Vibroverb 6G16

[lowell]

My coworker built this amp. He got the transformers, pots etc... from Weber. We used a Hammond head/cage enclosure and drilled the chassis/layout ourselves. We eliminated the NORMAL channel. So we got this thing working. THEN, smoke on its first gig. Now I got it back on the bench. Here's the issue. There is an amber machine-oil-like substance near the output jacks. I've confirmed that nothing has exploded, don't know WHAT that is or where it came from. The PA seems fine. The bias is good, plate voltage etc is all good. NOW, when i insert a plug into the input and turn the volume up a bit and give that plug a tap w/ my finger, the amp begins to oscillate. It sounds like the vibrato is on, it's a slow, low frequency thumping. The power tubes then begin to visibly pulse. This happens even when I remove the vibrato tube!! The intensity, speed, and volume controls all have an effect on the pulsating. If I throw it back in stby, then back on again, it's gone, until I inject noise w/ my finger. I tried a new filter cap for V4 supply as well as checked DC resistance of the PS dropping resistors, they're good. I also disconnected the left side of the INTENSITY control (per the schem), didn't help. Disconnected the NFB, didn't help. The only way to get this issue to stop is to ground pin7 of V4 which is the last gain stage before the PI. This led me to think that there's a bad connection from V4's grid to ground, but it's all good, I even tried replacing the wire. If I let the noise continue and monitor voltage on V4 pin6 the voltage is all over the place, which makes sense w/ this oscillation. But why THIS tube? And why is this tube modulating the bias for the power tubes? I've checked the circuit over and over and can't seem to find the issue. I've tried a new tube in V4 as well as new power tubes too. I'm hoping someone has dealt w/ this issue and has some advice.

http://taweber.powweb.com/store/6g16_schem.jpg

[Tom Phillips]

Sounds like you have "motoboating" oscillation. Things to check include:
Decoupling caps
Grounding (Loose /bad grounds and overall grounding scheme)
Wire dress
Scope the power supply rails and determine if the LF oscillation is riding on the B+.

You have both a new build and a custom layout so don't rule anything out until you check it.
The amp could have been on the verge of motorboating right after the build was completed or something came loose or a new component experienced an infant failure.

Suggest you look up troubleshooting motorboating.
At least you know that the amp can operate correctly based on your first testing.

[g1]

Sure sounds like a cap that leaked it's innards. Doesn't necessarily have to explode.
Smoke + mystery amber oil like fluid + motorboating.

[Tom Phillips]

Also check for electrolytic caps that may have been installed backwards. (I.e. reversed polarity)

[lowell]

Thanks guys. I agree with both of you. The odd thing is, none of the caps have any of this mystery fluid on them... at all. I actually discovered a .1uf coupling cap on the PI grid that was supposed to be 500pf. I swapped it and it's fixed. I guess there was too much low end reaching the PI?? Not sure really. In any case the vibrato is weak now so there's still something going on. At least the amp is working now so troubleshooting the vibrato shouldn't be too tough. My buddy played a 3 hour blues gig with it last night and it held up well.

[tubeswell]

I tried the 6G16 trem on one of my builds the year before last and found it to be less strong than suits my taste. I took the plate-to-cathode shunt cap out of the oscillator and it was better. In the end I went with the 6G9B LFO and adjusted the output voltage divider until I got the slam I was looking for.

[lowell]

thanks tubeswell cutting out that cap helped.

Does any one know of any other ways to increase vibrato intensity??

[Tubenit]

thanks tubeswell cutting out that cap helped.

Does any one know of any other ways to increase vibrato intensity??

There is information here on modifying vibrato's and increasing intensity.

Hoffman AB763 Tremolo 6L6 mods? - Hoffman Ampifiers Guitar Tube amp forum

With respect, Tubenit

[tubegeek]

Some detailed info here too:
Aikenamps.com: Phase Shift Oscillators

Looking at your schematic, I would guess that lowering the value of R56 and raising R57 (the intensity pot) would mix more oscillation into the bias point and I don't think it would affect your ability to get the "no tremolo" default bias adjustment correct for the power stage.