Fender AB763 - Tremolo Fades as Speed is Turned Up

[Bobwith2Bs]

Hello Everyone!

I am currently working on a 1966 Fender Super Reverb that has an issue I haven't encountered before.

As the speed on the tremolo is turned up past 5 it begins to disappear until there is only clean, non-modulated, signal at a value speed setting of 10.

I have tried a few different optocoupler "bugs", have replaced the cathode bypass caps in the oscillator circuit with new 25uF Mod caps (amp came in for a cap job so this was already done), tried replacing the 10M, 220K, and 100K ohm resistors that connect to one leg of the "bug" lamp in that part of the oscillator circuit, replaced the two 0.01 and single 0.02 uF caps in the oscillator, and tried a few different 12AX7s.

Seems like there isn't much more that can be done at this point other than trying to increase the gain of the first stage of the oscillator by adjusting the cathode bypass cap value but I am a bit stumped on this one.

Any ideas?

[sluckey]

Replace the oscillator tube's cathode resistor/cap (2700Ω and 25µF) with a 5mm red LED.

[Bobwith2Bs]

Oh interesting.

I was just reading about this mod on Merlin's site. So it is a matter of the bias cap not being able to keep up with the demand on the tube for the faster oscillation speeds?

[sluckey]

Using the LED will boost the gain of the LFO considerably. This increased gain will ensure the oscillator will not drop out at either extreme of the speed pot.

[Bobwith2Bs]

THANK YOU!

I can confirm that removing the cathode bypass cap and resistor (25uF / 2700 ohm) and replacing with a red LED has fixed my issue.

The tremolo is tremming quite nicely now with otherwise all original components reinstalled.

Appreciate the heads up on this very much Sluckey!

[solderhead]

Based upon your description it did sound like your lamp driver was weak. For future reference, there is an easy way to diagnose this problem without resorting to hanging parts and hoping for the best. Just take some voltage measurements.

To verify the integrity of the lamp driver's function you only need to check voltages on the cathode and plate.

Pin 8 on the lamp driver should have a DC value of 0 volts when the trem is off, and 13V when the trem is on. With the speed control at max (with no signal) you should be able to measure a drop of about 80V at Pin 6. Normal voltage would be 420VDC that reduces to 340 for about 1/5 of the duty cycle (when the lamp is on). If these conditions aren't met then your lamp driver is defective.