IMO, you should abandon the pentode switch idea.

Pentode mode requires the screen grid be connected directly to B+.  For best fidelity, the screen grid B+ should be regulated at approx 0.5X of the anode supply.  Also, connecting SV83 screen grids to the anode supply is out because of dissipation limits.

The O/P impedance of pentode mode is HIGH.  NFB of some kind is essential to lower that O/P impedance and obtain a tolerable damping factor.  Are you up to the engineering challenge?

THE way to get more "sock" out of a SE84 is the use of 2 Ohm speakers.

Well, it's a doable thing, Steve did it.  I used to have an amp that way. . . but I think he abandoned it when he made Rev. A possibly because of the change to SV83, probably because few preferred this setting.

I have an amp that was first made before Rev. A (#27 of Zen amps) and then revised. When I had the triode/pentode switch I really preferred the triode by a wide margin.  The amp went from laidback to forward, and I'm just NOT a fan of forward sound.

In your shoes, I would sound out Steve about doing a mod or allowing you to have the information to do the mod. . . if it's feasable now with SV83s.

Sorry, I should have mentioned I was planning to pull the sv83s and use EL84s.

[quote author=veryoldcat  link=1167936373/0#6 date=1168022109]I'm not sure about the electrical particulars of what happens to the sv83 connected in pentode mode, but geesh, it sure sounds EXTREMELY GOOD connected as a pentode in the Taboo.

Couldn't help but say...

Karl

[/quote]


Karl, FWIW the operating voltages noted on Steve's original assembly instructions for the kit version of SE84 series, even after converting to SV83 device and eliminating the pentode switch, are more than 100V in excess of Svetlana's recommended maximum for the screen.


http://www.tubezone.net/pdf/sv83.pdf


I have no argument that both the SE84 and Taboo are great sounding amps. While not owning either, I've heard both in my personal and other very familiar systems.

Assuming that much of the character of the earlier amps and Taboo etc, are derived from the selected operating points, it might be opined that long term service life could be shortened when operating these tubes beyond max ratings.  

[quote author=Chris_G  link=1167936373/0#8 date=1168025388]Thanks. I have the datasheets for both the EL84 and the SV83, so I have all the max ratings in front of me.

Does anybody have any information on adding a triode-pentode switch? [/quote]


contact Steve  -  he'll certainly have the required circuitry and his own observations to make.