Old Ampeg reverberocket tremolo problem

[Dehn]

Hi All,

I have a R-12-rB reveberocket on my bench, and when the tremolo is used the speaker moves back in forth so much it makes a banging noise.
I've checked out the osc. circuit with a scope which seems to be fine. I've never had many problems with tremolos in the first place and most have been with Fender type circuits. It seems odd that no bulb and light resistor are used. The amps sounds normal until you use the tremolo effect.
I've done a bit of troubleshooting but have found no suspects, anybody got any idea's

[Jazz P Bass]

The LFO circuit is running at about 3 to 10 Hz.
And it has to go through a 270K resistor & then the 1K grid resistor before the output tube grid responds.

So first check that the amp is biased well.

It's also possible that the amp was 'modded' or miswired.

Check to see that the coupling caps (C28 & 29) are not leaking Vdc.

[g1]

Also try another speaker.

[Dehn]

Hello Jazz P Bass,

The amp was modded some time ago to use 6L6's instead of 7591's, but the customer says the tremolo worked fine after that.
C 28 and 29 are new orange drop caps. Most of the caps are new in this amp. I will check the bias, but the amp sounds fine until you use the tremolo.

Thanks Dehn

[Dehn]

Hi g1,

I did that, no difference

Thanks
Dehn

[Chuck H]

It's interesting that the 6L6's should be less sensitive to the trem shift than the 7591's. So what's causing this? As mentioned, check the bias, check the cathode circuit resistance, cap, etc. Check to see that C13 isn't leaking and causing a strange bias condition. I would even try paralleling C2 with another cap to see if that helped. Basically, the trem is causing a voltage modulation at the power tube grid that is being translated to the plate. That's what it does. Right now it's doing too much of it and a lot of the amps wattage is being expended in the low Hz range of the trem. Ideally this will be a smooth change in DC voltage that is relevant to it's relationship with the cathode voltage. So this, I think is where to look. The trem voltage and it's bias relationship and the AC decoupling for that power supply.