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

Post Subject: Optimizing the (NOS) 2A3... "KM-style"?Posted by floobydust on: 2/15/2009

Adrian,

Fair enough... it's not about power or gain in your case, just making it sound as clean as possible.

Well, if you go back to my earlier post on findings, I list the basic operating values. Just to recap them here:

Plate supply = 350 volts
OPT Primary impedance - 3.5K
Cathode bias resistor = 866 ohm
Cathode bypass capacitor = 50uF
Grid resistor to ground = 249K

If you're up for making the above changes, you need to select a new OPT.... this obviously can get expensive and I've no idea on what you're looking at regarding iron and associated cost. Looking at the Moondog circuit, you have two driver stages which provide a non-inverting drive to the grid of the 2A3. This implies that the 2A3 provides inverted output to the OPT. Most SE OPTs are not phased this way and you would need to reverse the secondary winding for correct phase. The Hashimoto SE OPTs are phased for this so no reversing is needed. These are also my preferred OPTs in general unless you're planning for something custom-wound. I would suggest the H-20-3.5U which is rated at 20-watts output and has the typical 4-, 8- and 16-ohm outputs. Specs and such can be found here:

http://tube-amps.net/HP_H_20_35U.htm

You may also want to consider some changes to the input/driver circuit.... have you measured the amount of drive you are able to push into the 2A3 grid?

Regards, KM

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