It's time to go full Karna Kay

[Paul Barker]

Ok that makes sense Nick. So is that where the ccs comes in you were talking about to Dave was it?

Max yes secondary left open.

Nick, so actually I could use a two power resistors of the value required and b+ to centre tap? Or just use the ccs and have a horizontal. Loadline?

Not that Im goingto build it.

[Nick]

Nick, so actually I could use a two power resistors of the value required and b+ to centre tap? Or just use the ccs and have a horizontal. Loadline?

I would have to see a schematic to know what exactly you are suggesting.

[Paul Barker]

[Nick]

So remove the center tapped choke and use to resistors. Ok, standard PP driver stage. Not sure where the CCS's come into it?

[andrew Ivimey]

Those ccs. as sold by Kevin, I thought that too so I didn't, but later maybe. In other similar applications a ccs on the anode or cathode added great sound quality, so perhaps they are ( always) better!? I'll never get round to it.

[Nick]

Those ccs. as sold by Kevin, I thought that too so I didn't, but later maybe. In other similar applications a ccs on the anode or cathode added great sound quality, so perhaps they are ( always) better!? I'll never get round to it.

That sounds a bit like a version of:

No one I think is in my tree
I mean, it must be high or low
That is, you can't, you know, tune in, but it's alright
That is, I think it's not too bad

[andrew Ivimey]

Mmmmmmmmmm maybe - nice lyrics

[Paul Barker]

So remove the center tapped choke and use to resistors. Ok, standard PP driver stage. Not sure where the CCS's come into it?

At the b+ end two ccs one for each pole.

[Nick]

The major advantage of chokes over resistors or CCS as loads is they will swing above as well as below B+. So you need to have more B+ for resistors. In that circuit you may have enough B+ as you are dropping some of it in the 5k resistor. The other advantage is the added balancing action of the linked flux in the shared core.

[Paul Barker]

Good to know.

[Thermionic Idler]

OK so I think we're pretty close, if not there, with finalising the signal circuit.

Input stage - current-sourced and gas VR arrangement a bit like the original amity but to a centre tap choke load on 12SN7's instead of the 5687 and first interstage.

Then cap-couple to Nick's suggested parafeed driver stage with EL84's, Phil's donated "Russian-Engineered" 4uF parafeed caps and LL1660PP interstage. Then the 300B output stage is more or less the same as before with Lynn Olson's ultrapath-ish capacitor arrangement.

Probably need to open screenshot in new tab and zoom in to see it properly.

[Nick]

Any reason you are not using a shared cathode resistor for the second stage?

[Thermionic Idler]

Any reason you are not using a shared cathode resistor for the second stage?

It's much easier to buy 2 x 270R resistors to get to the 250V/34mA operating point specified in Mullard's EL84 datasheet, than one 135R resistor.

Okay, so maybe 1 x 130R would be close enough? If it's being linearised by the CCS's I guess it doesn't matter too much.

But, with the CCS's I figured it didn't make any difference whether I connected the cathodes together before the cathode R, then flowed through two 270R's in parallel, or tied together at ground per the schematic you see here.

Is there a reason for the cathodes to be tied together before the common cathode resistor instead? II know I'm going to have missed something here).

At first I thought that the 270R's would reduce gain because of the cathode voltage being allowed to vary from one EL84 to the other, but the CCS means that voltage drop stays constant. I can't see how the voltage at each EL84 cathode can possibly change regardless of how it's wired.

Does it matter one way or the other?

EDIT - actually one good reason I've thought of - noise. Lower value resistors = lower noise, and also phase cancelled with a common cathode.

So yes, I should do two series resistors to add up to 135R rather than paralleled 270R. 2 x 68R - close enough.

Leaving this all here to show my 'workings out'.

[Nick]

Belt and braces tbh. Just use the two 270 and try connecting the cathodes and see if there is any difference.

[Max N]

I think that the anode ccs’s don’t keep the cathode current constant because signal current flows through the load?
If you have a single shared cathode resistor then the current through that might be constant?