Three PP KT88 designs. Which one ... (please;-)

[WE91]

Thank you Brett; I see that the 6C45 is actually much better suited than a 7119, 5842 or 6H30 (6N30) and seems to fit directly in the Cuiffoli PP KT88 schematic in stead of the 5842 as driver. Are there any modifications that would need to be done at all to use this tube here?
What would be your suggestion as the best solution for a PP KT88 amp, SY? What would be the best way to do phase-splitting - and the best way to configure the output tubes?
The KaT88 uses long tail phase splitting, as does the Ella, and the KaT88 uses penthode coupled output, while Ella uses UL coupling as does the Mono Bills. How does this compare sonically? The Cuiffoli output is triode coupled ... What are the pros and cons?

My thoughts now is to go for the Cuiffoli KT88 PP, but with Plitron/Amplimo OPT and perhaps substitute the Lundahl IT with a Plitron/Amplimo also (if they have, or can make one with equivalent data).

Aril

 

[SY]

I don't think there's any one "best" solution. But I would pay attention to drive requirements, balance, and overload recovery. If I were scratch-designing one for myself, I'd probably use an input transformer to convert from single ended to balanced, then a couple of diff amps or a cross coupled stage. Before the output stage, I'd use a cathode or source follower to prevent blocking. Moderate (15-20dB) levels of feedback.

I am not a fan of interstage transformers for circuits that don't have unusual drive requirements.

 

[Cobra2]

I'd say it was impressive for a device biased at 1.3mA swinging 80Vpp open loop

S***, I did not read the details...sorry.

Arne K

 

[dhaen]

Arne,

don't worry. It's good to see your interest in valves grow over the past year.

 

[Cobra2]

Thanks, I just wish I could remember more from the early 80's, when I was a trained tactical radar-technician/engineer...
(Super Fledermaus ++)

Arne K

 

[Brett]

Thank you Brett; I see that the 6C45 is actually much better suited than a 7119, 5842 or 6H30 (6N30) and seems to fit directly in the Cuiffoli PP KT88 schematic in stead of the 5842 as driver. Are there any modifications that would need to be done at all to use this tube here?

The 6C45 can be a bit prone to RF oscillation if you're not careful, otherwise it's a very good tube, and incredibly tough. There are lots of designs out there using them as 'spud' amps that have details on minimising potential for oscillation. The 5842 has these issues too from what I've read (I have limited experience with it).

 

[Eli Duttman]

The 6C45 can be a bit prone to RF oscillation if you're not careful, otherwise it's a very good tube, and incredibly tough. There are lots of designs out there using them as 'spud' amps that have details on minimising potential for oscillation. The 5842 has these issues too from what I've read (I have limited experience with it).

Yes, the super high gm triodes darned near oscillate in the cardboard box. Put a Carbon composition stopper resistor on all grid socket connections, tie the "free" ends of the stoppers together, and use that junction as the I/P point. Place ferrite beads on the heater power wires close to the socket. Put a 100 Ohm Riken brand Carbon stopper resistor on the anode socket lug and use the "free" end as the O/P connection. With all that, vigilance is still required. Use a broadband o'scope to check for parasitic oscillation. These tubes can oscillate well into UHF land.

 

[Brett]

Yes, the super high gm triodes darned near oscillate in the cardboard box.

Damn, that made me larf.

All the rest of your post is excellent advice.

 

[beamnet]

If you don't want it that elaborate, you could put a css in the LTP phase splitter. That gives you good balance too and eliminates most con's of this type of splitter...