cathode follower calculations

Pentode

guess

(.25w

Got a schematic?

What's the mic capacitance?

A low signal could be due to a number of causes, but lack of bottom end suggests that there is too short a time constant formed by the microphone capacitance and the resistance effectively across it. Is the grid leak going directly to earth or is it bootstrapped to the cathode? How are you feeding in the polarising voltage?

For a capacitor mic you need hundred of meghohms between grid and earth (and the mic and HT+). Either you need special high-value grid leak and feed resistors or you need to bootstrap them so that the 'earthy' ends follow the cathode signal and there is effectively no signal voltage change across them, so they draw no signal current. Even with a bootstrap. 10 megohms is a bit low.

Circuit 1 at:

...shows bootstrapping for the grid and the HT+ with some typical component values.

An alternative system which uses fewer components is shown in Circuit 2; but this has the disadvantage that the microphone backplate (and possibly casing) will be live. It also demands a very well smoothed and decoupled HT+ supply.

If you have plenty of HT to spare and you only need 40v across the microphone (which seems very low to me, as I would have expected about

I'd forgotten how low the Gm of tubes were. Loading the transformer output could drop the gain of the cf and change the bootstrapping, especially if it's a step-up transformer.

Toobs are really awful. Why not use a nice jfet and an opamp?

Gm booster for toob cathode follower:

Way back when I was a technician in MIT Building 20 I was able to demonstrate that plate drive to 50 Ohms was more accurate that the typical cathode follower.

I've often been puzzled that the toob crowd haven't discovered that a tranny under a cathode can do wonders... and allow HV operation with cheap semiconductors. ...Jim Thompson

Another neat example, a "hybrid op-amp":

But those semiconductors have to handle B+.

Duncan amps...

has nice tube models.

I downloaded them ages ago, but never did anything with them.

Maybe I'll have some fun and see what I can conjure up ;-) ...Jim Thompson

These are on my bookshelf....

Note the price ;-) ...Jim Thompson

Le Tue, 26 Mar 2013 19:12:54 -0400, bitrex a écrit:

Huh!

R2 serves no purpose. Q1 has its BE junction reverse biased.

In case you meant an NPN, it doesn't work either...

Sorry, made a mistake in the schematic. Input to U2 should be taken from the top of R2.

Practice indicates at least 30V and not much more than 100V across the capacitive mike element. However, theory indicates that a large voltage bias will cause more non-linearity than a smaller bias; in the limit (zero bias) no non-linearity - but then no signal either!

Most capacitive mikes i have seen use 300 megs, and i think i still have a lot of those, so if you need one or two i would need a name and address..

check the transformer, if you drive a transformer from a source Z that is too high, you will loose the bottom end. What Z is the transformer designed for?

Mark

How loose? Will it fall off? ;-)

It wouldn't be a follower if you did that. Just pull a smidgen of grid current in U2 and it works fine.

?-)

I suspect that it rolled off.

?-)