Deluxe Reverb FAILED at gig!

[Steve Casas]

I played a gig on Friday night and was extremely disappointed in the performance of my 71 Deluxe Reverb. This was only the third gig in a year played with this amp. One was outdoors, in a backyard, and it was great. I had it up to about 4.5 on the volume knob, great breakup, and loud enough. I rehearse with this amp weekly and it's always been great.

So... Friday night, during soundcheck, everything was fine... empty room, 20 foot ceiling, so I figured things would change when it filled up... figured I'd have to turn it up and add some treble. But after we started, with a room full of people, I turned it up to about 5.5 (I've done this before, knowing I'd get more breakup with little, if any, volume boost) and I was getting the most distorted, washed out crap I've ever heard out of this thing! I turned it back down to about 4.5 and it was still very distorted. One thing though, I was getting great sustain! just no clean headroom. I really should have just mic'ed it, but I had no time by then.

About the amp: It was a stock early silverface when I got it with fried heater tap resistors, a fried screen grid resistor, and a few bad solder joints on brass-plate grounds. After all new caps (ALL of them) new JJ tubes, a 5v4 recto (to lower voltages a bit) and quite a few new CC resistors, just to be sure, this thing has been the greatest DR in the world for two years! (the Tone Tubby AlNiCo hempcone helped a lot!)

So... this morning I pulled the chassis out, contemplating swapping in some GT 6L6's and a solid state recto I have laying around, and thought I'd check the bias as is first. The Left tube reads 32ma @ 463V on the plate and the right tube reads 19ma @ 463V.

Did my matched JJ 6v6's go out of spec? 19 ma was where I originally biased them at (I think). The left one was noticeably hotter also...like REALLY hot!

I have read that the new JJ 6v6's are the best current tube to handle a DR's over spec voltages but I have also read that the quality control is spotty and many come defective out of the box. But mine worked great for two years. Any Ideas?

Also... I have a little 47uF 100v electrolytic cap coming off my bias pot. It's not on the AB763 ckt and I don't remember why I put it there. I did change the bias to blackface specs but left that cap. I don't remember why and I can't find any info on it... should I take it off?

[txstrat]

463 vdc is high, even for a DR. 19ma (per tube) is high for that voltage (around 75% dissipation).
Two years of weekly rehearsal and some gigs in between might have worn 'em out.
I wouldn't blame the tubes.
Have you tried new tubes? If you don't trust JJs 6V6s you could have a look here;
6V6 Tube Type Review
the Tung Sol might be your solution.
Regarding the cap, if it's the original one you could "refresh" it with a new one. On the other hand, if it doesn't hum - you might leave it in and wait until it's making noise.
A 100uF/100v cap would be fine.

[Steve Casas]

Thanks TxStrat...The Tubes are only about a year old... Gonna steal another matched set of JJ's I put in my princeton recently and re-bias... Also, I have a pair of grey-glass small bottle RCA's I got out of an old Hammond Organ I parted out... don't know if they were ever matched or how many hours on 'em... But I'll try 'em out and see.

That Cap is new... I remember replacing it when I re-built it. Just don't remember why I didn't remove it cause it's not on the AB763 schemo...remember, it's not the bias cap on the little phenolic board, it goes from the center wiper of the bias pot to ground.

[txstrat]

Sorry it took me so long to answer. Missed your post.
Some brownface amps had this cap (5nF) on the (tremolo) intensity pot wiper which changes the bias of the power tubes (Deluxe 6G3, Vibrolux 6G11). Might be there as a filter to prevent oscillating?!