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More hints on Dc amp optimization.....

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Lets take this opportunity for me to make some design comments on what I see is not optimized with your amplifier, in terms of op points, and the DC couple. This is all just my humble opinion.

You already know, IMHO, the best op point for the 2A3 is 245-250 VDC at 41 to 43 mA, which is usually -49 to -50 VDC of self bias. This is THE best trade off I know for long life and unstressed, relaxed sound. 'Takes good internal wiring to optimally pull off.

What you listen to the MOST in a DC two stage amp, is the DRIVER tube, and its op point has to be NAILED and optimized up the whazoo. The 2A3 is acting more like a unity coupler. You MOSTLY hear the driver tube.

In your case, with 420 VDC on the 2A3 plate, and 170 VDC on the cathode, this means, for -45 VDC of self bias, you have ONLY 125 VDC on the 6SL7 plate. This 125 VDC Ea is simply too low, unoptimized, to have a brilliant sonic presentation.

This too-low of an op point will forever hinder the ultimate level of your new amp's sonic performance. Ohh, it will pass a signal, last forever, but it will NOT sound as good as it can.

I find the absolute lowest B+ on the 2A3 plate in a DC 2A3 amp is 450 VDC, and I really prefer 480 VDC on the plate of the 2A3. This allows you to have a B+ on the driver tube's plate of about 175 VDC minimum, and sometimes as high as 195 VDC (maximum) which gives a far more brilliant presentation of the music than your 125 VDC Ea on the 6SL7 plate.

It is paramount to get the operating point of the 6SL7 driver NAILED and optimized.

Additionally, your self bias on the 6SL7 needs to be OVER 1.5 VDC, and likely over 1.75 VDC for optimal sonics. If you have efficient speakers, one 6SL7 section, at low current, will usually sound better than a parallel driving a 2A3, as it will avoid a time smear from two not identical sections.

One 6SL7 section will easily drive the 2A3 in a direct couple. You have assumed that a 6SL7 in parallel will outperform a single section, and I say, YOU better listen to both and decide.

The wire between the DC needs to be really good, try three in parallel, one run of Kimber AGSS, one of Kimber TCSS, and one run of teflon coated Percy 21 AWG solid silver. That works well.

The final 6SL7 bias point you alone need to determine BY EAR, by inserting a 10 turn pot as the Rk, and adjust the bias till it sounds the best to YOUR ear, usually at 1 mA. or less, as per all the Loftin White professional designers I respect, Isamu Asano, Nobu Shishido, and Dennis Fraker. Replace the pot's Ohmic value with two fixed value Rks in parallel, matched to .1% if you can measure that.

Have fun, it is a rewarding design, which can perform nicely. I've heard it do so, many times, and I prefer a two stage DC amp above all other tube amps I have made.

Jeff Medwin

Jeff Medwin

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