AB763 deluxe reverbs

[martan]

Hello all! I'm trying to trace down some trouble with two Hoffman board AB763 deluxe reverbs using the Hoffman schematic with bias wiggle trem and full size board. I apologize if I missed a document that addresses my problem. As much as I've read there is a chance that I overlooked something. Both amps play and sound great when hooked to a lightbulb current limiter, but then blow a fuse after about 5 minutes without the limiter. Using a Genelex GZ34 rectifier and new Genelex 6V6's. Probing the 6v6 pins only give continuity on the heater filaments.

I'm using this ClassicTone PT: http://www.classictone.net/40-18016.pdf

and the first layout at the top of: https://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf

Here's the latest pic of the power section:


and the full kit


and here is a pic of the light bulb limiter (200W bulb) when playing through it:



Here are the voltages using a Fluke 87V (filament pins to ground measure ~ 3.2 AC):


Bias currents are 34.4 and 28.9 mA (tubes matched by supplier)

Power node points are:
A   376.8
B   373.8
C   298.6
D   252.8

Observations:
The bias circuit and filaments read continuity with ground on the Fluke yet show voltages. I assume it is the small resistors not being accounted for by the meter.

The pilot light and the light bulb limiter pulse with the trem circuit, although the trem does work well audibly.

I could see no problems under the board using a flashlight and dental mirror.

The highest voltage I've read on point A immediately after power down was 17-18 volts

All high voltage nodes make a pop sound when measuring voltage

Any help or ideas would be greatly appreciated.

Thanks,

Marty

[shooter]

Bias currents are 34.4 and 28.9 mA

were those made without the limiter?

my crude math show you ~~ 80% plate dis, maybe try biasing colder and see if you quit blowing fuses, or keep increasing fuse size til you see smoke 

[sluckey]

Was the amp connected to the lamp limiter when you made those voltage readings? We need to see voltage readings with the amp plugged directly into the wall. And set your meter to read millivolts when measuring voltage for V7 and V8 pin 8.

I would expect the bias voltage on pins 5 of the 6V6s to be a bit higher, probably around -36v.

[martan]

Thanks for the responses, guys.
Here are the voltages with and without the current limiter. I forgot to mention I did the Robrobinette site lead channel mod to make it more "Marshall-esque"
"For the mod you increase the V1A cathode resistor from 1.5k to 2.7k (1/2 watt), reduce its bypass cap to .68uF (micro Farad) 25v, increase the plate load resistor from 100k to 220k 1 watt and reduce the big V1B .047uF coupling cap to .0047uF 450V (or higher voltage and yes, that is with two leading zeros)." That is why V1A looks relatively lower voltages



 I'm confused about lowering the bias range (if that is what is needed). Would lowering R67 (470/3 watt resistor) accomplish the same thing as lowering  R66 22k of the bias pot?

Thanks again,

Marty

[shooter]

with 433vdc * .048 = ~~ 21W plate dissipation, even the new 6V6 will probably consider that smoking!

with the PA tubes out NO limiter, what is your bias pots voltage range, CCW=?  CW=?

[sluckey]

You're killing those output tubes! They are biased way too hot. Put the amp into standby mode. Monitor the voltage on pin 5 of a 6V6. Now crank the bias pot for maximum negative voltage at pin 5. What have you?

[martan]

 Even at 41 years old, I am still sometimes and awe of the sheer scope of my own idiocy. Will get back to you when I set the bias using the right measurement

[sluckey]

Here's a nice, easy to read method to set bias...

http://www.duncanamps.com/technical/lvbias.html

[shooter]

Even at 41 years old

wait 20 more years, the brain starts becoming a very interesting organ 

[martan]

So yeah, that did the trick for both amps! Both biased to around 20.5mA and not tripping fuses. Just need to box them up and will have a good stereo amp rig for the first time. Thanks for the course correction, guys. I think the stressed might have gotten to one of the 6v6's though. On one amp I get 20.6 and 21.0, where on the other pair I get 20.5 and 16 mA. Had bought a matched quad. Oh well...

Here's a look at one of the pair boxed up:


Thanks again,

Marty

[sluckey]

That looks great! Under the hood looks very clean and neat too.

[mresistor]

Very nice looking amp Martan - and glad to see you got it straighted out.. 

[martan]

Thanks again, guys!

[Ed_Chambley]

Very nice work!