Input capacitance when tube is not conducting

[Eli Duttman]

It's a standard anode follower, doesn't have much juice, would appreciate it if you can tell me if it would work well. Something like this:

An externally hosted image should be here but it was not working when we last tested it.

Rg - 560kΩ (paralled with 50kΩ potentiometer}
Rc - 240Ω
Rp - 8.2kΩ
Cp - 2.2µF
Cc - none

6H30 datasheet:

http://www.hifitubes.nl/weblog/wp-content/electro-harmonix-6h30-eh.pdf

Probably lacking "stones", even though the triode is excellent.

I'm having PC trouble and using my wife's laptop. Currently, I can't upload graphics. Scan the archives for my posts about the tweaked RCA phono preamp. That shows how to DC couple an enhancement mode MOSFET to a triode.

As you will be driving a LOW impedance load, a LARGE O/P coupling cap. is needed. A 15 muF. metallized polypropylene part bypassed by a 1.5 muF. PPFX MultiCap allows for an obscenely low 2KOhm downstream load. Running an IRFBC20 with 25 mA. of drain current will get you an O/P impedance well under 100 Ohms. A 1 KOhm net downstream load, which is worse than obscene, is quite acceptable.

 

[Navyblue]

The 2kΩ load, I assume you are refering to the MOSFET? And the large coupling cap is to go between the tube and the MOSFET? So no DC coupling?

When I was building the F4 (actually, still am), I contemplated building the JFET input stage into the line stage instead of the power amp. It is basically a the B1 buffer. But I decided against building funky stuffs to minimise interoperability problems.

If the cable capacitance is the only issue, I can consider switching the output, and/or try to limit the cable length. I haven't build this so all options are on the table.

 

[Eli Duttman]

The big coupling cap. connects the MOSFET to the O/P. Dig into the archives for the phono preamp schematic. Except for the gate stopper, the FET's gate is connected directly to the triode's plate. That most definitely is DC coupling.

 

[Navyblue]

I see, so I don't really need to have large output cap then? I am only driving 50kΩ with both amp in parallel. But the problem is, I am short on current, I'd need a larger transformer.