Univox 1221 Tremolo only works with FET "tube"

[glebert]

Amp working but no trem. Pretty much just like this thread: https://music-electronics-forum.com/...on-t-oscillate

Tried a bunch of different tubes, no change. Trem does work using a Jet City FET tube replacement. Seems like it may not have full range of speed. Would the leaky cap diagnosis fit with this? Wondering if the FET device has more gain than a typical tube.

TIA

[Jon Snell]

FET tube?
A spec sheet would be useful.
A Tube (or valve) is a vacuum thermionic device. A FET is not a vacuum thermionic device.

Please explain what a FET tube is and where you placed it.

[loudthud]

Strange arrangement on the footswitch. The sleeve is not grounded. The 12AU7 is just being used as a resistor to ground. Strange circuit.

[Helmholtz]

The tremolo uses 3 triodes in total. One for the LFO and the other two (12AU7) are used as a (bidirectional?) modulator.

Which one is the problem?

[glebert]

The LFO tube is the one that is the problem. I don't have specs on the FET tube, this is the description "Developed and Patented by Roberts RetroValves and built by Jet City, RetroValves are 100% analog vacuum tube replacement devices offering the warm, fat, punchy sound traditionally associated with only glass vacuum tubes. RetroValves are pin-for-pin compatible with 12AX7 tubes in nearly any guitar amplifier, offering instant modification of preamp gain characteristics." IIRC there are (were?) three different Retrovalve models with low, med, and high gain. I'm pretty sure this one is med or "standard."

[Helmholtz]

Did you follow Enzo's advice about repairing the LFO? Especially the cathode cap is critical.

Also does a real tube heat? The substitute might not need a heater voltage.

[Helmholtz]

Another question (not problem related): Is the right side grid of the 12AU7 really floating, i.e. not connected?

[glebert]

Did you follow Enzo's advice about repairing the LFO? Especially the cathode cap is critical.

Also does a real tube heat? The substitute might not need a heater voltage.

The reverb works on the amp and one of the triodes of the 12AX7 is used for reverb recovery so I know the tube is heating, but I will check and make sure both triodes are conducting. I haven't done the cap rework yet, the way the circuit board is wired in looks like it will be a PITA to lift to rework, so I want to have a plan before I go there.

[Helmholtz]

You may be right about the Fetron having somewhat more gain.

All oscillators work by positive feedback and depend on sufficient (loop) gain.
Leaky phase shifting caps lower gain as does a cathode cap not having enough capacitance.

If gain is too low, the circuit can't oscillate. There's a critical minimum gain.

[glebert]

as does a cathode cap not having enough capacitance.
.

This was a super useful bit of knowledge. I alligator clipped a 33uF cap from the LFO tube cathode to the chassis and I got oscillation with a garden variety tube. Unclip the cap, oscillation dies out, hook it up again and it starts oscillating again. The great thing about this is that it is a "fix" I can do without removing the circuit board. Don't worry, the alligator clips won't remain....

Oh, and both halves of the 12AU7 are wired up full parallel, so presumably it will then get 1/2 the impedance to ground for the off of the trem effect. It actually is a pretty great sounding trem. Not necessarily a gentle warble, more aggressive than that, but can go slow enough (about 1Hz) Enzo would have liked it.

I have a friend with an identical amp with the same problem, hopefully I can easily fix both.

Thanks for the help!

[Helmholtz]

Oh, and both halves of the 12AU7 are wired up full parallel, so presumably it will then get 1/2 the impedance to ground for the off of the trem effect.

But note that the triodes are wired anti-parallel. That's necessary because a tube can conduct only in one direction (like a diode).
So one of them conducts at positive signal halfwaves, the other at negative halfwaves.

[glebert]

So it is.

[Helmholtz]

The reverb works on the amp and one of the triodes of the 12AX7 is used for reverb recovery so I know the tube is heating,

Don't forget that each triode has its own filament.

[g1]

Don't forget that each triode has its own filament.

Good point. I have seen 12A*7 type tubes where only one side of filament is burnt out.

[Helmholtz]

I have seen 12A*7 type tubes where only one side of filament is burnt out.

Now if that happens, both triodes would be dead with 12.6V (series filaments) wiring but not so with 6.3V (parallel filaments) wiring.