High gain pentodes

[Miles Prower]

Originally posted by Geek
Some of those pentodes are pretty noisy/microphonic for a V1 position though.... 6CB6 for example, only a few brands are suitable.

But that could be said for most though *shrug*

Until someone invents a plastic resin that is conductive, and can stand up to red heat without deterioration, and is clear (dunwanna lose the glowey bottle coolness) that one can use to fill VTs instead of filling 'em with vacuum, microphonics are (more or less) going to be a problem.

Of course, you see something like "VHF amp" in the spec sheet, look out. Since these are intended to operate at RF, all due care to keep 'em from making noise at AF wasn't taken.

[ray_moth]

The EF94 / 6AU6 seems to be a a very underrated pentode- it's usually much less microphonic than most EF86s. I rate it VERY hightly.

I rate it highly too, but quality was variable and you have to find a good example. Both my GEs are very microphonic, whereas my RCAs are not. On paper, 6AU6 is superior to EF86 in many respects, aqltohugh it is rather noisier. It has very flat plate curves at low current, making it a good candidate as a CCS provided that you keep its plate-cathode voltage above 100v.

[Boris_The_Blade]

I've used a 6SJ7 with success, though they are microphonic as well, so it may not be suitable depending on you tubes' location. Two 50V zeners stacked create a nice screen supply, but bypass it with a high and a low val capacitance for noise issues. Lots of gain with this, super high input impedance, you can usually get these NOS cheap off ebay, plus the metal shell is good shielding.

[coresta]

Originally posted by ray_moth


I rate it highly too, but quality was variable and you have to find a good example. Both my GEs are very microphonic, whereas my RCAs are not. On paper, 6AU6 is superior to EF86 in many respects, aqltohugh it is rather noisier. It has very flat plate curves at low current, making it a good candidate as a CCS provided that you keep its plate-cathode voltage above 100v.

or have a try to the real 6AU6 for AF : the 7543 , NOS can be found for cheap on ipay

[sparrott]

I have good luck with using triode strapped 6JC6A pentode with CCS source as an initial gain stage in a couple of amps (not guitar amps).

I believe it is has a GM of 15.

Scott