'66 Fender Deluxe Reverb Low Output

[ca7922303]

Checking all the tubes for voltages, output is about 12watts. Do any of these readings look like they are off? Thanks

[nevetslab]

They seem reasonable. Preamp tubes have nominal plate/cathode voltage readings, Reverb tube has the high voltage for the 12AT7 to driver it's xfmr-loaded primary to drive the tank, Tremolo tube has high voltages. You have the same bias voltage on the two power tubes, plate and screen voltages seem nominal. You're only getting 12 watts out. 8 ohm dummy load or at full output into the speaker, SPL levels be damned? 12W into 8 ohms is 9.8V RMS. A little low. 22W is what the nominal output level is stated at.....13.3V into 8 ohms, so you're not far off. Tweaking the bias for best waveshape may increase this. Do you have means of measuring the plate or cathode current of the power tubes? If they're substantially mis-matched, you will have low output.

[Leo_Gnardo]

Checking all the tubes for voltages, output is about 12watts. Do any of these readings look like they are off? Thanks

Sometimes, that's all ya get. I've seen Deluxes, Princetons too, vary 12 to 22 watts. Even with new output tubes.

One item I ran across years ago, on MEF or TAG can't remember which, is Deluxes often deliver more power to a 16 ohm load although factory equipped with 8 ohm speakers. Another mystery. But I check all that find their way onto my workbench and in some cases - not all - it turns out to be true.

First thing to examine is the state of the screen grid resistors. Sometimes they sneak up in value & that will limit output power. After that, I'd try a fresh set of output tubes on your Deluxe, maybe a fresh rectifier too. If they don't bring a power improvement then "that's the way it is."

[g1]

Power tube plate and grid voltages seem a bit high. Maybe biased very cold?

[Greg_L]

Power tube plate and grid voltages seem a bit high. Maybe biased very cold?

That's what I was thinking. Some of those voltages seem pretty high for a Deluxe Reverb.

[pdf64]

It’s always a good idea to include the mains VAC at the wall outlet, and the VAC across the pilot lamp terminals (ie heater voltage 6.3V).
I the mains is high, everything else will follow suit, so it provides an essential context.

[Helmholtz]

How did you determine output power?
What signal frequency and equipment was used?

Generally higher B+ should result in higher output, everything else being the same.
The influence of biasing on output power is very small. A class B amp doesn't put out significantly less (if any) than a hot biased class AB amp.
A cold biased output stage requires more grid drive, though.
Apart from B+ (measured at full output) the other significant influencing factor is the "saturation voltage" of the tubes used.
Saturation voltage can vary considerably between individual tubes of same type and tends to increase with age, thus lowering available output.

[ca7922303]

Installed new JJ 6V6s and GZ34, now 22w using 8 ohm dummy load, signal generator @ 1000hz/.30v sine. Oscilloscope starts to distort about 6, getting around 13.2vac on dmm connected to dummy load. I'm calculating at around 5-6 volume. 13.2x13.2÷8=22w.