4XKT88 drive

[Salas]

Hello all

Do you think that a 12AT7 3+3mA phase splitter is enough to drive 4XKT88 triode strapped decently?
If not, what would you suggest, a beefier splitter or to add a 6CG7 buffer for instance?

[Eli Duttman]

Salas,

DC coupled IRFBC20 source followers between the splitter and "finals" buy you immunity against Miller capacitance and also keep the splitter stage gain up. The modest value grid leak resistors needed in combination with the individually adjustable bias set pots. indicated for PPP are not a problem.

A Mullard style circuit using a 6GK5 voltage amplifier and 12AT7 LTP splitter offers plenty of open loop gain. High gm small signal tubes provide resistance against slew limiting.

Is an octet of "reissue" GEC KT88s part of your plan? It's too bad that you can't source decent EI KT90s, which KILL in triode mode.

[Salas]

You know I do have developed a parallel 12AY7 - 12AT7 ccs tail split - KT88 triode amp, with Audionote class B OPT that I underestimated initially as just a good PP amp, but after bringing high rated stuff around it holds its own very well over time and challenge, so I thought of being able to build more power on it wasn't a bad idea. Yes Genalex KT88 is the best current power tube I know of. And I would use those. I thought of the mosfet buffer too, but a ccs and a mosfet buffer is ''powerdrive'' and Tube Lab will litigate!

Anyway, I have 10 NOS 5687 GE and isn't it a good alternative to a mosfet buffer? What kind of a grid leak would you use per KT88 in PPP?

As for KT90 I have used Harmonix and good vintage EI in triode mode on that amp and they are not that great. More romantic, beguiling, but KT88 is surely more real.

[Eli Duttman]

I thought of the mosfet buffer too, but a ccs and a mosfet buffer is ''powerdrive'' and Tube Lab will litigate!

Power drive DC couples the FET to the "final". I suggest you DC couple the FET to the splitter. The requisite individual bias adjustments for PPP make (IMO) a single FET and individual coupling caps. more attractive than a single coupling cap. and multiple FETs.

As for KT90 I have used Harmonix and good vintage EI in triode mode on that amp and they are not that great. More romantic, beguiling, but KT88 is surely more real.

The EH KT90 is not a good sounding tube. I suspect that which O/P trafo is used affects whether properly made EI KT90s or "reissue" GEC KT88s sound best. The EI tube is golden in combination with the Freed trafos used on H/K Cit. 2s.

What kind of a grid leak would you use per KT88 in PPP?

Follow the recommendation on the GEC data sheet of not more than 100 KOhms total, when "fixed" bias is employed.

[gianis]

An amplifier ..10 kt88 pp UL mode, by GEC

[SpreadSpectrum]

...a ccs and a mosfet buffer is ''powerdrive'' and Tube Lab will litigate

I don't understand. Is powerdrive patented?

[leadbelly]

Originally posted by SpreadSpectrum
I don't understand. Is powerdrive patented?

I don't see how. People were building such things long before tubelab showed up here.

[Salas]

Originally posted by leadbelly


I don't see how. People were building such things long before tubelab showed up here.

Joke, joking, humor. I just remembered our good member Tubelab and Powerdrive the way the circuit was going. Hence the hammer emoticon.

[tubelab.com]

I don't claim to be the "inventor" of PowerDrive, I just gave the circuit a name. I posted it on my web site for all to use, modify and generally do whatever with. I can say that I had not seen the circuit or the CCS - mosfet combination before I "discovered" it. That doesn't mean that it didn't exist. I have used a single triode with a CCS load feeding multiple mosfets to drive multiple tubes. It works good and allows for individual bias adjustments. I can say that I have not tried 10 KT88's at the same time yet!

[Salas]

Originally posted by Eli Duttman

*
Power drive DC couples the FET to the "final". I suggest you DC couple the FET to the splitter. The requisite individual bias adjustments for PPP make (IMO) a single FET and individual coupling caps. more attractive than a single coupling cap. and multiple FETs.
**
The EH KT90 is not a good sounding tube. I suspect that which O/P trafo is used affects whether properly made EI KT90s or "reissue" GEC KT88s sound best. The EI tube is golden in combination with the Freed trafos used on H/K Cit. 2s.
***
Follow the recommendation on the GEC data sheet of not more than 100 KOhms total, when "fixed" bias is employed.

Thanks Eli

*Still a heavily biased 5687 buffer leaves you cold vs the FET?

** Yes the EH KT90 sounds forward and soft. The EI KT90 did not sound on par with Genalex reissue KT88 on the Ultralinear Jadis DA30 that has OPTs specifically made for EI KT90. The EI si consistently darker, less informative, and slower on the Jadis and on my triode amp vs the Gen KT88. With other trafos it may be superior in triode mode, since you have experienced it.

***
No problem for buffers to drive 100k and maybe 60pf per phase then.