This capacitor filters/smooths the voltage to the tube plates from the voltage multiplier output and at 47uF or
more, hum particular to the tube path (blend cw) and due to ripple originating at Vcc to IC1 is minimized.
Then, if not enough 'Drive' is obtainable to get the tube drive LED lighting,
3) Substitute 100k,1% resistors for the 33k, 1% ones at resistors Rs 20 and 39.
This increases the gain of the MIC input stage, presenting more signal to the tube.
Then, if hum is apparent in the audio output when a condensor mic is connected with blend ccw for solid state,
4) Substitute a greater value capacitor with a 50V or more rating for capacitor C12.
This improves the filtering/smoothing of the voltage used as the phantom supply to the mic (again, from the
voltage multiplier/Vcc ripple) and I've heard reports of users going as great as 1000uF here.
I'd say try just 1 and 2 first, then do 3 and/or four if needed. With the Sovteks, the dc-filament mod isn't needed
and can make hum apparent where it wasn't before due to the increased ripple currents associated with the
added resistor/capacitor filter. This mod came to be back when we stocked tubes labeled, Made In China.
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The setting of the Symmetry trim can be 'calibrated' as follows to optimize the s/n / headroom through the tube.
A signal generator is connected to the input and the output is viewed on a scope (or in a pinch, monitored by
ear) with the blend set to 100% tube signal (blend fully post/cw) and the Drive control and the Symmetry trim
alternately adjusted.
Start by setting the Bend fully post and the Drive and Output at about 12:00 to 1:00. Connect the input signal
source and vary it's level externally from minimum up until the tube begins to clip due to overdrive and stop at
the onset. Most likely, it will be either on the top or the bottom of the waveform extreme (oh yeah, use a sine or
triangle here so you can detect the flattening due to clipping). Adjusting the Symmetry trim one way will increase
this effect and the other will remove it. Adjust to remove it and go back and increase the input to the tube by
either increasing the external signal input level or the Drive control to the onset of overdrive again, and, readjust
the SymTrim to remove it. Go through this process again and again and eventually, you'll notice that no more
input signal or drive can be applied to cause overdrive that is removeable with adjustment of the SymTrim. This
is the 'calibrated' setting for the trim.
related,
http://www.prosoundweb.com/recording/tapeop/paia.php