to daisy chain grounds. You definitely want all the preamp grounds in the same place and separate from power amp grounds.

Even factory Marshalls have suffered oscillation problems. Veering away from the normal layout is really taking a chance unless any differences are planned to improve stability. You'll probably need to do a lot of poking and moving leads around. Perhaps even relocate or separate some grounds.

Also, I've used normal pots for bias circuits too. I don't do it anymore though. The quality of currently made panel pots is nothing you want to trust your bias to. Try to use multi turn cermet pots if you can. Or at least a very high reliability part that is clearly better than a typical panel mount pot.

Now, about that lead with the arrows on it?

Chuck, please forgive me but I can't see a lead with arrows on it....???
I did take a pic of the encircled general area. Plz see attached.

[Chuck H]

I blew up the portion of the image I put the arrows on. If you click on the image it will expand.

[pdf64]

There's what looks to be a piece of folded over tape with some writing on it, above the left arrow head that Chuck kindly added; what is it?

Marshalls seem to operate with a low margin of stability, eg http://music-electronics-forum.com/t40521/

[Jalamaroz]

I blew up the portion of the image I put the arrows on. If you click on the image it will expand.

Ohhhh I see now. That is a tap from the OT. My output selector uses the 4, 8 and 16ohm taps.

I wasn't sure what to do with the unused tap so I taped over it as a temp fix while I was testing my amp out. My long-term plan was to put heat shrink on it and zip tie it out of the way.

I'm sort of leaning towards rewiring the pots and tubes on the amp except make the wires shorter....etc

[Jalamaroz]

There's what looks to be a piece of folded over tape with some writing on it, above the left arrow head that Chuck kindly added; what is it?

Marshalls seem to operate with a low margin of stability, eg http://music-electronics-forum.com/t40521/

Hey, I tried a grid stop resistor on terminal 2 of v2. I used a 15k resistor because that was the only available resistor I had on hand close to 22k.
It didn't help the situation....I'm sort of considering replacing some of the wiring. Not all of it, just rewire things like the tubes and controls.....try to make my wires shorter and ran better...

[Chuck H]

Ohhhh I see now. That is a tap from the OT. My output selector uses the 4, 8 and 16ohm taps.

I wasn't sure what to do with the unused tap so I taped over it as a temp fix while I was testing my amp out. My long-term plan was to put heat shrink on it and zip tie it out of the way.

I'm sort of leaning towards rewiring the pots and tubes on the amp except make the wires shorter....etc

That lead, the presence lead and any long grid leads are almost certainly causing your problem. You really need to keep power amp leads away from the preamp. In fact, get them as far away as practical. Move that OT tap to the other side of the chassis and move the presence lead toward the power amp end instead of under the tone pots as it is. Then see where you are. Don't trouble with a lot of rewiring yet.

[pdf64]

Yes, that was about the worst place to tuck the 100V wire!
There will be a massive output signal on it, and it's right by the input stages!

A key point with lead dress is to minimise coupling between inputs and outputs that may be in the same polarity; the addition of gain to such coupling is likely to result in oscillation.
There's a lot of phase shift through an OT, especially via the 100V output which will have a lot of turns, so it doesn't take much coupling for there to be a frequency at which input and output are in the same polarity.

In the light of the above, I hope it's now clear how pertinent Chuck's point is, regarding the wire to the presence control being in too close a proximity to the wiring etc for other tone controls.

[Jalamaroz]

Yes, that was about the worst place to tuck the 100V wire!
There will be a massive output signal on it, and it's right by the input stages!

A key point with lead dress is to minimise coupling between inputs and outputs that may be in the same polarity; the addition of gain to such coupling is likely to result in oscillation.
There's a lot of phase shift through an OT, especially via the 100V output which will have a lot of turns, so it doesn't take much coupling for there to be a frequency at which input and output are in the same polarity.

In the light of the above, I hope it's now clear how pertinent Chuck's point is, regarding the wire to the presence control being in too close a proximity to the wiring etc for other tone controls.

Hey, this is in response to both (PDF and chuck) of your wiring advice.
First, I just want to say thank you. Both of you have helped me (a perfect stranger) tremendously with my amp. Some of the advice and tips are knowledge that experienced amp builders learn over time. Again, please know that I'm thankful for the help.
I'm gonna follow the advice you guys have given and will get back to you in a few days. I feel really good about this plan....

[Jalamaroz]

Yes, that was about the worst place to tuck the 100V wire!
There will be a massive output signal on it, and it's right by the input stages!

A key point with lead dress is to minimise coupling between inputs and outputs that may be in the same polarity; the addition of gain to such coupling is likely to result in oscillation.
There's a lot of phase shift through an OT, especially via the 100V output which will have a lot of turns, so it doesn't take much coupling for there to be a frequency at which input and output are in the same polarity.

In the light of the above, I hope it's now clear how pertinent Chuck's point is, regarding the wire to the presence control being in too close a proximity to the wiring etc for other tone controls.

(See pics) Hey guys, I took your advice, word for word: reroute presence lead, reroute OT unused lead, shortened most leads on V1/V2/V3.

That definitely fixed the problem. I was finally able to play through the high treble input. Also I can play through input 2 and adjust volume 1, without sound going away.

There is a little buzz but I think once I permantly solder all inputs and remove alligator clip jumpers, the amp will hum less.

What can I say, you guys have taught and helped me alot with this. I really do appreciate the help!

[Chuck H]

If the buzz continues after the pots are soldered to their proper, and probably shorter leads, don't forget the above advice on grounding. Buzz is very often from having preamp and power supply grounds sharing resistance via the chassis or some daisy chained connection. Best to keep preamp grounds separate and all grounded near the input with power supply grounds nearer the AC plug. One ambiguous issue would be the preamp power supply filters. Which should be grounded with the rest of the preamp BUT on their own, unshared leads. Also, and especially on new build with "new" tubes, you can reasonably suspect a preamp tube might be buzzy or hummy right out of the box. So after hooking up the pots, if the buzz is still there, try a known quiet preamp tube in each position first. If that doesn't fix it you can chase grounding issues