JTM45 Clone - distorting heavily

[fdesalvo]

Hello, all!

I'm troubleshooting a JTM45 clone that's distorting heavily with the volume from 2-onward. The distortion sounds like a biasing issue, where the sound is very spitty and fuzz-like, with a quick drop to clean once the note begins to fade. Maybe a bit of upper octave in it. There's very little background noise - just a touch of some 120hz. Voltages line up with this chart @+/- 10%. Please note this chart is not my amp, but of a healthy clone with similar iron. On this particular amp, the PI grids are: pin 2 @ 29vdc, pin 7 @ 32vdc.



I've tried all new tubes in addition to validating all component values. Maybe I've made an error here. No caffeine. TIA for any guidance!

[Helmholtz]

Table says V5 pin 3: 3.147Vac. As pin 3 is the plate you should have about 400Vdc there.

Readings on LTPI grids will always be too low and depending on meter resistance.
Real grid voltage should the same as the voltage at the lower end of the 470R bias resistor (upper end connects to cathodes).

Did the amp ever work fine?

Do you have a scope?

Please post schematic.

[fdesalvo]

Table says V5 pin 3: 3.147Vac. As pin 3 is the plate you should have about 400Vdc there.

Readings on LTPI grids will always be too low and depending on meter resistance.
Real grid voltage should the same as the voltage at the lower end of the 470R bias resistor (upper end connects to cathodes).

Did the amp ever work fine?

Do you have a scope?

Please post schematic.

I should have been more clear in that the chart isnt this specific amp, but a similar clone using he same iron. When I received the amp, it wasn't working. There was massive oscillating and buzz. Now it's stable with very little background noise, but the amp distorts with the smallest of input signals.

[Helmholtz]

So what is V5 pin 3 voltage?

Scope?

Schematic?

Try the amp with presence fully up.

[g1]

The symptoms sound like what you often get from oscillation. Have you tried flipping phase of OT primary?
(which may be what Helmholtz was aiming at with turning the presence up)

[Helmholtz]

Yes, I thought of oscillation but with correctly wired OT.
Some JTM 45s like to oscillate at low presence settings with frequency depending on the OT (may vary between 40kHz and 200kHz).
Reason is the extremely high amount of global NFB used in these amps.
Turning up presence lowers NFB at HF and typically stops the oscillation.

Is this the amp?
http://valvestorm.com/sites/default/...5_KIT_V2.1.pdf

[fdesalvo]

Apologies. Lots of fam action. Plates are at 400v. No audible oscillation. Amp arrived with the ot primaries reversed and correcting it resolved the low level oscillation. Presence control isn’t impacting the issue unfortunately.

The kit is very similar to that, if not identical, but it’s sitting in a chassis punched from a dumble clone, so it’s got one input jack instead of 4.

Edit! Was just told the OT is a diff model than I was led to believe. There are no labels or markings visible - according to the diagram I’ve found, the OT’s ct is currently tied to a plate and the plate lead is tied to the B+!

TY for the suggestions. Will update tomorrow am!

[nosaj]

Known good speaker?
nosaj

[The Dude]

......according to the diagram I've found, the OT's ct is currently tied to a plate and the plate lead is tied to the B+!......

Well that'll certainly be a kink in the iron. To be sure, I'd use your DVM to confirm which wire is the CT. Since it's not the specified part, don't trust diagrams and wire colors. Trust your meter.

[fdesalvo]

Thanks guys ^. Done and done. She sounds beautiful.

[nosaj]

Thanks guys ^. Done and done. She sounds beautiful.

So speaker was good or you used another one?
nosaj

[fdesalvo]

Only outstanding issue was that miswired ot. My bench test speaker rig is a 412 cab loaded with gk12-100’s haha.