Tremolux Amp repair FINISHED

[xtian]

I am rejuvenating a '65 Tremolux Amp AB763. I have replaced all e-caps and nearly two dozen bad parts that previous modders had installed.

Voltages are now very good, tremolo is working well, and amp sounds pretty good. Except for an ugly "fizz" distortion that shouldn't be there.

I'm getting only about 10v p-p output before distortion (1KHz sine at 1v p-p input). And the distortion is really ugly (see photo).

This issue is in both channels, so I'm focussing on the PI. Not sure what to try next.

Could this be an issue with the extremely poor lead dress of the wires (marked X and Y on the layout) that jump from the two channels to the 220K mixer resistors?

[Firestorm]

You've got to go to the true and proper Fender lead dress. Inputs on both channels and especially the way the power supply leads are dressed behind the Vib. Channel. Even then, the Vib. Channel tone leads sometimes have to be shielded.

[Firestorm]

Double (dammit). But as long as I'm here again, Fender lead dress isn't perfect, but it worked the way the amps were intended. If you restore one, you have to do it just the way they did. Or improve it.

[xtian]

I improved this monster 90% by replacing some important runs with shielded cable and otherwise improving lead dress. Discovered a bad Treble pot and replaced it. Still have a little, annoying fizz, that begins and ends abruptly, not like the easy onset of tube distortion. Can't send it out like this.

Here's a list of what it's not.

Not tubes. Tried two sets, swapped positions, with and without tremolo tube, with and without V1/V2 depending on channel used.

Not a grounding issue. Retouched every ground point. Can't influence fizz by chopsticking.

The fact that it's in both channels made me think it was the PI. But I replaced the PI plate resistors (100K/82K), replaced the 0.1 plate coupling caps (temporarily, then put the blue ones back), no changes.

Again, all voltages look good.

Hawlp.

[Gibsonman63]

A shot in the dark.

Carbon traces on the power tube sockets from previous tube failure due to overmodding?

[Firestorm]

I've heard that sound, but only on the Vib. Channel. Both? I don't know. On the Vib. Channel, I've solved it with a 3-conductor shielded cable for the tone leads. The power leads back and forth to the doghouse are often problematic. Twist. Maybe even wrap them Silverface style.

[jjman]

I resolved a similar issue on my '71 DR with shielded wires to the grid stoppers on the power tubes. I had removed the grid-to-ground caps there and the noise developed after some years.

[fredfretter]

According to Fender layout, output transformer leads (brown and blue wires)
seem to be reversed.

[Randall]

I'd inject a sine wave in various locations such as the grid of the PI and scope around. Does it distort on the plates of the outputs? How about the plates of the PI?

[telenut62]

Check the 10K's in the B+?

[xtian]

Thanks, everyone for your suggestions! I'm back from vacation and digging out. Will get to this in a few days.

[Structo]

Well Andy has done a lot of work in this area.
He says especially older Fender amps are vulnerable.

If the screen voltage is not below the plate voltage it will make the output fizzy.

Simple enough to try, solder a resistor (1K) in series before the two screen resistors.

BTW, are those screen resistors on the power tube sockets truly 470R?

Early Marshalls had what they call a floating resistor the drops some voltage before the screen resistors.

[FUCHSAUDIO]

add 50 or 100 pf across the PI ?

[xtian]

add 50 or 100 pf across the PI ?

Already did this one. Darker, but same fizz. Thanks!

Tom, your screen voltage idea sounds very worthwhile. I did notice that the plates and screens are at the same voltage. I'll let you know!

[xtian]

According to Fender layout, output transformer leads (brown and blue wires)
seem to be reversed.

Thanks. I did think of this, but I disabled the NFB, and it made no difference with the fizz problem.