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RE: Principles and compromises....

Posted by tube wrangler (M) on August 6, 2012 at 12:37:21

In Reply to: Principles and compromises.... posted by andy evans on August 3, 2012 at 01:00:52:

Hi Andy. Cool discussion!

I'd like to shed a bit of light on this-- just for completion's sake-- with full understanding of what you're doing-- and respect for that.

However-- there's more to this:

(1) All DHTs. Who can argue with that? Not me! You do, however, have to make it work. We know a few things-- (1) Nobody makes THE High-Gain (amp-factor in the 70-100 range) DHT to give an ideal gain factor for a modern amp. That gain factor should be 18-20 db-- total amp. (see "Get Better Sound"-- Jim Smith.
The reason for this is that now we have sources that output 3-5 volts S.E. modulated, or up to 10V balanced. Therefore, the amplifier DB range that's needed.

(a) DHTs always sound really good with balanced A.C. for the heater voltage. A simple center-tap on a filament transformer allows full fidelity at the cathode. Using D.C. heating perverts/distorts 1/2 of the musically modulated waveform every time the modulated signal switches-- which is always occurring, and is at varying rates and frequencies-- as is in the recorded music.
A set D.C. bias on one end of a filament structure is not the same as at the other end-- yet the whole thing should-- ideally-- have a uniform response to grid input, and a uniform radiation pattern to the tube plate. Cathode-to-grid, and grid-to-plate is affected. Balanced A.C. heating is simply superior. Can you do this on every stage and get it to work? This is where the IDT helps you a bundle!

(2) No cathode bypass caps. That's certainly ideal, as I have yet to find the World's first barely usable capacitor! However, if you use only a resistor on the cathode-- or a bias supply-- which contains capacitors or a battery- (a battery IS a capacitor-- AND a sputtery-variable resistor as well--!)--- now, you've got a whole new set of distortion-causing issues.
The advantage of low plate current on a driver stage is monumental-- grid admittance is so superior that it's ridiculous to consider anything else. How you gonna run-- say-- 6 tenths of a milliampere (ideal) on a driver stage if you have only a resistor on the cathode? I suspect you'll use a more powerful tube-- and whoops! up goes that plate current, and DOWN goes grid admittance-- and dynamics-- big time. Cathode bypassing, then, becomes an art-- out of necessity, unfortunately. But it's worth pursuing-- primarily because nothing else works as well. Can one do this cheaply? NO, and that's the disadvantage.

(3) No coupling caps. Certainly agreed! A cap that is in series with a signal is the worst possible thing that one can do to a musical signal. This absolutely guarantees that the stage that is driven by the cap will favor certain things in music and will be impervious to other things that are occurring in music at the same time! Further-- the low-plate current driver won't work here. The power needed to drive the series cap is large. Coupling caps are DISASTER #1-- they are a serious problem that MUST be eliminated.
Well, how about transformer-coupling? Same problem to a lesser degree. Energy loss. But at least, you don't have severe favoring or complete elimination of only certain parts of music like series coupling caps do.

(4) Fastidious output tube! And, why not? There are some great DHT outputs available today-- and they're NOT 300B's!

(5) You gain with two stages. (as opposed to 3 or more). That's true, but the largest single gain here is Direct Coupling-- which simply has no equal. Think of the two stages operating as a Single Stage. That's really what is happening with D.C. coupling, and is why you can drive a large DHT with far less grid input and still have plenty of power to run the output DHT... you're not losing both signal amplitude and current in a coupling cap, you're also not having to overcome the cap admitting SOME forms of energy and blocking others-- this WASTES ENERGY in spades.

(6) "I don't like horns". Correction: you don't like the AMPLIFIERS that you have heard driving horns. Speakers that load the amp more also lose tons of musical detail. They COVER UP for bad amplifiers.
It's the same problem as the grid-admittance energy losses when a coupling cap is in an amplifier. That non-horn-loaded speaker is going to deliver mostly what the power response of the system is-- it will deliver very little TRUE musical detail.

Here is right where the big argument begins: The Low and Middle-Eff. speaker guys will counter that they have tons of detail-- they just use a high output H.F. driver and drive it with a lot of power.
Yes, they do get a lot of "detail". Trouble is, it's not like being there with the artist-- something is missing. In the best of those kinds of systems, the full frequency range is there. Rhythm, Pace, and Timing can be spot-on as far as what you're hearing is concerned, and alignment to the room can be exemplary. Even a form of dynamics is present-- but it is Power-Driven, and bears only the High-Energy resemblance to actual events at the recording venue.
The small stuff- that which makes music present, real, lifelike, present-- is GONE. Oh-- the proponents don't THINK it's gone-- they have tons of detail-- but it IS GONE. For that reason-- loss of micro-signal energy-- those systems can impress, but they can't actually play music.

IF you're having a listening problem with well-designed horn-loaded transducers, it is ALWAYS the AMPLIFIER!

I know this from years and years of theatre construction-- and much more. The entire reason that I designed the first good S.E. amps was a search for musical reality from Horn-loaded transducers-- which we KNEW was THE BEST way to load a speaker driver.

---Dennis---

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