fender ab763 tremolo troubles

[alexc]

I did a super reverb ab763 build a few years back - it all came out great and sounds a treat.
It has a genuine reissue vibrato/reverb footswitch.

A while ago, the tremolo began to 'tick' and 'thump' quite strongly at all slower speeds and fade away at higher speeds.
MF! 

I include a grab of the circuit below.

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So I did the reading on the issues and set to rework it - I did the usual things :

- re 'dressed' the wiring
- added the 0.01uF to 0.022uF 630V poly cap from the tremolo oscillator gain stage plate/neon junction to ground
- tried same cap across the neon
- tried different ocillator/gain stage AX7 tubes
- tried different oscillator caps

After all that, the only things that helped really was adding the cap and/or re-dressing wires.
That improved things but the noisy tick/thump still very far from fixed.

Changing the caps made no difference to the oscillator 'die out' at higher speeds.
I also slowed the oscillator down by changing the 0.01, 0.01, 0.02uF oscillator caps to 0.02, 0.01, 0.02uF.
That worked well but even more 'die out' happening at higher speeds.

OK - so time to get radical.

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I removed the optocoupler bug, opened it up and replaced the neon with a led.
I tried a few leds - yellow, red, white etc. Verified it lit up and oscillated.
Ticking and thumping virtually gone.

I found the high-brightness white leds worked best, so I sanded the lens flat, polished it a little with wet-dry and re-shrinked up the opto assembly then installed.

I could still see the lighting up at the back end of the opto, so I could visually check how well it was working.

OK - on the right track.

I noticed it would still die off very quickly and was a little anemic in general.
I removed the 10M resistor from the plate circuit driving the opto and that did the trick.
Tried some other experimentation but nothing else significant.

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So final mods :

1.replace neon with high-bright white led
2. remove 10M
3. changed one of the two 0.01uF oscillator caps with 0.02uF


A nice, strong tremolo signal almost fully silent.

Fantastic - finally the thing is sounding like I wanted.
Fullsome intensity, good, steady rate control at lower speeds. No die off at higher speeds.

So I closed her up, put back in the room and tested -  all good and quiet, sounding great.
Even with the reverb up (pos '2') and the intensity at max.

BUT AGAIN higher speed temolo dies away.  MF! MF!

I think I must have done some more wire dressing for lowest noise after I verified the fast oscillator rate before I closed her up and screwed up the fast setting. Very sensitive is the positioning of the blue wire to the 'intensity' pot.

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So, as far I can tell, it seems like wire dress and possibly oscillator cap selection is the critical thing in oscillator die off.

Anyone have any tips on how to improve the oscillator function ??

Thanks!

 

[alexc]

Just a note about the oscillator gain stage with the 10M removed and neon replaced with white led :

- I found the cathode voltage was around spec at 17V or so on the 100K cathode resistor when the tremolo was running strongly in the lower speed range. This suggests some 0.17mA current.

- The bias would reduce to around 5V when tremolo was dying off at higher speeds and the led would stay faintly lit.
  When speed reduced, the oscillator would fire up again strongly after a few seconds

- B+ was around 467V at the 100K feeding the led and around 450V or so at the plate. Obviously these things were moving around with the oscillation.


The LED was lighting up strongly but nowhere near it's limits. It can take 15mA or so with no probs and gets very bright. This was being used at quite a low current.

As is often the case, there is more going on in this simple circuit I thought! For example, changing the 100K resistor made surprisingly little difference as did changing the cathode resistance.

I need to think about it a bit more

 

[PRR]

The tube is critical. Not all 12AX7 have enough excess gain to work when the rate pot is turned down as far as 100K.

 

[alexc]

That makes sense - the more gain, the better in a feedback phase shift oscillator circuit.

I did try a bunch and settled on the one which was quietest from the tick/thump point of view, which was an old china tube so that is probably at odds with the requirement for higher gain.

I'll try some more.

Thanks

 

[SteveF]

I've found increasing the cathode bypass cap to help in some instances.