Tube direct box cicuit

[paul tumolo]

i am looking for a circuit diagram for a tube direct box. something with an
input impedance of at least 10Mohm. thanks.

[Justin Ulysses Morse]

Scott Dorsey <kludge@panix.com> wrote:

> Go to the RCA Radiotron Handbook and look up "cathode follower." Put a
> transformer on the output to balance it. The input Z is entirely due to
> the leak resistor, so if you use a 10M leak resistor, you have a 10M input.
>
> Very clean and transparent if it's biased right. The transformer will
> be pretty much all of the coloration and it'll be damn near impossible to
> overload.

How do you calculate the output impedance of a cathode follower? I
think I know how to measure it, but I'd rather do the math. It can't
be higher than the cathode resistor, right?

Also, I know any cathode follower will have a gain less than unity, but
what can you typically expect from a 12AX7 or 12AY7? I know a 12AY7
has lower gain than a 12AX7 in a typical plate-loaded circuit, but is
there a substantial difference in the signal level you'll get out of
them in a cathode follower circuit? Seem like a good idea to use both
halves of a 12AY7 in parallel and thereby use a lower-ratio output
transformer.

Sorry, I dunno where Johnny put the Raditron. So I ask and I ask.

ulysses

[Scott Dorsey]

Justin Ulysses Morse <ulysses@rollmusic.com> wrote:
>Scott Dorsey <kludge@panix.com> wrote:
>
>> Go to the RCA Radiotron Handbook and look up "cathode follower." Put a
>> transformer on the output to balance it. The input Z is entirely due to
>> the leak resistor, so if you use a 10M leak resistor, you have a 10M input.
>>
>> Very clean and transparent if it's biased right. The transformer will
>> be pretty much all of the coloration and it'll be damn near impossible to
>> overload.
>
>How do you calculate the output impedance of a cathode follower? I
>think I know how to measure it, but I'd rather do the math. It can't
>be higher than the cathode resistor, right?

The math is in the Radiotron Handbook. For a triode with a single
cathode resistor, Rk, the output Z is Rk/1+gmRk. For a pentode you
fudge the gm for whatever the operating point you have the screen set
at. I don't remember the other configurations but they are all in
the book.

>Also, I know any cathode follower will have a gain less than unity, but
>what can you typically expect from a 12AX7 or 12AY7? I know a 12AY7
>has lower gain than a 12AX7 in a typical plate-loaded circuit, but is
>there a substantial difference in the signal level you'll get out of
>them in a cathode follower circuit? Seem like a good idea to use both
>halves of a 12AY7 in parallel and thereby use a lower-ratio output
>transformer.

You can get as low an output impedance as you want, as long as you don't
want much voltage swing. From a 12AT7 you should be able to drive a
50-ohm load up to -10 dB without any linearity issues. Look at the output
stage of the Harman Kardon Citation II for a nice example of a well
designed follower output stage.

>Sorry, I dunno where Johnny put the Raditron. So I ask and I ask.

You need it! It has everything you need to design tube or fet circuits.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Chris Hornbeck]

On Tue, 02 Sep 2003 18:21:01 -0500, Justin Ulysses Morse
<ulysses@rollmusic.com> wrote:

>How do you calculate the output impedance of a cathode follower? I
>think I know how to measure it, but I'd rather do the math. It can't
>be higher than the cathode resistor, right?

The active device's output impedance is the reciprocal of its
transconductance. Circuit resistances all contribute as you'd
expect 'em to.

>Also, I know any cathode follower will have a gain less than unity, but
>what can you typically expect from a 12AX7 or 12AY7? I know a 12AY7
>has lower gain than a 12AX7 in a typical plate-loaded circuit, but is
>there a substantial difference in the signal level you'll get out of
>them in a cathode follower circuit? Seem like a good idea to use both
>halves of a 12AY7 in parallel and thereby use a lower-ratio output
>transformer.

Lightly loaded high-mu devices have gains of about mu/mu+1, IOW,
pretty darned close to unity.

A good way to think about loading is that loading should not be
heavier for a cathode follower than for common cathode use. The
device operates on the same load-line, same plate curves, etc.


Chris Hornbeck