Dynaco ST-35 Newbie cathodyne question

[rotaspec]

Ok, trying to understand DC coupled cathodynes like that in ST35.
I only have a schematic that shows no voltages for the ST35.

Say I want to redesign the input stage for a tube that needs more or less current.
How can I do that without upsetting the phase splitter, or will the self bias in that stage mean that it doesn't matter. I mean, for example, if I pull more current through the 1st stage and drop the plate voltage in the process, how does the splitter stage react to that? What about raising the plate of the first stage by drawing less current? What if I want to alter the bias of the splitter without altering the input stage?
Sorry if this has been beaten to death before. I can see how each stage functions separately, but the plate-grid DC coupling is throwing me.

Gary

 

[kavermei]

Headroom may be reduced if the phase splitters input voltage is way off, since it will be able to swing farther in one direction than in the other. If this causes problems, you can modify the plate resistor of the 1st stage so you get the same grid voltage for the cathodyne as before.

Changing the bias of the splitter will have not so much influence because of the cathode degenerative feedback. Just keep the ratio of plate resistor vs. cathode resistor roughly the same as it was.

 

[rotaspec]

Thanks. I'm changing a cathodyne designed for 12AX7 to use a 6N1P which seems to like a lot more plate current than the 1mA or less that was being run.

I told you that I was a newbie

 

[kavermei]

You're welcome, and yes indeed I think the the 2 cathodyne resistors should be reduced for that tube to sound good in that circuit. Otherwise the tube will generate too much distortion. Same for the 1st stage's load resistor.

The most difficult to do if you're new to this is to determine the correct cathode resistor for the 1st stage. Of course you could cheat and use a trimmer to determine the value that gives you adequate plate current

 

[revintage]

If going 6N1P or 6922 in your Dynaco I suggest you go for 4-5mA through both driver and concertina. Remove the positive feedback and add a CCS on the driver. You should also make the concertina resistors equal.

 

[rotaspec]

Thanks guys. It's not a Dynaco, it's a mixture of designs that happens to include a DC coupled cathodyne modeled on the Dynaco. Output stages are PP 6P1.
I have just done a conventional gain stage with a 6N3P and found it needed at least 3mA plate current before it sounded decent.
I have to work by ear judging distortion, as I don't have much in the way of test gear (poor quality handheld scope, multimeter and dubious AF gen).