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In Reply to: RE: SE 6V6 triode : effect of plate to plate feedback ... posted by Triode_Kingdom on August 26, 2016 at 08:05:03
The power level for THD% measures in triode mode was just over 1/2W before onset of clipping.I get more power in UL mode, over 1W.
The PT in this case is a bit undersized, probably better for a preamp than a stereo SE amp, so the biasing is conservative.
Edit: in SE UL mode w/fbk, Max Power Out was 1.67W @ THD = 1.78%Vin at max power = .74V , Voltage Gain In/Out = 5
In SE UL without feedback @ 1W power out , THD = 3.88%
I'll have to see it improved THD% with feedback = a better listening experience or not.
Edits: 08/26/16Follow Ups:
I measured my little 6V6 SE amp last year while hand-picking tubes. Don't remember the distortion numbers being nearly that high. I need to find my notes...
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Buy Chinese. Bury freedom.
I am curious as to how you dertermine "max power out" for each variation? Also why are the distortion numbers for the feedback version different from the non-feedback version? I would think that the term "max power" would be referenced to some arbitrary distortion number and that referenced matched in the two cases so a better comparison can be made.
dave
For max power out, a scope probe is hooked up across a load resistor on the OPT secondary.
I run a 1KHz test signal from my soundcard into the amp and look at the point where the waveform clips, then back off until the sinewave looks undistorted.
I also have a true RMS voltmeter across the load resistor to record the AC signal voltage and use the typical ohm's law equation for power.
As for why the THD% is lower with local FBK than non-feedback version, that follows every SE pentode class circuit I have ever tried building.
I guess the conflict I see here is the visual clipping which is a very subjective thing and the harmonic distortion measurement which is purely objective are not agreeing with each other.
In order to make a "fair" comparison either the output power or the harmonic distortion (clipping) should be held constant and then the other result documented. Of course this doesn't take into account the distortion spectra but for a SE amp where 2nd order dominates the gross distortion figures things should line up fairly well.
dave
One problem is that the internal sine generator in ARTA is in 1dB steps.
I would need a way to fine tune the signal going into the amp so the output signal over the load resistor can be held with more precision.
Then I could get the same power output and compare THD% when in non-feedback vs. feedback modes.
Or I could use my external signal generator, but that tends to output more distortion than the soundcard.
Thanks for your comment.
Arta is limited to 1dB increments on the audio generator? Wow...
Even so that is in the voltage realm and the increase of 1dB of voltage is about 25% more power leaving the maximum variation between any two settings at 12.5% and statistically you will probably end up in the 3-10% range of variation.
dave
You should get about 2 W. from a Class "A", triode wired, 6V6 family tube.
Building a modest cost, high performance, B+ PSU for a stereoblock using SE "12" W. tubes is pretty easy. "Full wave" voltage double the O/P of a Triad N-68X isolation trafo. 2 A./600 PIV Schottky diodes will be switching noise free and more than tough enough.
Eli D.
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