tube output

[AL1]

Anyone know the wattage output of the jj 6v6 tube? Or, maybe I should say maximum output? Regular 6v6's are rated at 12 watts but I think these must be rated higher. Any one know for sure?

 

[AL1]

Old Tele Man, thanks for the info. Could you give me your opinion on these #,s. 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.

 

[WailinGuy]

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.

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.

But since this is a Princeton Reverb, which has a rather small power transformer with limited current capabillity on the seconday (B+) , you're simply not going to get more than around 12 watts of output power at clipping. Paul Cochrane has described a couple of mods to the the phase inverter circuit that might bump this number up by a couple of watts, but that's going to be it as far as what a Princeton can put out.

No matter what the specs of the output tubes are, an amp's maximum power output capability is ultimately determined by its power supply.

 

[AL1]

Jim Salman

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.

 

[Blue Strat]

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

where:
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?).

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.

 

[AL1]

What about the fact that the voltage is going to 485 volts and the current is going to 30 ma when the amp is first turned on. Is this normal and safe? Thanks to all for responding.

 

[WailinGuy]

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.

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.

If the multii-section filter cap in your amp has been replaced, then it's probably rated at 475 volts per filter, so 485 volts is probably OK for a few seconds. If your amp still has the original filter can, you might be asking for trouble, since it may be pretty dried out by now. The tubes, if good quality, shouldn't have any trouble dissipating a few extra watts beyond their max. plate dissipation rating for several seconds.

I don't know whether or not any significant cathode stripping would occur during warmup. (This is where a tube's useful life is shortened because the cathode coating gets "stripped" while it is still cold when high voltage is applied to the plate).

A cool and easy mod for these amps (I did this to my '69 Princeton Reverb) is to get rid of the polarity switch and replace it with a standby switch (so you don't need to drill a new hole in the chassis). The standby switch should be wired to go between the output of the rectifier tube and the first (main B+ supply to OT primary center tap) filter cap.

 

[Wakarusa]

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.

 

[AL1]

what would this cap do ?

 

[WailinGuy]

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.

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.