Tube op-amps...

[Eric at Svetlana Electron Devices]

At 05:46 PM 4/20/98 -0400, you wrote:
>   Okay, here's something for you tube guru's...I was helping my boss 
>clean out the physics "bones" room and I ran across an old Heathkit (they 
>used to have their factory just down the road aways so the University got 
>tons of their stuff) tube op-amp lab demonstrator.  The thing has 4 
>"op-amps" plus a booster amp (which looks like just another op-amp to 
>me).
>   Anyone use one?  Am I right in thinking that I can use this beast to 
>put together a seriously wild audio processing unit?  And I think I know 
>where a second one is...Hmm...Eight tube opamps??!
>   Thanks for any comments or suggestions for its use.  :)

  It's possible to use such op amps for audio. Keep these essential
facts in mind:

a) their gain-bandwidth product tends to be low. They will usually
give a full 20 kHz frequency response with closed-loop gains of
10 or less. 

b) for this reason, beware of jump resonances when using them in
a high-Q bandpass or state-variable filter or similar circuit.
This will produce effects quite different from thos intended (though
you MIGHT find the distortion interesting---who knows---)

c) Each tube opamp MUST be DC offset nulled. This is tube-world,
not chip-world.

d) be careful--probably these work on +-300 volt power supplies.

e) Output current is limited to a few mA--you can't use these
to directly drive LEDs, lamps, speakers, etc. (except for the
"booster amp", which is probably just a cathode follower. It MIGHT
be usable for such drive applications.)
Otherwise they can be used in EXACTLY the same circuits that we
use IC opamps today.

e) if still interested, see my article about using tube opamps
in the 3/92 issue of GLASS AUDIO magazine. It is available from
Audio Amateur Publications, PO Box 176, Peterborough NH USA 03458,
phone (603) 924-9464.

If you need further help, e-mail directly to me. I do have
schematics and other info about Heath (and many other tube)
opamps.

Eric Barbour
Svetlana Electron Devices
Portola Valley CA USA