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Philbrick opamp bases

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kevinkr

kevinkr

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I have a bunch of these, might be useful in a high performance tube power supply as they're basically high performance op-amps. So far I have not figured out how to open them without trashing them totally.
I have K2-W, etc.

Robert Pease (one of my engineering heroes) has written extensively about these vacuum tube op-amps, and you can read about them on the National Semiconductor site here: http://www.national.com/rap/vacuumtubes.html

Particular emphasis is given to K2-W and K2-XA as well some earlier types.

V1 in both is 12AX7A, V2 is 12AX7A in K2-W, and 6BR8A in K2-XA, note the K2-XA also uses a transistor, but without closely studying the schematic I have no idea what it is doing.
 
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Eli Duttman

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V1 in both is 12AX7A, V2 is 12AX7A in K2-W, and 6BR8A in K2-XA, note the K2-XA also uses a transistor, but without closely studying the schematic I have no idea what it is doing.
Click to expand...


I'll venture a WAG. Opamps have a differential gain block at their I/Ps. Differential amps work well with constant current sinks in their "tails". My guess is that the transistor is serving as a current sink for the I/P diff. gain block.
 
anatech

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Hi Kevin, redrabbit,
They would work as a front end for a preamp or amplifier. I know of one tube preamp that uses this concept but not those assemblies (as you would expect). They used a bipolar 150 VDC supply for this arrangement.

So, why not give it a whirl? Mind that the output terminals are high impedance points, also as you would expect. This commercial design used 6922's instead, not surprising from this particular designer.

-Chris
 
kevinkr

kevinkr

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I abandoned global feedback designs about 16yrs ago, and ain't going back.. :D The K2-W has an open loop gain of 70dB or so - far more than I am comfortable with in most applications.

Actually the transistor in the K2-XA performs a different function, but I have not bothered to figure it out yet.

RAP knows quite a lot about these devices as he worked at Philbrick in the very early days - and his articles are extremely entertaining. I'd read them.

The link I provided in my earlier post provides a lot of information on these devices, and is well worth the time to read..
 
anatech

anatech

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Hi Kevin,
I abandoned global feedback designs about 16yrs ago, and ain't going back.. The K2-W has an open loop gain of 70dB or so - far more than I am comfortable with in most applications.
Click to expand...
That's fair enough.

No reason not to play with it though. Here you have the sub circuit prebuilt and you could use a short feedback loop.

I have read those articles, many years ago and then a refresher recently.

-Chris
 
jan.didden

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denny9167

denny9167

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Audio-x-press has several tube based power supplies in this article, and I believe one is based on the Philbrick design, and I have read some US Navy articles that that list power supplies like this to give at .1% regulation! The actual Philbrick design is slightly different, Ive spiced out the original, pretty d....n good!

http://audioxpress.com/files/bicknell2890.pdf
 
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Tubologic

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The K2-XA is an improved version of the K2-WA and can be used in the same circuits and applications. These early opamps should be considered as historic pieces and have very poor specs by comparison with modern opamps. (even the old venerable µA709 has far better specs). The main limitations are low gain/bandwidth product/slew rate and poor offset stability. They could be used in audio but don't ask for too much gain or they won't even pass the 20KHz audio frequency requirements. Still, these are fun for experimenting and you can find more info's about their use in audio applications in the 1992 Vol.4/Nr.3 issue of GLASS AUDIO. ("Vacuum Tube Op Amps Applied" by Eric Barbour, p.12)
 
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