12AU7 Phono Stage

[JohanB]

Originally Posted by piano3

That is tremendous work JohanB! Many thanks for that and I hope many members read it. I had estimated by ear on 75 microsecond RIAA stage that the values for these triode connected pentodes had to be in this region though I had not suspected quite as high as 5.6 pF!


your last post was while I was writing this-yes that seems more believable!

Yes, the g1 to anode capacitance is only some 30-50 micro-pico-farads when the shildings are grounded, so anode and g3 should not be measured together with g2 for capacitance. It is the stray capacitance between the shildings inside that is left floating, that makes the higher readings when the anodes (and g3) are included.

By the way, do you have problems with microphonics with these tubes?

Many people say that they are, but I suspect there in many cases are VHF self-oscillation, that's causing the exaggerated microphonic rumors......

[Salas]

Thanks for the work Johan. They need Rg stopper in most cases, which works against best noise in a head amp to a point. Kinda prone to oscillation sometimes, yes. When cranked for gain with much Miller at play you know how it goes when stray inductances are meeting the grid.

[piano3]

I used trioded E180F in a mic preamp and you would certainly get a nice silvery shimmer if you banged the case but in actual use there was never a problem. I am currently using trioded E810F(and I can drop in E180F,E280F or EF184 as I have switchable capacitance in the HF EQ stage) in the second stage of my phono amp without any problems whatsoever-you have to really hit the case near the valves to get any ringing at all. I always use grid stoppers with these kind of valves.