Best compromise screen R for 6L6 and EL-34 operation

[greiswig]

I have a fixed bias D-style amp that is currently wired for 6L6s using 470ohm screen resistors. I'm interested in making it also able to run EL-34s. Trouble is that putting 1.5k in there is fine for the EL-34s, but seems to make the 6L6s sound kind of poorly-defined in the bass.

Anyone had good luck with a compromise, or is it too much to ask?

[mhuss]

This begs the obvious question: did you try 1k?

--mark

[Gaz]

1k should be fine, especially because the D amp has a master, and the power tubes won't be slammed. 1k on the 6L6 will lower power a little, but in my experience it's negligible.

[greiswig]

This begs the obvious question: did you try 1k?

--mark

Heh, it does beg. But the mean is not always the best compromise, hence the question.

I'm basically looking for the "how low can I go" question, since to my ear that sounds better for the 6L6 types.

[diagrammatiks]

el34s should be ok with 1k right?

maybe try 690ohms.

[ToneMerc]

My current four 6v6 build is setup so if I pull them, I can run two EL34's. I used 800 ohm screen resistors.

TM

[trane34]

Why not just leave the 470 in there and try it? I've run EL34s with this resistor and it sounds good to me.

[JamesHealey]

680r

[greiswig]

Thanks, all, I'll try a couple of the lower values and see if anything catastrophic happens.

[loctal]

Another solution is to use two sets of tube sockets, One set could be wired for 6L6s and the other quad or pair set up for the other choice. These sockets may be spaced very close to one another because you would not have both output tube choices used at the same time.

I have used this idea in an EL84 or 7B5 amp. These tubes require different cathode resistors.

[Roe]

measure the voltage drop over the screen resistors

[rdjones]

measure the voltage drop over the screen resistors

More importantly, what is the screen voltage compared to the plate ?
If it's a 'high plate' D-style where there's a large difference between then screens and plates, then yes you can get away with very small screen resistors.
If the screens are at or near the plate voltage and the amp will be run hard, the screens will want more current protection, the EL34s moreso than the 6L6s.

rd

[greiswig]

measure the voltage drop over the screen resistors

More importantly, what is the screen voltage compared to the plate ?
If it's a 'high plate' D-style where there's a large difference between then screens and plates, then yes you can get away with very small screen resistors.
If the screens are at or near the plate voltage and the amp will be run hard, the screens will want more current protection, the EL34s moreso than the 6L6s.

rd

Hmmm...okay, I'm clearly past my point of understanding, but interested in learning. This is in a pair of D-style amps, with 510V on the plates in one and closer to 430V on the plates in the other. The lower voltage one is the one I'm interested in running either 6L6 or EL-34s in. The other amp runs either KT-88s or KT-90s.

Seems like I need a lesson in the relationship of screen voltage to plate voltage. So I go to my usual source, aikenamps.com, but the tech section is under construction.

[MGW]

I've got 1K in mine and have swapped back and forth with 6L6 and EL34s. They both sound great. I haven't noticed any loose low end.

[greiswig]

I've got 1K in mine and have swapped back and forth with 6L6 and EL34s. They both sound great. I haven't noticed any loose low end.

My experience is that you may not notice it until you try. Then all of a sudden you think "Wow, that's been giving me some problems in some venues." It's not dramatic, but the change in feel is noticeable.