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In the Forum: Melquiades Amplifier
In the Thread: Single-stage Melquiades vs. DHT amps
Post Subject: My experience with DHTsPosted by nl on: 11/30/2008
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I am interested that Romy has found some interest in DHT amplifiers.

Many, many amp builders have used many, many tubes over the years, including many IHTs, triode-wired pentodes, etc. Japanese builders have used big transmitting pentodes driven in deep A2, for example. They all go back to DHTs. The 6C33 has some adherents, including Lamm and also the well-regarded Almarro amps. However, even Lamm is now using the GM70 for his $140,000 statement amplifier.

DHTs have a special character. I wouldn't call them "neutral." But, if I wanted "neutral" I would use a gainclone-type amp. DHTs can be among the lowest-distortion amplification devices ever made. However, most DHT users like the funny addition they make to the sound, and tend to emphasize this aspect.

In general, DHT amps are lacking in power, dynamics and bandwidth. This is exacerbated by rather sloppy powersupplies and other such engineering decisions. The problem is, active powersupplies, huge capacitance and other such methods tend to detract from what people like about DHTs. Many DHT amp builders have used the kind of generic large-value electrolytic capacitors used by Romy in his Milq amplifiers, and took them out in favor of other solutions.

Once again, a simple gainclone amplifier, which can be built for $200 or so, will crush a DHT amp in terms of dynamics, bandwidth and headroom. It will crush the Milq amp too, despite Romy's possible protestations otherwise. It will crush any amp that uses an output transformer, as regards those qualities. However, it just doesn't have that DHT (or Milq) tone.

I have used Russian high-gm tubes like 6C45 in amps before, including single-stage designs very much like Romy's single-stage Milq. These have many attractions. However, in my experience it just didn't have the tone. Jim de Kort of VT52.com came to the same conclusion, regarding his WE437 spud using amorphous output transformers, on his 107db horn speakers. He went back to his 26-10-45 combo, three stages instead of one.

In my experience, a very simple 6sn7-6sn7-45 amp destroyed the 6C45 spud, even with the disadvantages of three stages instead of one.

I don't have time to try every possible combination. The high-gm tubes tend to have a certain glassiness which doesn't appeal to me. I even bought some 6E5Ps to give it another try. It would surprise me if the outcome was much different than my experiences with the 6C45 however.

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