Deluxe Reverb - weird vibrato oscillator voltages + choppy trem

[waspclothes]

Hey guys,
I have a Deluxe Reverb (silver face, converted to AB764 with the original 5AR4 still in place). I measured all of the voltages to compare to the fender schematic and nearly all of them were 10-20% higher, but the voltages on the vibrato section are -way- off.

All the other voltages being 10-20% higher, I'm guessing that's maybe to do with higher wall voltages (Around 125VAC here)? But the vibrato voltages I don't understand (392V at one plate instead of 270, and 443 at the other instead of 345. At the cathodes I'm only reading +0.1V as well instead of 2.1 and 17.0.

Attached is the schematic with all of the voltages.



Also attached is a scope shot of the amp going into an 8 ohm dummy load. The blue trace is the amp's output with a 440 Hz test tone, and the yellow trace is the vibrato oscillator (tapped off of the speed pot). Speed is low and intensity is at max. I can't help notice that the tremolo isn't smooth, it is mostly at max volume, with just a very quick dip in volume and an even quicker rise back to full volume. The oscillator itself looks to be pretty smooth, and reasonably sine-wave-ish.



FWIW, the amp is biased at 68% idle dissipation, power tubes are matched and an extra capacitor has been tacked onto the LDR as the standard Fender service bulletin "tick fix".

[TimmyP1955]

My guess is that both(vibrato tube) cathodes have lost their ground, and thus aren't drawing any current.

What's the voltage at B?

[waspclothes]

My guess is that both(vibrato tube) cathodes have lost their ground, and thus aren't drawing any current.

What's the voltage at B?

My mistake! Looks like those two voltages at the vibrato oscillator tube cathodes are only present when the Vibrato is turned on!

[Chuck H]

Yup. You need to check the vibe tube's voltages when it's NOT oscillating since a cutoff condition eliminates current and you end up with a net reading of the conduction and cutoff states.

Regarding the overall high voltages... Some is due to higher AC mains, but unless the transformers were changed you need to use a 5u4 rectifier since that is what the SF amp was designed with. Changing the circuit to BF doesn't make the amp qualify for BF voltages. That depends on the transformers. And on that note...

Oscillator circuits are more sensitive about gain/voltage than signal amplifier stages to operate properly. It may get better if you bring the voltages down. Otherwise, it's not uncommon for old vibe circuits to need tuning up because the oscillator and it's interaction with the circuit are not only sensitive to specific voltages, but also component values. And the crappy old caps in those amps are known to get hinky as well as the notorious drift of CC resistors. Check for drifted component values and cap leakage in the circuit.