• 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.

For what are high mu triodes still useful?

For what are high mu triodes still useful

Other than guitar amps where one wants a lot of gain and plenty of distortion (12AX7 properly biased e.g.) where would one want to use a high mu triode?

It would seem inappropriate for the input tube of a phono preamp because of the high Cmiller. Maybe second stage??? As a LTP PI maybe but the miller and high Zout would seem to be a problem.

Would high mu be advantageous for driving some low mu output tubes?

Just curious as to why one might intentionally choose high mu unless they wanted to use gobs of feedback for some reason.

mike
 

ray_moth

diyAudio Moderator Emeritus
2004-01-27 8:55 am
Jakarta
It does depend on the tube itself. I wouldn't use a 12AX7 for any purpose (as you say, that's a guitar amp tube), but a 6SL7 is good. It's useful as the input LTP phase-splitter stage of an all-differential PP amp.

It can also serve well, in resistor-loaded (or SRPP or mu-follower or mu-stage or CCS-loaded) configuration, as the input (voltage amplifier) stage of a Mullard-style PP amp where you want to apply a large amount of NFB and so need plenty of open-loop gain, e.g. for a pentode-mode PP output stage.
 
Mike,

Your focus is too narrow. Other types, in addition to the high mu, low gm, and high RP 12AX7/6SL7/5751 family, that have high mu are available. The 12AT7's high mu, high gm, low RP, and distortion spectrum make it extremely well suited to LTP service.

The nicely linear 6GK5 is another high mu, high gm, and low RP type that's useful both in phono preamps and in power amp voltage gain blocks. The low Miller capacitance of the 6GK5 does not adversely affect MM phono cart. performance. A large Miller capacitance (as in a 12AX7) can combine with cart. inductance to yield mediocre results.

BTW, stage gain from a 12AT7 section and a 12AX7 section are usually quite close, in spite of the mu differential. The low RP of the 'T7 allows relatively large load resistors to be employed.
 
Re: For what are high mu triodes still useful

mashaffer said:
Other than guitar amps where one wants a lot of gain and plenty of distortion (12AX7 properly biased e.g.) where would one want to use a high mu triode?

I've used high-u duals (6SL7) for an LTP phase splitter/voltage amp. (For another project I wanted mogain, and used cascodes to form an LTP phase splitter.) You could also use something like the 6SF5 as a "VAS" for a preamp with gNFB. They'd also work to make "anode followers" (another type of feedback amp).

It would seem inappropriate for the input tube of a phono preamp because of the high Cmiller. Maybe second stage??? As a LTP PI maybe but the miller and high Zout would seem to be a problem.

That Cmiller needs to be watched. It can become problematic; same deal when using transistors. If worse comes to worse, you can always drive it with a cathode follower to get the Zi down.

Would high mu be advantageous for driving some low mu output tubes?

Not unless you included a grid driver, something like a cathode follower or a source follower. These high-u, small signal triodes don't have enough current sourcing to drive these low-u power triodes. The big problem with those is Cmiller, and that they can start pulling grid current even before Vgk actually goes positive.