• 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.

5687 long tail pair

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I used the "preferred current source" found on Rod's page.


Scroll down a bit to find it. (the actual current source is the part above the "output" of course) This is a pretty common design--you'll need to run it from a negative supply of -20V or more, and use the complements to the transistors shown.

Rod gives the formula for obtaining the current:

Some measurements on your chosen transistors (I used BC546) are in order, but you know that about 55-65 ohms is right for Re. The resistor to ground should be 100K.

I'm still refining this circuit, but it does work well, and with 5% resistors and no attempt at matching provides 24dB CMRR. Omit the current source and CMRR drops to 3dB in this configuration.


This circuit was developed with the aid of quite a few people on this board. The thread where much of the discussion occurred is "Plate follower and applications".



The circuit was designed for transparency. The "tube sound" will come through more and more as B+ and bias current are lowered--more and more harmonic distortion will result.

For instance, running the circuit on my bench supply with B+ of 30V and bias of 3mA resulted in harmonic distortion of about -48dB, almost pure 2nd harmonic to the noise floor of -110dB. Although one might suspect this sounds awful it really isn't as bad as would be expected. (I still recommend building the circuit as described)


2001-03-06 3:07 pm

ok, I thougt it was some audiophile reason not to make it balanced.

I got some and e88cc (telefunken goldpins) that I think I will try to design a LTP for them with current sorces. btw should the currentsorces be Mosfet or BJT based ? any prefereces there?

Or maybe I´ll just rescale the bride of son of zen for tubes.

Anybody knows where to find a Pspice library with e88cc?

12AX7/12AU7/5687 are all quite different, with steadily decreasing mu and Rp of ~60k/7k/2k. Using the 5687 helps to maintain a low output impedence.

You cannot directly substitute one for another. There are plenty of circuits published for 12A?7, including the LTP circuit in the "plate follower" thread.

Or, you could use the student version of Pspice. The simulation models work with that without modification.
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