Re: Biasing JJ E34L valves




"White Spirit" <wspirit@xxxxxxxxxxxxxxxx> wrote in message
news:e5bbf97e-0fcf-4777-b8b2-66338734fb39@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
mykey wrote:

EL34L runs hotter and it's preferable to change the screen grid
resistor
to 1250 ohm 5 watt before biasing
either that or use EL34's
lowering the bias (slowly) until the tube just starts to turn red,
then back
again slowly until the red just disappears.
that gets you to an approximate "hotter" bias setting, the amp won't
sound
good unless the tubes run on the hot side (but not red).

I don't like the sound of EL34s biased hot. I set mine at 17.5W
static because I read from several sources that the E34L is a 30W
valve and 17.5W would be fine for a max plate dissipation of 25W in
any event. It seems the confusion has arisen because the Groove Tubes
version (which I don't have) is intended to be a 30W valve, and Groove
Tubes' specs have been parrotted for the non Groove Tube E34L.

using an oscilloscope to check for crossover distortion (too cold) is
preferable, but even with a scope it becomes necessary to bias the amp
on the hot side to eliminate the crossover distortion.

Using an oscilloscope won't tell me whether the current draw is within
safe limits. Besides, don't all Marshalls have a certain amount of
crossover distortion when biased within safe limits? Eliminating
crossover distortion would be more than on the hot side - do you
really suggest going hotter still?

Of course the tubes don't last as long when running hotter, but the
amp
won't sound that great unless they ARE hot.
It's a compromise.

As I said above, I don't think EL34s sound good when running hot. I
took the static dissipation down to about 15W. I'll see how that
sounds later today. Contrary to what I said the other day, the dirty
channels were sounding quite muddy. I'm trusting my ears and blaming
the thinness of the clean channel on the GE 5751 I have in the clean
hole. Having had these valves in two different 6100s, I don't think
they're the right valve for this amp.

Do yourself a favor and purchase a matched pair of Chinese (Shuguang) EL34B.
These are inexpensive tubes that sound good. Not for the cork sniffers, but
can be just fine in a guitar amp. Gven the price, your risk is really quite
low. Though, it might be that your GT issue is really what I just
suggested. You should probably try to find that out first.

Personally, I wouldn't push any tube to the red plate point. That's just
silly. You know the spec and can run the bias hotter or colder without this
kind of brinksmanship. And you don't need a 'scope to set bias. Follow
your instinct. Use your ears.


.



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