• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Philbrick opamp bases

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#3
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.
 
#4
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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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.
 
#6
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
 
#7
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.

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..
 
#8
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.
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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
 
#9
Hi Chris,
One area where they might be useful is in a high performance power supply design. IIRC Rap shows such an application for these op-amps in one of his articles. You'd have to wonder though at how good any of those carbon resistors are likely to be at this point. 😀
 
#10
Hi Kevin,
Those resistors are probably not useful anymore. You could rebuild it, but then you may as well simply build it as a new design.

More than one high performance regulator used a tube op amp type circuit.

-Chris
 
#16
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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