• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Anyone deliberately dial in 2nd harmonic?

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Big 'nope' here. Second obviously gets preference over higher harmonics but that's simple logic based on auditory masking curves. For example, the current SE preamp in the system runs roughly 0.01% 2nd at a volt out with 3rd and 4th 90+ down and higher harmonics better than 120 down. I can get lower second at the expense of seeing high harmonics poke out of the noise floor but don't consider it a good trade.
 
Big 'nope' here. Second obviously gets preference over higher harmonics but that's simple logic based on auditory masking curves. For example, the current SE preamp in the system runs roughly 0.01% 2nd at a volt out with 3rd and 4th 90+ down and higher harmonics better than 120 down. I can get lower second at the expense of seeing high harmonics poke out of the noise floor but don't consider it a good trade.

This reminds me of what I wanted to mention earlier. Often the things we do to reduce overall distortion actually increase higher order distortion while reducing f2.

A good example is balanced push-pull topology, which doesn't simply cancel f2, but converts at least some of the f2 to f3.

Lower overall distortion may sound worse if f2 decreases and f3 increases. I don't know if that's the same as dialing in f2, but it is the equivalent of not dialing it out...
 
Kevin -

May I open another thread about these mods? I have one of these and would really appreciate your insight. thanks.

Yes, go ahead and I will try to contribute what I can. What I do remember as this was a while ago is that I replaced the original quad op-amps with National LME49740 - see here: LME49740 - Quad High Performance, High Fidelity Audio Operational Amplifier

Beyond that I had to cobble up a new supply as my eBay score had the wrong wall wart.

The supply in this thing is one of the real weak points IMO and a separate toroid and full wave rectification of the rails would be a real enhancement. A separate winding for the tube would be good too.

I haven't recorded with this beast yet, have yet to purchase some microphones. Probably going to get a pair each of condenser omnis and cardioid or perhaps a pair with switchable patterns. (Nady, MXL, CAD, Audio Technica, Behringer - usual inexpensive suspects. I'm not a serious recordist.) MXL also has a really inexpensive ribbon that looks rather interesting.
 
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