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Original Message

RE: It can, but

Posted by JKT on February 10, 2017 at 11:13:18:
"Second thought is to swap in a 6H6n tube to replace the 6H30, this move may need to change the valuse of 1st stage resistor values because the 1st and 2nd stages are direct coupled"

Without seeing the schematic this is only a guess. If the 1st and 2nd stages are differential and direct coupled check to see if there is a voltage differential between the two grids of the 2nd stage. Any voltage difference there will "eat" into the voltage headroom of the second stage. If this condition were found to be worse at start-up that could explain the problem. If this is the case changing to 6N6Ps would make it worse. At the same plate voltage and bias current the grid bias voltage of the 6N6P is about half that of the 6N30 (less headroom).

I have a fully differential phono pre where the first and second stages are direct coupled. The first stage is a MOSFET/tube cascode with CCS plate loads (very high output impedance). Some of the plate load current source current is shunted to ground across a resistor (part of the RIAA network). This sets the plate voltage of the 1st tube and the grid voltage of the second. Matching this voltage for the two halves is very important. When they are not matched this does result in a hard sounding distortion. The second stage of this preamp is a 6N30 (150Vp/k, 25mA/side). Originally I used a 6N6P. In this application the 6N6P was more prone to brittle sounding highs and harsh transients. Your milage may vary