Bias modulated trem help

[jzucker]

I bought a Vibroclone amp off ebay. It's made using Doug Hoffman's AB763 board kit which is based off the Fender AB763 circuit but with the 6G16 brown face vibrato which uses a single 12AX7 tube and oscillator circuit to modulate the bias on the power tubes. I'm having a few problems with the amp and the trem seems to be acting a little funny so I thought I'd try soliciting some advise from some of the amp gurus here.

The amp was professionally done and the wiring is very neat and tidy using high quality sprague 715 or 716 caps, metal film resistors, yada-yada.

The problems I'm seeing are:

1) Vibrato is weak
2) Amp fades out occasionally

I've gone over the trem circuit and it's exactly like the 6G16 other than a 270k resistor in series with the two .1 caps. (This is listed as a performance mod on Doug's site in order to increase vibrato intensity). He's using the correct value pots including a 3M RA Taper speed control.

Using ohms law and measuring the DC mv across the 1 ohm cathode resistor with the tubes set to idle at 35ma, I'm seeing the voltage swing from about (approx) 31ma to 47ma with the trem on. I also see the odd behavior of more swing as the speed control is increased. This seems odd but it also seems like a fairly narrow swing for this type of circuit. However, since I've never owned a 6G16 amp, I'm not sure what swing to expect here...

Another interesting phenominon is that the guy who made it installed a switch which adds a 2.7k resistor in series with the stock 820ohm neg feedback resistor. When the 2.7k resistor is switched into the circuit, the audible vibrato is increased though when I measured the voltage swing, it's unchanged. Perhaps the neg feedback is "negating" some of the audible oscillation when using only the 820ohm resistor?

Regarding the amp fading, I've checked the bias supply which is giving me a very consistent -52v and I've replaced all the tubes, including swapping out the rectifier tube for a weber copper cap, all with the same behavior. I've looked for cold solder joints in the power and bias supply and could not find anything....

Just wondering what other suggestions you folks could give me to help debug the amp...

Doug does not offer a schematic for the amp but here's the layout diagram:

http://www.hoffmanamps.com/images/Ab763.gif

And here's the 6G16 diagram:
http://www1.korksoft.com/~schem/fend...6g16_schem.pdf