Reverb?!?

[ElectronAvalanche]

Hi gang,

after working on my second Slo-Clone I finally got back to my 75W ODS with Reverb.

I still have one minor (major?) problem with the Reverb section.

The driver (12AT7) gets distorted by the clean/od quite easily.

The ckt is as follows:

Preamp relay to Preamp out Power Amp in jacks.

From Power Amp In to 500pf Reverb input cap and 100k mixing resistor.

250k Dwell trimmer to grid of Reverb driver.

Rest is as Gary´s or BobI´s ckt.

Even if the Reverb tank unplugged I get distorted sound riding on top of the signal. Turning down the Dwell gets rid of this.

The B+ for the Reverb driver is derived as follows:

Screen B+ to 5k6 to 50uF cap (B+ for CF post PI), to 5k6/50uF (Reverb driver node) to 2k2/16uF Reverb return section.

Voltages are where they should be for a Fender style driver.

Grounds:

All grounds from the Reverb driver are going directly to the corresponding filter ground.

Turning down the Reverb recovery level control does not change the distortion sound, only turning down the signal level to the driver will.

If I slave some other signal into the Power Amp, I get undistorted signal. Even when putting in quite some large signal (headphone out of a Behringer mixer board all the way up).

Any ideas? I am close to ripping out the Reverb altogether. Maybe installing the FX loop.

Thanks in advance.

[glasman]

Even if the Reverb tank unplugged I get distorted sound riding on top of the signal. Turning down the Dwell gets rid of this.

Sounds almost like it is modulating the power supply lines or there is a ground loop. If available, I would hand a scope on the B+ of the reverb driver and see if there is some garbage getting back into the supply. A filter cap just before the reverb tranny might help.

I normally drive my reverb directly from the screen supply with no adverse effects. Ala Leo...

Gary

[ElectronAvalanche]

Hi Gary,

thanks for your answer.

Well I took the Reverb driver B+ node and put it directly to the screen grid (5k6 dropping resistor 50uF filter cap) and still had the problem.

Maybe it is indeed the grounds???

I even changed the layout and put the 500pF input cap further away from other caps and this did not solve the problem.

Maybe it is time to get the scope out?

Any other ideas?

Thanks in advance,

Dominik

[FUCHSAUDIO]

Do you have a 470-Pf cap from plate to cathode on the reverb driver tube ? Sometimes the reverb driver circuit (especially) can oscillate on transients.

[ElectronAvalanche]

Hi Andy,

thanks for your answer.

Yes I installed this cap, the problem is still persisting. The only real problem I can not adress is that I had to install the Reverb driver tube under the board. It under the board close to the PI section.

Maybe I will pull the whole Reverb section. I do not seem to dig the Tube Reverb tone that much anyway.

Thanks again,

Dominik

[swt]

I had a problem like that one once. It was an oscillation. Check ps, layout, and shield some wires to see if it goes away. sometimes the rev driver tranny can saturate and distort by itself, but don't think it's the case, because you still have under no load condition. I'm almost sure it's an oscillation...ground the wires going to the input of the rev tube after the 500pf cap, or at the grid, and see if it goes away...be sure there are no potential voltage in there (there shouldn't be).

[ElectronAvalanche]

Hi there,

finally got around to measure some voltages.

The raw B+ going to the Driver is about 400V, but drops quite a bit when feeding a signal into the driver.

If I ground the input (after 500pF cap of Reverb driver). The voltage stays at 400V.

So the Reverb driver must be drawing a LOT of current.

Using a different Reverb tank resulted in a little less sag.

The Reverb tranny does indeed seem to ring, although I already installed a cap from anode to cathode.

Any ideas?

Cheers,

Dominik

[ElectronAvalanche]

Hi gang,

I finally had enough and pulled the Reverb. Man, the amp sounds even better without the Spring-Reverb.

My first clones had Reverb and I rebuilt them many moons ago to be non Reverb. I think Gil said a while ago, that one should not fuss with the original design. How right he is.

cheers,

Dominik

[Normster]

Just for the heck of it, you should try Gary's 1-tube reverb circuit. It's quite a bit more subtle than the Fender circuit (won't do Dick Dale), but it doesn't effect the tone nearly as much. That said, I much prefer running a digital reverb pedal in the effects loop. To my ears, it's much more natural sounding than a spring reverb.