807 SEP

[Cressy Snr]

Sorry Paul, deleted the last post, but I'm certainly more interested in the effect of the CF.

[Cressy Snr]

Well do the math to direct couple the cf to the 807 with a level shifter i.e. absorb voltage in series diodes bypassed with a tiny cap if required for noise reasons. To go a2 aswell.

For the cf use EL84 recommendation of Nick’s. Its a very unusual tube makes it ideal as a cf driver. Ive found EL82’s almost as much transconductance 10 instead of 12 but much cheaper. Im planning using them in my 801a driver one day to benefit from a2.

The higher the transconductance the more suited to driving grid current direct. Lower output impedance you see. El84 pretty perfect. On paper far better than famous circuit used 5687 in ongaku for example. Better sounding valve than valves similar to it used elsewhere like E88CC but would be “move over” in presence of EL84 and similar numbered valves with high transconductzance. Ongaku should be bettered by it. Everything else left the same.

Cheers Paul.

[IslandPink]

If I find any 1624’s they’re yours.

Ooof !
I have a box upstairs marked 'Tabor'.
I was close to getting some 1624's from someone Chris was in contact with, but I haven't heard from him in over 2 years...

[IslandPink]

No. but seriously Steve can have any you might have - it will be ages before I get around to this amp.

[Paul Barker]

There might be enough for both. I think theyre military designation from memory, in brown military boxes, well protected.

[Cressy Snr]

Have transplanted this lash-up into the box:

807SEP.jpeg (115.65 KiB) Viewed 3374 times

A few components have been changed from Alex's circuit, but it is essentially the same.
Coupling caps are 330n Obbligato copper tube jobbies, I bought from Brian Cherry (can’t remember what his firm is called) a very long time ago.
Cathode follower valve is a common or garden Mullard ECC82.

[simon]

DIYHiFiSupply?

What happened to Alex Kitic? I recall Paul sending him stuff as a collaboration but don't remember anything after.

[pre65]

I seem to remember he got the hump when discussions about filament supplies went against his thoughts.

[Cressy Snr]

DIYHiFiSupply?

Aye, that’s it. Thanks Simon.

[Cressy Snr]

Revised circuit:

Screenshot 2023-02-09 at 19.47.31.png (57.35 KiB) Viewed 3278 times

[Nick]

Looks like a job for a ecl82

[Paul Barker]

Like

[Dave the bass]

Awww, sweet memories, you lot introducing me to ECL/PCL/82/84 got me started all those years ago

BOBA amp.

[Cressy Snr]

Looks like a job for a ecl82

Nick...are we talking about replacing the ECC81/82 CF combination with an ECL82? In which case, why bother as the 807 in pentode mode doesn't need that sort of heft behind it.
OTOH are we talking about replacing the ECC81/82 combination with the ECL82 in order to direct couple the pentode section to the output stage as a CF in order to drive grid current, ie class A2.

Option 2 would necessitate a redesign of course.

[Nick]

Well it was just the triode and cf that the ecl would do. No big gain as the same number of envelopes ends up getting used. I a bit confused though you saying the 807 doesn't need that heft when you are using a cathode follower anyway.