OK, where was I ?
Well, I had D3a in there .
Now, I realised that E180F has the same pinout and similar filament current . I had Mullard E180F's and Russian E180F-equivalents in my stash . Now, Mick Maloney, these days , seems quite fond of 6688 ( ie. E180F ) above the FET :
http://supratekaudio.blogspot.com/searc ... results=50
So, I thought , I'll lob in some Russian E180F's, and see how it sounds . Oh, this is still in triode-mode, just to be clear .
Luckily , the current and gain are almost the same, as it's the FETs that are in charge here, as far as the voltages and current go .
Ok, the Russian E180F's sounded BETTER . Some more tone , less aggressive, more musical . Still not great , but better .
Ok, then I put in the Mullard E180F's . This was worse again, but not as hard and charmless as with D3a's .
So , in the last couple of days, I thought "what is the order of linearity in these three valves , in triode mode ? - surely it is : E180F Russian , E180F Mullard, D3a Siemens , going upwards" . This was in opposite order to the sound quality .
Also , Nick ( and Allen ) have used ECC88 , which is not known for pleasant distortion spectrum either - in the higher harmonics .
Now , OK, then I read a very useful snippet from JC's articles on the microphone pre-amp ( part 3 ) :
"the next change has to do with noise. in order to minimize distortion, i increased the voltage across the FETs by biasing the 12AY7s up, until i got the lowest 3rd harmonic. in this configuration, that turned out to be 18 volts. i would have guessed 7 or 9 would have done it. but the combination of these FETs with this tube worked out that way. this is NOT the lowest noise arrangement."
Now JC was working with a balanced input gain-stage here, so 3rd and 5th-harmonic would be very important to him .
Breather again ....
"Once you find out ... the Circumstances ; then you can go out"