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

Post Subject: One-stage DHTsPosted by nl on: 12/3/2008

Gordon Rankin of Wavelength Audio did a series of one-stage DHTs with a step-up transformer input about ten years ago. I think they were called the Mercury series. By all accounts, they sounded good, but of course had limited gain, not much power and had to be driven by a powerful preamp, which involves at least one and probably two stages if you use tubes. So, it amounts to a two or three-stage amp in the end, with extra complexity. My 6SN7-6SN7-45 amp had about the same overall gain and same overall output power, and the same overall number of stages.

I think using a DHT amp only over 3000hz would be somewhat self-defeating. I would try to use it at least over 600hz, and use a 45 tube, not a 300B. The 45 is a wonderful tube for running compression drivers. If you use a 45, however, you probably won't have enough gain even with a 1:4 stepup. Personally, I would forget about the input transformer, and just use a 6SN7 driver. RC coupling is fine. Many people like carbon comp resistors for the plate load, and paper/oil capacitors for the coupling cap here. PIOs take some time to break in. You might later try substituting an inductor for the load resistor. Of course you want to use a good 6SN7, not a new-production Russian or Chinese model.

A nice 6SN7-45 amp, RC coupled with carbon comp/PIO, and a decent powersupply that doesn't sound drecky (which probably means NO cheap high-value industrial grade electrolytics, or at the very least a film cap that is isolated from the electrolytics with a resistor or choke), and whatever tricky bias schemes turn you on, would be a good intro into the sound of DHTs.

Pay attention to the quality of the filament supply. The Tent Labs boards seem to get some acclaim, but I shy away from IC regulators in this position. You can try an IC current source filtered with a cap and choke such as Hammond 159ZC, which is 60mH and 2A, 0.7R.

You can use a 26 instead of the 6SN7, but be aware that the 26 takes a VERY high quality filament supply. I use batteries with a cap and choke with my 01As.

Then, I would use it on your S2 driver in the 600-12000 range, and turn off your other S2. I would also turn off the "injection channel." That would give you a better idea of how it sounds.

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