I am getting a dedicated third amp to drive my upper bass channel and I just figured out that when I go with my midrange channel at the line level passive filter then I would need 0.0000302uF capacitor against the amp's input stage, that means… 30pF… Holly cow! This looks like it is in the area of those fantastic air capacitors that I used with such a great success in my phonocorrector! The crazy S2 driver driven directly by ML2 behind a 30pF air cap would be insultingly interesting animal!

My tail is trembling….

I too have thought a while about exactly this... there is something that troubles me deeply though about this approach. I have a theory that too small a cap may not work out as well as one might hope.

Now, shall I do a Romy and delay revealing my thoughts, lest I prejudice the outcome of this experiment?

Actually, speaking of the ML2, am I right in thinking it uses a mosfet follower to drive the output tube?

cv

*** I too have thought a while about exactly this... there is something that troubles me deeply though about this approach. I have a theory that too small a cap may not work out as well as one might hope. Now, shall I do a Romy and delay revealing my thoughts, lest I prejudice the outcome of this experiment?

Well, my enthusiasm on the subject is purely theoretical and is based on my wishful assumptions. I have a very firm and a very clear vision what the midrange driver should do in the end (frequency response-wise) and I am quite confident that I will be able to make my amp to have the same slope  using  an ultra small cap. The ML2 had a capacitor shunted input that I took out long time ago.  Also, at the 30pF level the positioning of the cables, layouts,  the Miller capacitances, the grid's capacitances, the many other things are mater. Therefore my estimate of 30pF is purely speculative and is based on just simple circulations. In the real would I am planning to run my 12db/octave RTA against the midrange driver and to dial-in the correct response with an air variable capacitor. After I will be done whatever will be left will be a Result. How it will sound I have no idea and it is what I’ll be very interested  to learn. Yes, Chris’, feel very free to share your thoughts and doubts on the subject and I really would like to learn what I do/think (might do) wrong.  However, be advised that it if it turn out to sound good then I will take legal actions against you. :-)

*** Actually, speaking of the ML2, am I right in thinking it uses a mosfet follower to drive the output tube?

Nope, ML2 is a 3-stages cap-coupled amp: a paralleled 12AX7 at input, a paralleled 6N6P in the middle (a Russian version of 5687) and a regulated 6C33C at the end.

The caT

... it may well work superbly. Where it may not be ideal - well, let me explain.

The Miller capacitance is not constant but will have some dependence on level (this effect is much more pronounced in silicon). If we feed the valve from a small capacitor, it forms a potential divider (2 caps, one of which is varying a bit unpredictably with the signal).

This is mostly conjecture and neglects the fact that you have a resistor to ground there too which will smooth things out. Then of course, there is the feedback too, so please don't be put off at all! I shall call my attorney in the meantime...

Look forward to hearing how it works out.

cheers

cv

I see, I never thought about it... If you are correct then I will not be able to say, “how it works out”. I would definably be able to say “how it sounds” in regards of a speaker-level 3uf of “Vitamin Q” vs. 30pf at line level, but I will not be able to detect the minute fluctuations of the lower cut-off with a signal. And yes, ML2 runs a global feedback…

The good part in all of this that a friend of mine after reading this post contacted me and informed that with a targeted eclectic crossover point of 3500Hz and the amps inputs of 150K I would not need a 30pF cap but 300pF.  (I always loose something while convert the things from uF to pF) If so, then perhaps it would be a little easier and I would be picking up less capacitive mess, wouldn't it be?

So, Chris, when I try it (and I fell that I’m very intrigued) then what should I watch for?

Thanks,
The caT

Ah, those figures seem more reasonable - 30pF seemed a bit low.

Well, I'm not sure what you  should be on the lookout for - as you say, it's a completely different animal to having a 3uF in line with the S2.

What I would suggest, and this isn't too difficult given that you'll be using an adjustable aircap, is this:

1) Try the initial setup, ie with the 300pF

2) Add a 150k in parallel with the 150k to give 75k. Double the cap to 600 pF. See if you can tell any difference. I assume your preamp is just as happy driving 75k as 150k and the cables you are using don't have too much capacitance.

This assumes your aircap will got 600pF, which is getting quite big. If you don't mind changing the grid resistor to a higher value, you can try smaller values of capacitance. If R is the resistance in kilohms and C is the capacitance in pF, just be sure to keep the product, RC, at 45000. Eg try 150pF and 300k, 96pf and 470k etc.

I myself am building a 2 stage amp to drive some ribbon tweeters. The input tube will be E810F triode connected, which has an enormous Miller capacitance, so instead of an RC at the input, I decided to try a stepdown transformer which I wound myself. The filter is formed using a resistor in series with the primary inductance, and the stepdown really reduces the Miller cap of the E810F. The tx also enables direct coupling to the output stage (-ve supply) which will have an air-core tx, giving an additional rolloff at 600Hz to protect the ribbons. No caps at all and I do away with the transformer on the ribbons as well...

The  toroids are less than 2" dia. and made of a rather special cobalt amorphous alloy. Wound with Chimera labs single crystal copper (which is probably about a million crystals after the winding process).

Anyway, as I'm some way off from getting the rest of the project sorted, I could send you the transformers to try out for a few weeks... preamp would see a load of 20k and up. I'd be *very* interested to hear how they compare to the aircap in your setup. Lemme know.

cheers

cv

Thanks, I will take it all under advisement.

Yes, I  consulted with my magnetic experts about the opportunity to have an air-core output transformer driving the HF channels. They brought 1 hour-long technical justification why it should not be a good idea. Their evidences brought me actually to the state of… interest. I would like actually to hear it, because I do not have in this subject any tangible feelings between the application of those scientific methods of analyses and the subjective results. I will probably have the power amps that would be able to accept the different output transforms sometimes after the New Year. I would love to try your idea of the air-transformer and I 'm pretty sure I will.

Rgs
Romy