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Original Message

RE: The road ahead

Posted by tube wrangler on June 24, 2012 at 13:43:50:


You're not entirely wrong here. Unfortunately, it's your emphasis on only one part of an entire design that is way over the top.

The Miller Capacitance of a DHT is a very real parameter, as you state, and it must be considered, as you correctly point out.

When I build an amplifier, that design is supposed to replicate a studio recording in the best way that I can build.

ALL amplifiers are sets of compromises, and in all cases, one item in a design comprising many parts and calculations may have to be changed in one or another direction-- but not too far!-- in order to allow for another important consideration-- to NOT go ignored!

In this case, I'm not willing to give up the transparency and speed of a High-mu, low plate current driver just to satisfy a desire for the ultimate in powering Miller Effect inside a DHT, although I do consider DHT bandwidth very important, as I do slew rate.

I fully understand the truly awesome performance that comes from a high-mu, low plate current driver-- as opposed to higher-current stages which are much harder to drive themselves AND THUS SEVERELY COMPROMISE THE SIGNALS COMING INTO AN AMPLIFIER FROM MUSICAL SOURCES-- that are used to drive amplifiers..

In the case of the High-Mu driver stage, it is very important to avoid overloading its output... and I recognize that you have noted this.

Accordingly, I allow wide voltage swings with high-impedance loading on the driver stage output. Certainly, the Miller Effect of the DHT being driven is also loading this stage, but I am able to get the bandwidth I need anyhow by carefully evaluating all of the factors that impinge on it.

So, the simple calculation of the filter network that is always part of a driver stage's loading, is not the only consideration there.

In your own case, I would be cautious of overestimating Miller Effect in DHTs-- different brands are very different, and plate currents and voltages that are used in a circuit operating the DHT are also contributors to the same tube exhibiting DIFFERENT Miller Effects under different operating conditions.

Miller Effect Capacitance is NOT a numerical figure that one can simply plug into a formula-- it changes with operating conditions, and with brands of tubes.

Overall Balance is everything.

In this case, Miller Effects have been recognized, but the much larger other factors present in the circuit design have also been considered-- and kept close to THEIR optimums-- all the while still keeping in mind the limitations imposed by Miller Effect, and not going overboard on that, either.

We want ideal TOTAL loading-- of the driver stage as much as is practical so that Miller Effect-- which really isn't that bad at all-- is really the only thing left that negatively affects driver stage performance.

By using such a comprehensive, overall approach, the Miller Effect that is present here simply isn't a problem.

The final result of an entire amplifier's performance is what I'm after, and in this case I have it right where I want it.

---Dennis---