Small value Ck (like really small)

[Chris Hornbeck]

All of the small value resistors around the valve socket are for the purpose of keeping the valve stable. It's very much along the same line as a "grid stop". A really over-engineered valve stage of all sorts would include these, but mostly, everybody just cheats as possible.


A capacitor across a 10-12 Ohm resistor, with an order of magnitude larger 1/gm cathode impedance is just goofy. Somebody didn't do their sums.


YOS,
Chris

 

[Sam Magrath]

Sam, the discussion about the use of 10R is pivotal to the reason for it being shunted.

If the 10R was a means for monitoring cathode (ie. anode) current, then the shunt cap may well be to suppress the influence of noise on the cathode current measurement process. Many monitoring schemes get messed up by noise - without better appreciation of the use of the 10R all one can say is that the RC corner frequency is 1.6MHz.

Thanks trobbins.

Seems at worst the cap does nothing much, and at best it supplements something else which is unknown. If its purpose is not to shunt HF noise from cathode during normal operation (and with nothing else connected to cathode), I'll omit it from the build.

I will need to check current while adjusting B voltage, but this will be via test points and constant monitoring of current is not required.

Cheers.

 

[trobbins]

Sam, are you copying the NCP-1 schematic from post #8, or was that just an unrelated schematic that appeared out of nowhere? I just can't see in this thread where you actually define where the 10R and 10nF came from for starters.

 

[Sam Magrath]

Sam, are you copying the NCP-1 schematic from post #8, or was that just an unrelated schematic that appeared out of nowhere? I just can't see in this thread where you actually define where the 10R and 10nF came from for starters.

See schematic at post #11. Cheers.

 

[trobbins]

Yes but that schematic has no context. Did you draw/design it? If you designed it then why did you insert the cathode capacitor?

 

[jhstewart9]

6688/7721/7722/7788

A pair of 7788s drove the vertical plates of the HP 1402A 20 MHz scope plugin on the HP 140 Series scopes. The HP 140 Series Scopes where a very successful product line, we sold lots of them.

 

[jhstewart9]

Thanks to all.
I'm at collecting the parts stage.
Please see below.

What will the circuit be when it grows up?

 

[gorgon53]

My take on the small value shunt cap.
10nF is used to short out rf, measuring leads can inject rf-noise and amplifying this could falsify measuring results.
A value of 10nF is a good compromise because higher values turn at rf to soon from caps into inductors,
smaller values would not provide enough shunting.