Ampeg R-12 Tremolo problem

[olddawg]

I have a 1960 Ampeg Rocket R-12. The schematic seems to be the same as the R-12A: http://members.aol.com/portaflex/schems/r-12a.gif
After replacing th filter caps the tremolo is operational but seems a bit weak. I have replaced the .1uf cap to the intensity pot with no change. The strange thing is if I replace the 25uf electolytic to ground in the switch circuit the trem ceases to work at all. I do not believe the problem is with the three .05uf oscillator caps because it seems to be oscillating and the speed control works. The problem is the intensity is about 1/4 what it should be. Any suggestions? Thanks.

[52 Bill]

Weak tremolo in this style circuit can be caused by two things. Either the oscillator is weak or the fluctuating voltage is not getting to the output grids.

Turn the intensity control to 0 and measure the voltage at the top of the control where the 0.1 uF cap attaches. You should find a fluctuating voltage that varies with the speed control. If there is little or no voltage here then check the 0.05uF caps and all of the resisitors in the phase shift circuit. Replacing the 25 uF cathode cap should not shut off the oscillator, in fact it should make it stronger. Maybe a bad replacement cap?

If you find the fluctuating voltage is ok at the top of the intensity control then, slowly turn up the control and watch the voltage reading. Does the voltage change to a small but steady positive reading? If it does, check the 2 - 0.02uF phase inverter caps. If they become leaky, the dc voltage will override the tremolo signal and the depth will be effected. Also check the 2 -270K resistors while you're at it.

Hope this helps.

[olddawg]

So you are saying that the oscillator may not have sufficient amplitude? I know it is working and can vary the ferequency with the speed control and the amplitude with the intensity. I don't have any idea what the ideal amplitude should be. I guess the best thing to do at this point is just shotgun all of the caps. I probably should have done it first anyway. Not a big expense or problem in this amp.

[Enzo]

If your car has a dead cylinder, it will still drive down the road, accelerate, charge the battery and do pretty much everything it should, but it will run rough and be weak.

Your trem oscillator may be running, but it might be barely running and not strong enough to power the drive it needs.

The FIRST things I'd do would be replace the three feedback caps. I would not try to talk myself out of them being bad.

Scope the osc signal at the plate. or monitor with an AC voltmeter. Does the osc come right up promptly like it should? Or does it take a few seconds to build up? Does the osc weaken at the higher frequencies? Or if you spin the knob from one end to the other, does it kinda balk and take a second or two to settle into the new speed? Any if those symptoms points at weak caps.

[MWJB]

ditto what 52 Bill & Enzo say...but do you think that this tremolo is weak for this model of amp, or are you comparing it to another type?

I have found different 6SL7 can have an effect.

These trems can be quite sensitive to plate current in the output tubes, drifting cap values at the PI could have an effect...if, after following the suggestions so far, you still have no joy, try reducing plate current a little, maybe see if larger PI coupling caps help.

[olddawg]

Thanks for the quick responses. The intensity of the trem starts out stronger but reduces after a couple of seconds. I am going to replace the three .05uf oscillator caps and a fresh 25uf electrolytic. If this does not solve the problem, I'll fire up my scope and take a serious look at it. (It's a matter of clearing off all of the crap my wife and kid have piled on my bench, I'm sure some of you have been there) I've been using this amp for years. I finally replaced the 4X20uf filter cap when it stared to hum excessively. I still sounds great except for the weak trem. I guess I shouldn't be so lazy and do a complete cap job.