Thank you for valuable information Paul.
Since I'm running 6CG7 in Foreplay II (I think) with c4s with power voltage of about 157dc, all resistor values in circuit and c4s boards are as per manual, I have separate trafo to produce 6vdc for heathers and another for 157dc, do you think I would gain anything sonically if the voltage in Foreplay was increased ? If so, what would have to be changed in circuit?
Thanks,