[sdiy] STUPID tube questions

[metasonix]

>is it true that e.g. a triode stage large signal
>characteristic measured at DC differs from
>those measured at -say- 10kHz or 20kHz?

No

>I read something like this every now and then.
>What is the reason? Parasitic capacitance or
>time of flight of the electrons?

If the tube has low capacitance in general it is not a problem. 

Where did you get this "electron time of flight" crap??
Transit time in a small signal tube only becomes important
at more than 10 MHz.
(Langford-Smith chapter 23 section 5.)

I thought you were building a guitar amp.

>I found that output and PSU transformers for tubes
>are quite expensive, arround 70 EUR.
>Is this shameless abuse, or justified pricing
>for low volume production?

Perfectly justifiable. The market is very small.
Output trans are difficult to make.
Unless you are going to buy 10,000 pieces, don't complain.

>If you have a different characteristic at 20kHz 
>already you have a problem I would say. 
>(Typically a 12AX7 stage with a 
>220k plate resistor trying to drive the next triode stage....)

Caused by Miller effect. Use a pentode.

>Since beauty is no criterion, I could
>perhaps use two of them back to back.

Yes

>So far I've only seen ELA transformers
>with one primary coil, not suitable
>for push-pull pentode stages.
>Of course I could use two of them in series...

No. Must use a center-tapped or double primary on the same core.

>I saw an old app. note where a zener is used as a "grid voltage regulator".
>It replaces the cathode resistor and bypass cap. 
>It allows you to adjust the tube operating point, using only a single supply.
>It seems necessary if a tube circuit is to handle +/- 5-10 volt signals at
the grids.

<groan> I give up. 



uncle eric
metasonix.com