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Hi I want to know if anyone has tried this before. I have run my mark 2 in ultralinear and triode straped. I like the sound of it much better in triode as I think it sounds closed in the ultralinear mode. I need more power and think pentode/tetrode might be a good comprimise.
It looks as if I will need to supply the screens with about 350 volts and adjust bias as needed. Also to keep it stable I may need to limit forward bandwith and adjust feedback.
Any thoughts ?
Regards Fred

I have a eico st-70 that sounds very nice and uses the same basic circuit configuration that the updated mark 2 does.

freddymac406 wrote:Any thoughts ?

Try it, and see how it sounds. I think all you need to do to run it pentode is to disconnect the screens from the UL taps on the output transformers, and connect it to a screen grid supply as you have described. I think the traditional way of deriving the screen grid supply is to tap off the B+, and use a resistor to drop it down to the 350V or so that you want.

Often, when people describe the relative sound of the three designs, ultralinear is somewhere in between triodes and pentodes. I'm going to guess that whatever it is that you dislike about the sound of ultralinear, you're going to find more of it when you go pentode.

But try it for yourself and see.

I would read up a bit on tube regulators, as if you just put a resistor between B+ and the screen, you're still in triode mode. Generally, you will need to add a regulator tube in series with the B+, and you can either bias that with zener diodes or gas regulators. This is not an easy thing to do if your tight on space, so you may want to build one on a piece of wood outside the chassis to see if it's what you're really into.

In my opinion, it will be too much work for a backwards step. After listening to the ST35 for a few years I acquired an Eico HF-86, a similar 6BQ5 PP amp but pentode mode instead of UL. I was instantly disappointed after restoring it. Just my 2 cents, though. Adventure on.

It would seem to me, however, that it wouldn't be super difficult to do a 3 way switch to toggle between them all. I suppose the U/L and pentode modes would be pretty close in terms of OPT primary impedance, but I'd be that triode mode might not be so hot.
-Paul