Circuit indentification

[georgehifi]

Guys, what this 6DJ8 circuit called, and is there somewhere that explains how it works and how good/bad it is, it drives (C14 .47uf)a 10megohm video buffer called a BUF03, I lowered the 10meg input resistor to 510k as it was flapping the woofer a bit at <1hz

Cheers George

[oshifis]

I don't know the name, but it is a high-input impedance FET gain stage driving a common-grid tube stage. This configuration has high gain with very high bandwidth due to the low Miller capacitance of the tube.

[planet10]

Looks like a variation on a cascode to me....

dave

[KaDe]

George,

that cascode-stage should work fine as phono input.
Very low noise and high gain.
I use a similar in one of my phono-preamps with good results.
The FET is sonicaly invisible.
What´s your application to it ?

Klaus

[SY]

Originally posted by planet10
Looks like a variation on a cascode to me....

dave

Not even a variation, it's a plain old cascode.

Its potential bandwidth can only be realized if its output loading is very light- output impedance is roughly equal to the plate load resistor. A BUF03 is welcome there.

[rfbrw]

Originally posted by KaDe

What´s your application to it ?

Klaus

A Counterpoint SAx000 ?

[KaDe]

Yes,

Counterpoint SA-9 Phono and the SA-11 Control amps from 1990
had this gain-stages also.

- some full house equipment btw -

[rfbrw]

Originally posted by KaDe
Yes,

Counterpoint SA-9 Phono and the SA-11 Control amps from 1990
had this gain-stages also.

- some full house equipment btw -

Thought they were all glass, mostly octal, right down to the psu.

[KaDe]

Here is a scan from an old magazin with the Conuterpoint SA-9 Phono case.
Most of the glass is working in the psu at the back side.
In the front four 6DJ8 with solid state assistance.

[georgehifi]

RFBRW get's the cigar for "id"ing first, yes it's a Counterpoint SA3000 that I snaffled. it actually has six in total BUF03's, 4 x in the phono section along with 2 x 6922's and two on the output along with 2 x 6922's.
I haven't tried the phono but the line stage sounds superb very powerful transparent tight and smooth. This is one of the best tube sounds I have heard.
I lowered the input resistor from 10meg to 680k because of wandering subsonic dc getting through the .47 cap , SY seems to think that maybe the bandwidth has been compromised now that I've lowered it to 680k. It was flat out to 300khz before, but I didn't check it after the resistor change. Still sounds like it's going very high in frequency.

Cheers George