el34 vs 6l6 bias question, primary Z 4k 6k6...

[ampkitchen]

somehow I am confused about the way things work
el34's and I do not seem to get along, maybe just this question
Why When I try to bias a set of el34 in my 6l6 type amp 4k and like 475 volts..
would they redplate and I cannot seem to get them into a stable state
they also draw screen current like mad..

I really thought that with higher transconductance they "drive easier
so they would need more negative voltage, to slow down
current
then I see that a typical marshall is 3k4
and I know they run higher voltages ...
I would think that 6k6 or higher would make more sense..?
it seems I can have a stable amp with my 4k iron and el34 as long as I stay under 425 or so, do I have this backwards ?
or maybe the bias circuit would need more range?
I must be missing something 6l6 are more forgiving
but not as much fun sometimes..
sorry to ramble..

[stoo]

Are you aware that the suppressor grid in a 6l6 is tied "internally" to the cathode BUT in an EL34 you must connect them physically on the tube socket?
Just a thought at 5:45 am
stew

[ampkitchen]

yes I did know about that I am not a total newbie
at tube amps just have not done a lot with the el34's
and I have a bunch of that schumacher 4k iron
you are talking about pin 8 and 1 must be tied on the 34's

[stoo]

Sorry AK . No insult intended. Yes I meant pin1 to 8.
Stew

[Zippy]

Why When I try to bias a set of el34 in my 6l6 type amp 4k and like 475 volts..
would they redplate...

or maybe the bias circuit would need more range?

Have you measured the current that you are drawing on these tubes?

Yes. Having more range on the bias circuit seems like a possibility given what you've shared.

[ampkitchen]

I did not get that far to measure the current I went back to 6l6 in those amps
I guess I am just trying to get the basic concepts down of the differences
I know the wreck amps use a higher primary Z and I was thinking based on
my difficulties that it would be easier to bias them into a higher load
say 6k6 then 4k no? assuming the voltages stayed the same
and no offense taken I was just trying to get these things in my head
I think I have the whole thing sort of reversed because I think about the
transconductance and the fact theat they "drive easier so I would think
larger primary Z and more negative grid voltage would slow them down..
ie stop the red plate and the burnt screen resistors..

[stoo]

I wouldn't be too concerned about that stuff at the biasing stage as we're dealing with DC only. After getting the op tubes biased then you can decide if you want to consider changing the op impedance to match the primary impedance.
stew

[ampkitchen]

ok well I guess I am asking too much at one time here
sorry to clutter up the forum, I have to restrain my rambling posts..

[Zippy]

No worries about asking too many questions or rambling. Speaking for myself, I'm most concerned that you first get your amp biased correctly and stop redplating the output tubes.

Read your currents and adjust the bias range if necessary to get the currents within appropriate operating limits of the tubes.

G'luck!