Fender AB763 - Tremolo Fades as Speed is Turned Up

[Robert Technology]

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?

[Robert Technology]

[glebert]

When you say it "begins to disappear" are you talking about the effect or the oscillation itself? Have you probed to see what is going on with the LFO waveform?

[Robert Technology]

I haven’t probed it with a scope but measuring voltage at the lamp side of the optocoupler the voltage oscillates at speeds lower than five but stays steady at ten on the speed knob.

Not only does the tremolo effect audibly disappear but the oscillator seems to quit oscillating.

[glebert]

I would focus on the oscillator side of circuit first. Maybe even disconnect the lamp driver from the oscillator until you know that is working well.

[Pedro Vecino]

Have you checked the 3M (speed) potentiometer and the attached resistor?

[Justin Thomas]

Funny, I've had this happen to every Fender I've slowed the tremolo speed on by putting in larger caps. It always slowed the slow really well but if I wound the speed up to 7 or more the effect would go away.

Amps were a 61 Concert, 65 Showman, & my 73 Vibrolux Reverb. Don't recall it happening with my 79 Vibro-Champ but that was a long time ago that I owned that amp.

Following with interest.

Jusrin

[Mick Bailey]

I wouldn't increase the gain. It may resolve the problem , but won't necessarily fix what's wrong.

Imagine you have an older car with a carb and its leaking air from a failed gasket, making it run lean. You could resolve it by altering the mixture screws or changing jets to richen things up, and it could run fine afterwards. It hasn't fixed the problem though.

When the oscillator stops, does it resume right away when you turn the control back below 5?

Also, does the trem kick in right away when switched on/off, with the control turned to the point just below where it usually stops?

[Robert Technology]

I have checked the 3M pot and associated 100K resistors (the 100k measures around 120K though i would think that would help resolve this issue more than hurt it). I have continuity checked all other oscillator connections to ground and their associated components.

Oscillator starts right up at amp power on and does resume immediately when the knob is turned back down to the midway point.

After reading an article on Merlin's site about "Tremolo Oscillators" I am starting to be convinced that at the higher speed setting the RC combination of the 100K resistor at the pot and the first 0.01uF cap is creating too short of a time constant causing the tremolo to speed up to the point that the effect is no longer audible. I don't know why this would be the case with stock values... i have a 1970 Super Reverb that is all stock in the trem circuit and it wobbles just fine!

Going to try a few more things this morning and will pull the output of the actual oscillator and get it on the scope so that i can actually see what is going on.

[Helmholtz]

I might be an LFO gain issue.
Are cathode and plate DC voltages good?.
Did you try a new tube?

Also new caps aren't necessarily good caps. Could be leaky.

[Robert Technology]

I have tried a couple of new tubes but all were the same model of JJ 12AX7s.

Voltages on the oscillator side of the tube have been fine. They were a little high with one of the "bugs" installed but not on the oscillator side of the tube, only at the lamp connection of the optocoupler "bug" (closer to 440 than the listed 390V - This amp has one of those solid state plug in rectifiers so all voltages are about 30V above listed value, bias has been dialed in correctly though).

[Robert Technology]

Okay, that same article on the Merlin Blencowe website, and a member of another forum, have pointed me towards replacing the oscillator's cathode resistor and cap (25uF/2700 ohm) with a red LED to boost the gain of the oscillator tube. Going to try that on my lunch break and will report back in case it helps anyone else in the future.

[Robert Technology]

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.

Hope this helps anyone else that may encounter this issue in the future!

[g1]

I wonder if the higher B+ with the rectifier conversion created the problem. It must have worked properly at some point.

[Helmholtz]

"sluckey" over there gives good advice. And of course Merlin: https://www.valvewizard.co.uk/trem1.html is always a good source.

I don't think the problem was due to increased B+, which actually should somewhat increase gain.
Still suspect a poor (e.g. high ESR) cathode bypass cap.