GoodSoundClub - Romy the Cat's Audio Site

In the Forum: Melquiades Amplifier
In the Thread: Single-stage Melquiades vs. DHT amps

Post Subject: DHT driversPosted by nl on: 6/17/2009

Why do we use DHT tubes? it is not because of their technical specifications. They have low transconductance, a problematic direct filament, and they cost a lot.
Obviously, it is because of their sound. Some people like it and some people don't.
The DHT's low gain makes a driver stage necessary.
In Romy's case, I would suggest experimenting with various driver stages. The primary quality is how it sounds -- how it combines with the DHT sound to make an appealing overall result.
Many, many, many experiments have been done regarding different driver stages.
People keep coming back to the 6sn7 over and over again. It is a good sounding tube. They try the 9-pin series (12ax7), high-gm drivers (417a), pentodes (C3m), etc. etc. and come back to the lowly 6sn7.
Why are we experimenting with DHTs at all here? It is because of the possibility of a different sound character than the 6E5P. Using a 6E5P as a driver, with DHT output, compared to a 6E5P alone, will tend to result in "something that sounds like a 6E5P but with DHT coloration, and the fuzzyness caused by a second stage."
Thus, it doesn't make a lot of sense to me to keep the 6E5P as a driver tube. Why not try something else?
I would suggest a 6SN7 or possibly a signal DHT like a 26, 112, 01A, 30, etc. etc. Direct coupled with resistor load (as already exists) is fine.
Some people like the combo of a high-gm tube like 417, 6H30, 6S45, etc. etc. as a driver tube. Other people find them hard, glassy, and not emotionally involving -- the typical complaints of the high-gm series.
The main advantage is that they allow the elimination of a gain stage. Also, they provide a rather low Rp to drive the output tube harder.
However, in this case, there is not much need for gain, so whether you use a 6H30 or a 6SN7 or a 26, you end up with two stages anyway.
The second thing I would suggest (again) is to try your DHT amp running the large horn (600hz up) as far up as it will go -- ie no lowpass filter. That should give a better idea of the overall sound quality of the DHT amp without having to make two to run both horns simultaneously.

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