For what are high mu triodes still useful?

[mashaffer]

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

[Boris_The_Blade]

Don't get me wrong, I'm not a huge fan of them, but...

Phono cartridges usually have low output impedance, so Cga isn't a problem. Most popular vintage integrated amps use them.

[coresta]

Hi , I choose them for CF amps: for line driver stage or before output toobz : EF184, E282F , E186F are really nice for high swing / high current loads

[ray_moth]

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.

[Eli Duttman]

Mike,

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

The nicely linear 6GK5 is another high mu, high gm, and low R_P 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 R_P of the 'T7 allows relatively large load resistors to be employed.

[SHiFTY]

The high gain, linearity and low noise of an EF86 or EF37A pentode makes them ideal in the input stage of many amplifiers. If there is too much gain you can wire them in triode...

[rdf]

Followed by a CF driving big, transmitter-type output tubes.

[Miles Prower]

Originally posted by mashaffer
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.

[mashaffer]

Thanks for the information guys. It is a complex topic isn't it.

mike