Philbrick tube opamp.

[brad]

I've heard about these things before, but never realised how relatively simple they are.  Here are a pair of schematics:

Original datasheet copy: http://www.national.com/rap/images/CCC.gif
Contemporary redraw: http://www.ibiblio.org/obp/electricCircuits/Semi/03235.png

Could this be converted for modern, low voltage stompbox use?  Opinions, tube aficionados?

[The Tone God]

Could this be converted for modern, low voltage stompbox use?  Opinions, tube aficionados?

Low voltage, I doubt it. I don't think there would be much tonal advantage to go though all that but thats just my opinion.

Andrew

[brad]

It's not necessarily about tonal advantage, but more about it being unique!  Imagine if someone would have had the foresight to build a distortion with a tube opamp back in the 50s. 

Perhaps low voltage tubes like the 12U7 would work rather than 12AX7s.  I believe those circle dealies in the right hand corner of the schematic are acting like old style diodes, am I right?

[brad]

The later K2-SR looks even more straight forward:
http://www.philbrickarchive.org/k2-sr_operational_amlifier_for_r100-a_power_supply.htm

(check out those gut pics!  )

[Dolly Parton]

When I first started learning about JFETs I cooked up circuits similar to this.  Not that spectacular, but easier to do...

[Dolly Parton]

PS  Helps if you match the diff pair(s), especially if low gain devices are used.

[brad]

Do you mean discrete jfet opamps?  Would you mind sharing what you did?

[puretube]

don`t you just love the +/- 300V DC ( = 600V ! ) supply ? 

[brad]

Have you ever used tube opamps, puretube? (At less not at 600v I hope!)

[puretube]

not that kind...

[JimRayden]

don`t you just love the +/- 300V DC ( = 600V ! ) supply ? 

Yeah it's pretty cute. Well it's nothing a usual 2x250 tube tranny with a bridge recto couldn't handle.

Hmm, I wonder what the clipping looks like... much different from the tops-chopped-bottoms-squished triode stage?

-------
Jimbo

[Dolly Parton]

Do you mean discrete jfet opamps?  Would you mind sharing what you did?

Emphasis should have been on similar in my post.  I used a single ended JFET diff amp for a booster (only breadboarded for evaluation/learning purposes), so not quite the same.  Even more need for matching as the circuit was run without feedback.  Matching would still be advised for a full blown op-amp though, especially if you're using low gm devices and running the amp at moderate to high gain.

You could probably convert those schematics you linked to (thinking about the last one in particular) by using JFETs at a (lot) lower supply and scaling down the plate & cathode resistors to suit.

Overview of (discrete) opamps here:

http://www.passdiy.com/pdf/diyopamp.pdf

[brad]

Cool.  Thanks uh...Dolly Parton! 

Puretube:  do tube opamps even perform well enough to be used in these sorts of applications, or am I chasing a red herring?

[Peter Snowberg]

http://ed-thelen.org/dc_amp.gif

Note the voltage requirements of -320V, -250V, -200V, +75V, and +250V. 

http://ed-thelen.org/ifc_comp.jpg

The missile system I helped restore used 54 modular dual opamps built around a set of 5 tubes on a modular chassis (5755, 6AU6, and 5687). That's 108 opamps in 1957. The equipment was a descendant of the M-33 gun director which was a descendant of the M-9. The M-33 used a whole mess of opamps too, but fewer than the Nike Hercules' 108. The M-9 was based around something like 12 opamps.

Performance was good enough to steer a missile to within a couple of meters of a target which is 150 kilometers away. The same opamps were responsible for shooting down lots of bombers and V1s over London. 89 of 91 V1s in August 1943 to be exact.

They work very well.

[A.S.P.]

information lost...