At a little over 9 watts plate dissipation per 6V6, you have them biased at a good point for getting good tone and long tube life. For a bit fuller and warmer tone, you can safely go a little higher and bias them at perhaps 24 or 25 ma per 6V6 at idle.Ac voltage from wall socket 119.5 volts, plate votage on jj 6v6 tube 420 volts and current 22ma. The amp is a blackfaced '78 Princeton Reverb with a 5u4gb rectifier tube.
True, but the power limit of the output tubes is the final limiting variable. I'll bet this was what the original post was asking.Originally posted by Old Tele man
AL1--to get more power output (Po) from your amp, you need more drive signal (Vg) going into the grids of the output tubes...and that comes from the phase inverter...of course, that only becomes effective IF there's already some plate voltage (B+) "headroom" available.
...the power output equation is:
Po = Zo' * (gm * Vg)^2
Po = Power output, Watts
Zo' = Effective load impedance, Ohms
gm = Tube transconductance, Amps-per-Volt
Vg = Control grid AC-signal, Volts(peak)
...thus, you can see that output power is increased by either:
1) using a higher impedance (Zoo) output transformer...
2) using tubes with greater transconductance (gm) values...
3) using a phase-inverter (PI) that puts out more signal (think Paul C and B+ tap mods here)...
...however, as Jim Salman notes, none of these will do much good IF there isn't any MORE plate voltage available for the increased output voltage "swing" to occur over...ie: the amp is probably B+ voltage "starved" (limited?).
This isn't as big of an issue with the earlier (original blackface) Princeton Reverbs that have the GZ34 rectifier tube, since the GZ34 takes significantly longer to warm up and supply current to the rest of the amp, compared with the 5U4GB.Since this amp doesn't have a standby switch, the voltage on the plates goes over 485 and the current at about 31ma until the tubes warm up. Is this normal and safe? Thanks.
Yes, but in this case, the first filter (one of four in the multi-section can) is rated for 450 or 475 volts max. working voltage. While in standby mode, the voltage on it could go well beyond the max. value. That's why I recommended the "tweed" style setup instead of the usual blackface wiring.Perhaps splitting hairs, but I usually prefer to put the standby switch after the first filter (may require rewiring OT center tap etc.) to allow the first filter to charge fully before B+ is applied to the rest of the amp.