Sweeping pentode mode to triode mode

[stiltamp]

Hi,

I already asked this question in the SLO-Clone Forum:

Have anybody experiences with feeding in various voltage to the screen grid to sweep from the triode mode to the pentode mode?

It seems to be that no-one have experience in this there.

Have anybody tried on?

[tubeswell]

Haven't tried it but Merlin Blencowe describes this in Ch 3 of his recent book. There's a a few different ways you can do it according to him, but I don't know how much I can divulge without infringing something or other. Basically you can either make the screen bypass circuit variable like you would do with a variable cathode bypass cap, or you can use a pot and extra cap in the screen bypass circuit to achieve a combined mode switch/variable screen bypass.

[M Fowler]

You mean like this?

[David Root]

That looks like just a pentode/triode switch to me.

It's not clear to me if stiltamp is talking about a pentode preamp, which Merlin's book does address nicely, or a pentode power tube. If a power tube it seems to be almost like UL mode with a continuously variable primary screen tap, except not doing it that way.

[VacuumVoodoo]

Passive feedback from anode to screen grid as per Merlins circuit works only on small signal pentodes like EF86. Since it is a passive feedback screen grid is fed from high impedance. This has some limitations on sweep range.
Better results are obtained with active buffer in feedback from anode to screen grid. This has a disadvantage of buffer needing a supply voltage about 60% higher than anode DC supply.

A guitar amp utilizing sweepable triode-ultralinear-pentode in the preamps first gain stage is swedish made reVintage Pro

[img:320:240]http://www.revintage.se/Profront.JPG-for-web-NORMAL.jpg[/img]

LouderAndMore amp designed by myself in collaboration with Björn Juhl has a sweepable triode-ultralinear-pentode single end power stage. I am currently experimenting with a sweepable PP configuration.
A rudimentary circuit illustrating general principle was shown in one of David Haflers patents from early 1950s.

review here:
L&M PG review

[VacuumVoodoo]

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