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

Post Subject: Going down the DC pathPosted by floobydust on: 5/6/2009

Romy,

I have some (strong) opinions on DHTs concerning filament supplies versus specific tube types. For virtually ANY 45 triode, AC is my preferred option. Still, not all 45 triodes will be acceptable for my use, hence weeding thru many of them to find acceptable (matched) pairs to install in an amplifier. As you move up to other (DHT) tube types, this may not be a viable option as noted earlier.

If you have no other option beyond a DC supply, I still prefer a center-tapped transformer secondary using a pair of diodes (not a FW bridge which has 4 diodes) and a choke input filter. This generally results in better voltage regulation but requires additional filter stages to get the ripple low enough. Using half of the winding for half of the waveform reduces the heat build-up in the filament winding and results in a cooler running power transformer plus less voltage drop than a diode bridge. Most of the newer schottky diode rectifiers show larger voltage drops at higher current levels and as such create more heat from wasted current.

I'm still a bit leary of using too many solid-state components and regulation... either shunt or series-drop. When a new pair of WE300B tubes is over $800, having a $2 part fail and taking out one of the tubes is simply not acceptable (a rare occurence). I would prefer passive LC stages, but this may not yield sufficient performance (i.e., low ripple) for your required operation. I'm still undecided on a filament supply for my future 300B amplifier design for the reasons noted above. In short, I prefer the simplest approach to meet the requirements and performance levels.

I am interested to hear about your findings once you try the regulated supply. Perhaps you could also try comparing this to a more simplistic approach as outlined above.

Regards, KM

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