JTM45 Build

[Littlewyan]

Unfortunately I won't have access to a scope until later this week as I blew mine up!

Anyway few things to try in the meantime......

1. Double checking the bias using the transformer shunt method. Dangerous but my meter isn't exact enough to measure the voltage drop across the OT secondary windings.

2. Checking all voltages again to ensure there is no leakage anywhere.

3. Attaching a jack to some shielded wire with a grid stopper on the end to solder in temporarily to see if this solves the issue.

4. Moving the NFB wire so it goes behind the V4 socket and then down to the board instead of following the OT Secondary's and then going across the board.

5. Upping the PI Fizz Cap to 100pF. I did disconnect this cap to see if it was causing an issue but I am wondering now if perhaps its open and not doing anything. Hence the oscillation.

I'm also concerned that I've broken a wire on the board as I bent them round when I soldered them like George says to in his 50W guide, but some got twisted when I put the board into the amp . Everything checks out ok with the meter though.

[lovetone]

I think without a scope your trying to work blind because you cant see whats going on, your not going to find a fault like this with a volt meter...

I would wait for the scope.

[Littlewyan]

I probably won't have access to the scope until the weekend. HOWEVER in the mean time I did try a 100pF Cap across the PI anodes (as well as the 47pF Cap), across V2a Anode and across V1b anode and it didn't make a difference at all. Then I discovered another symptom, something that I probably should have noticed earlier.

When I turn on the amp it makes this noise. Even with the volume controls set to 0 and with V1 and V2 pulled, but stops if I pull V3. I can set every control to 0 except Treble and it will do it. However if I turn the treble down to 0 as well it doesn't do it. But if I turn the treble up to even just 1 it will happen. Also when its happening if I quickly turn the treble down to 0 it makes it sound slightly different but doesn't stop it.

I should also add that the noise stops by itself.

[Reeltarded]

Welp that is a weird one..

This is purely stupid, but does the amp do that with no tubes installed, like acoustically, as when a transformer sings?

[Littlewyan]

If I remove V3 its dead silent.

[Reeltarded]

That PI has caused us lots of thought. That kink in the image you posted... screaming..

What's the chance of that dump producing octaves of 100Hz? Tiny speaker here..

[Reeltarded]

290.6Hz

[Littlewyan]

I could try another filter cap for the PI. I used a 16uF+16uF Cap and bridged the two halves together. I'm sure I've got a 22uF or a 47uF Cap in my drawer that I can try.

[Reeltarded]

edited to nothing because had a better message. Lol

[Reeltarded]

Hey wait a second just unbridge the cap.

[Littlewyan]

Same issue. Trying a different cap altogether.

Edit: Dammit, same issue.

[Reeltarded]

I can't find any fault in your pictures, but I do have a question about filtering.

In my 50 that is similar I have less nodes than you. In my 45 similar to you, I also have 1 less node. Where is this schematic!

I confused myself once and ended up with a hot running tranny for a reason I couldn't figure out til when I gutted part of that amp for a redo of some things and found a fault bewteen the last two nodes. I can't remember what I had done there, but it induced a harmonic thing where it wasn't 60-120Hz here.. It was higher, like 3x maybe..

I just can't remember...

[Littlewyan]

The filtering is 32uf mains - Choke - 32uf screens - 8.2K + 10K - 16uF + 16uF PI - 10K - 32uF V2 - 10K - 32uF V1.

I'm wondering if its a ground loop or something.

[guitarmike2107]

Hi Ryan, I think this is all grounding issue you have. that amp should be dead quite with V1 and V2 removed

Firstly
Have you run a ground wire from the speaker ground to the PI ground? or where have you run it to exactly? or have you not run a ground wire back to the PI and the OT secondary is floating?

Secondly
I assume you have the presence control ground to the buss bar? I would remove that connection and run a wire directly to the filter cap ground that feeds the PI
Right now you have the cap can grounded through the chassis and are hoping that the buss bar connects to that without picking up any issues, so forget that idea and run a wire direct.

[guitarmike2107]

That is to say both the presence cap and the grounded lug on the presence pot should be connected directly to that filter cap ground

Also I see you have grounded the OT output to the chassis, I would run a wire over to that filter cap to to be sure, take all the guessing out of it.