Geezer's HoSo56 with SE KT66 and Ghetto UL OT.

[David Root]

Following up on a recent thread in which the Ghetto UL was unleashed upon us by matt h, my appreciation of the 5879 as an input pentode and all hail Geezer's HoSo56 design, how about this little piece of cut & paste with an extra 5879 replacing the LTP? (as Mae West said, "Too much of a good thing can be wonderful!")

Comments welcome. I will be building it as I have the iron and the tubes on hand, and a suitable chassis (Princeton Reverb knockoff in aluminum).

Update: Revised the schem as it had a major power supply screw-up in it. Had my head on backwards when I did it I guess. Also changed second 5879 to pentode mode (was triode mode).

[matt h]

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[David Root]

Thanx matt. I posted that schem before I simulated the 5879 in triode mode in TubeCad and it will only get a gain of 10 or 12 at best, although the 5879 data sheets all say 21. I did wonder if the plate resistance would be low enough and no it's not, it's still at least double that of a 12AX7.

Since that tube is acting as a driver, more current is probably a good idea too. With 250V B+ at that node that's not really possible.

I'll have to check that "pentode sandwich" schem of Merlin's, I have the book but don't recall it. I'll also check the tone stack out in that simulator from Duncan Amps which I have.

[matt h]

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[David Root]

What about doing the second 5879 in pentode mode? That takes care of the gain, but I haven't looked at the impedance match (mis-match) between Zo of the 5879 and Zin of the KT66, and what that would do to the effective gain transfer to the KT66.

If not there are other high mu high Gm pentodes eg D3A, E180F, which I have and could use as a driver.

[matt h]

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[tubeswell]

FWIW I tried a 5879 with Merlins morph control, first just as a pentode

[img:526:321]http://farm9.staticflickr.com/8373/8380 ... 3df5_o.png[/img]

then with a 12AU7 follower driving the morph control (to take the load off the 5879)

[img:575:339]http://farm9.staticflickr.com/8192/8381 ... 2959_o.png[/img]

yes there was a notable difference in gain and brightness in pentode mode

[renshen1957]

FWIW I tried a 5879 with Merlins morph control, first just as a pentode

[img:526:321]http://farm9.staticflickr.com/8373/8380 ... 3df5_o.png[/img]

then with a 12AU7 follower driving the morph control (to take the load off the 5879)

[img:575:339]http://farm9.staticflickr.com/8192/8381 ... 2959_o.png[/img]

yes there was a notable difference in gain and brightness in pentode mode

Hi,

Pentodes are quietest when they are isolated from the load by a cathode follower.

Best regards,

Steve