The heart of the SRPP is the 3 resistors used in this circuit (the upper one is generally omitted but it is bad practice if you want a perfectly balanced push pull). Easy to calculate they just depend upon tube parameters and AC load.
With a fully decoupled cathode all have the same value which is :
R = Rload + ρ / µ -1,5
Current through tubes can be determined by the following formula
Io = 1/2 [ Vht / 2 ρ + R ( µ +1,5 ) ] Ohm's law...
Gain calculation is way more complicated and is an extrapolation of the simple triode gain mathematical relation
G = µ Rload [ ρ + R ( µ +1 ) ] / ( ρ + R )2 + Rload [ 2 ρ + R ( µ +1 ) ]
An Excel file makes a very handy tool to calculate all these parameters. Just has to be filled with tubes characteristics and AC load.
I also use it to determine the DC load RaV1. Drawing this load line on tube curves helps a lot to check linearity and allows to precisely set the working point (NB: This load to not take in consideration voltage drop in the 2 upper resistors, it is necessary to add it to anode voltage found on curves to have the exact Vak value).
This load is Io formula's denominator
RaV1 = 2 ρ + R ( µ +1,5 )
It is almost like a 47K loaded single tube and data sheet shows that we can expect very low distortion. In the case of a classic anode follower we have 4.1% mainly second order at 50Vrms!. In this preamp we deal with smaller voltages and the even harmonics will be greatly canceled by the SRPP configuration. Furthermore the 1.3K value has been selected using a HP 3561A dynamic analyzer to get the best distortion figure.
The SRPP operation will be very close to the Philips values, with a 57K load we have a 3.07 mA current for a 4V cathode voltage and a calculated gain of 18,11.
Line preamp schematic
Like in the 6J5 line preamplifier just a few parts. Resistors are Holco H2, Philips (not Vishay) MRS25 and coupling capacitors ITT PMT/2R.
These are to my humble opinion (and to my friends ears...) some of the best caps for the price along with AEROVOX V161 and ERO MKC 1860 / KP1832. They give a very neutral and accurate restitution without spending hundreds on exotic parts for a somewhat slight improvement. I made tests with well-known pure copper or silver film and foil ones. There was an improvement in transparency or finesse on certain parts of the musical message but each time to the detriment of the overall tonal balance .....went back to the ITT's.
Looking inside...
Can't be a more compact wiring
Next step: power supply, fully assembled unit and tests
