First powerup of Brownface Deluxe (6G3) build

[JoeTele]

I just powered up my friend's first build, a stock Mojotone Brownface Deluxe kit. The only changes were bias test points via 1ohm resistors and adjustable bias (replace stock 22k resistor with 15k and 10k bias pot in series). All in all the amp sounds great. What is interesting to me is that with the stock -26 bias volts, the amp is running hotter than the 14w max 6V6 dissipation (it was actually pushing 17 watts). Right now it's at 11.3 watts (still around 80% dissipation) with -35 volts bias, which pushed plate voltage to around 390 (365 is what's on the schematic, but Fender schematics were of course low anyway). Is this acceptable? There is still some room to cool it off, but the voltage will obviously go even higher and the bias voltage that much further off spec, and I'm wondering where the best compromise is.

Also, once the "vibrato" intensity is turned to about 5, the amp starts to sputter. I now see that this is an inherent problem with these circuits and will likely do more research into it, but I'd appreciate any thoughts or possible solutions (if there is one) that anyone is willing to share. I know that the bias adjustments mentioned above are probably a factor. However, I did notice that unless the footswitch switch is connected to the amp and set to "off" the voltage readings on the plate and cathode of the vibrato triode are in constant flux. Is that normal?


Thanks as always!

JoeTele

[xtian]

In short, all sounds normal, and use your ears to set bias WRT tremolo.

[JoeTele]

So the plot thickens. Now that my friend has had a chance to turn the amp up higher than we can in my apartment, he is finding that there is a steady popping with the amp's volume above 5, even with the tremolo speed and intensity at zero, the footswitch in, and set to off. Any ideas as to what this might be?

Joe

[xtian]

Does the tempo of the popping change with Trem Speed control?

[JoeTele]

I had originally thought so but it turns out it does not. Knowing that, I'm guessing it could be a separate issue and will check the usual suspects.

Joe

[martin manning]

Is this one of those "I have this friend..." stories? ;^)

Don't pay any attention to the bias voltage. Set the dissipation on the cool side, maybe 60-65%. Another thing you can try is putting a diode across the intensity pot, cathode on the bias supply lug, to kill the positive half of the LFO signal. You can temper that by adding a resistor in series with the diode.

[strelok]

On the Deluxe I build a while back I had that tremolo popping issue as well. The cause was just the cathode of the tremolo oscillator stage coupling to the previous gain stage. Lifting it up and away from that stage fixed the problem.

This was a AB763 circuit so slightly different, but you might try poking and moving wires with a chopstick or pencil while the amp is making the noise to see if it makes any difference. Especially the wires associated with the trem circuit.

[JoeTele]

Thanks for the great suggestions! Will hopefully get over to my friend's place this weekend and try them (and yes, it really is my friend ).

Joe

[JoeTele]

Turns out there was one intermittent solder joint, but the tubes/bias were the main culprits. I happened to have a spare set of 6v6's from my orphaned deluxe reverb build which ended up being much cooler than the JJs that were in the Brownface deluxe. Once those were in, the bias adjust swung the tubes between about 55% and 70% all of which was useable. I think we ended up right around 65% (Thanks Martin!). We'll probably experiment with the resistance in the bias circuit to try to cool the JJs off because we both agreed we liked the sound of them better, but for all practical purposes the amp's a winner.

Thanks as always!

Joe