Express with E34L's

[Cliff Schecht]

So the other day I was in a rush to get my girlfriend to work and had to do some aggressive driving to get her there on time. I had my Express build in the back of the car and after taking a corner too fast, I managed to flip the amp and shatter one of the JJ EL34's that I had in there. I went to a local amp shop to get another matched pair of JJ's and he only had E34L's. I bought a set ($35 cash) and threw them in the amp for today's jam. When I first fired up the amp, I cranked the bias down to -30V and noticed that the output was low and the distortion was very weak. Since I have an external bias knob on the amp and the output tubes were the only thing I'd changed, I figured it was the grid bias. It only took about 1/5th of a turn to get the amp to come back alive and start sounding like it did with the EL34's. I was a bit weary about messing with the bias without a DMM handy but I kept a close eye on the tubes and noticed no red plating or excessive temperature rise. The sound was spot on and I really liked the small increase in headroom (the Express has enough distortion as is!). I'd like to take a few measurements tomorrow to make sure I'm operating within the limits of the design (i.e. won't be blowing tubes or screen resistors, etc..) but I have another jam earlier in the day so I may end up waiting until later to check things out. I'm not too attached to this set of tubes anyways.

Has anybody else tried E34L's in their Express? I'd like to know what other peoples results were and what bias they ran them at. I know the E34L's are supposed to be a bit more rugged so I'm not too concerned about running them a bit hotter but I'm not quite sure what bias to aim for (either grid voltage or plate current).

[Structo]

The ratings between the two tubes seem pretty similar.

The 34L's can take a bit more plate voltage. 800v for the EL34 and 850v for the 34L
The output is the same 25w.

So from my brief look at the specs it seems you are right, the 34L is just a more ruggedly built EL34.

[ckpop]

JJ tubes is the fomer and reorganized Tesla tube that I always had to apply a hotter bias to get the correct current draw from these tubes. Measure when you get time but more than likly you are not really running these tubes hotter it just takes more of a bias adjustment to run them properly