amp not outputting full wattage

[g1]

Check that your line voltage is holding at 117V inside the amp while the amp is running at full power.
The schematic shows 410V B+ to be correct, so 80W or so may be all you can expect to get for clean power output.

[Chuck H]

Thanks g for providing the schematic. Note that the bias on John's amp is icy cold compared to the schem.?. Ordinarily I wouldn't expect this to affect output power, but at -13 volts colder I don't know. The only other anomaly would be that John's amp shows the screens at 1V higher than the plates where the schem shows the screens at 15V higher. Regardless of the small B+ difference overall I wonder about that too. Could the OT be compromised with shorted winds? It seems unlikely that both sides would be shorted evenly. And, of course, all the major manufacturers absolutely DID fudge the actual wattage of their products favoring the high side.

[johnhoef]

Indeed,G1, thanks for the amp schematic. Chuck, I'm not surprised to see you say that Fender fudges the wattage output to the high side. I think a lot of amp companies do that. I also suspected the output transformer, so substituted with a known good one with the same results. Further, I did a pure resistance measurement of the output transformer, and both sides read identical. I feel confident the transformer is OK. OK, I'll look into the line voltage at the amp during full output, and will also investigate the screen versus plate voltages. Thanks for those tips. I'll report back soon with those results.

[johnhoef]

Did some more work on the Fender super bassman. Today, the lines voltage at the amp is 120v. This is with the amp running at full power with output waveform slightly distorted. I also decided to measure voltages with no signal as well as full output. All voltages drop at full output, which is no surprise. The schematic indicates such. Today I am getting 15vac at the output at full power with slight distortion and clipping. Doing the math = 56watts into 4 ohms. This is definitely low for a 100 watt amp. I understand that full 100 watt output is not expected, but 56? I would say this is low, especially considering today we have 120vac lines input. I am still at a loss what to do with this amp. Any further help?

[Randall]

This would be a great time to have a 4 ohm 100 watt OT on hand to clip in to see if yours is functioning properly or not.

[Chuck H]

re: the B52

I decided to do a thorough cleaning of all pots and switches and tube sockets, etc, and the amp is back to it's full loudness. The customer came over today to pick up the B52, and he was thrilled with how well it worked.

Did you confirm the output wattage after the cleaning? "loudness" is too subjective.

[mac dillard]

I'm pretty dumb about a lot of this stuff but I have to ask if anyone has actually measured 100 watt at the speaker with an amp that is running 4 6l6's? With a max anode dissipation of 30 watt per tube That gives you 120 watts total..My old rule of thumb has been that output power is actually about 60% of the total. I know that Fender, Marshall and other say 100 watts and I have never tried to measure output power but I doubt the 100 watts with 4 6l6's same as I doubt the 60 watts from two EL34's.

[The Dude]

Have we checked that all tubes are conducting? An amp will work with an open screen resistor, but you'll be short the associated tube.

[nickb]

Have you scoped the grid one's of the 6L6's to check that the clipping is not happening prior to the final stage?

Having a cold bias of -56.7V means the Gm of the 6L6's is lower and that means a bigger voltage swing on those grids for full output. This begs the question: what is the bias current set to? Do you see crossover distortion on the output?

[johnhoef]

Thanks so much for your thoughts. I have another unit on the bench at the moment, so haven't yet gotten back to the bassman.
Randall, Thanks for your idea, but have already tried that. No change. I do indeed have a spare output transformer here that I use for this very purpose.
Chuck H, The cleaning was with the B52 which has been returned to the customer. On this bassman, I did do a cleaning of all controls, switches, closed circuit jacks, and tube sockets. No change.
Mac Dillard, Indeed, I wonder the same thing. Several thoughts have been written here including drops in line voltage, and over specifying the power output by the manufacturer. What I have learned is that we can not expect 100 watts out of these amps.
The Dude, Indeed, all four 6L6s are new, and the driver 12ax7, I have swapped out with several known good tubes, no change.
Nickb, Good thought. That has not occurred to me to check the amount of drive necessary to get full output. Perhaps my next step is to build a simple adjustable bias circuit, and experiment with that. And no, there is no crossover distortion.

[Chuck H]

Do understand that the 100W rating is fudged by the MFG for good reason. Tube amps can peak well beyond what most SS amps will do and well beyond what the MFG rated them at! In the wash most tube guitar amps rated at 100W are notably LOUDER than equivalent SS counterparts. Some with peaks over 200W. Suffice to say that amplifying a sine wave can be a very different thing than playing a set with rests, breaks, dynamics, etc. (musical power) I'd put most 50W tube amps up against any 100W SS amp in a live, playing scenario any time.

[alexradium]

Fender declares max output at 10% distortion and that is pretty squashed waveform

[g1]

Fender declares max output at 10% distortion and that is pretty squashed waveform

For this amp? I've seen Fender spec at 5% but not 10% distortion. Is that what they are using now?

[johnhoef]

Hello, fellow techs,
I'm finally back, with the Fender Bassman on the bench. The first thing I did was install my tube extender to check bias. The extender places a 1 ohm resistor in series with the cathode of the 6L6. The bias was a very cold 12ma! Such was the case with all four tubes. I decided to place a 10k pot between the negative supply and the center tap of the bias centering pot. Contrary to the schematic, there was no 3.3k resistor there. Setting the bias is now a very easy task, and I set it to 45ma, which is in the "warm" vicinity. This did not effect the total wattage output of the amp. The low output persists. Checking the line voltage today, it's 115volts. I also checked the input to the phase inverter at the .002mfd capacitor, thinking that maybe the sine wave was being chopped there....no, sine wave stayed pure well into square waves on the output. Thought I would play around with the bias, now that I have the pot there, while observing the slight square waves at the output. No change as far as trying to increase the output amplitude. If I crank up the volume, and squash the output to almost square waves, I can get around 70 watts, but really, this doesn't seem right to me. I realize Fender specs these amps with a high distortion rating, but still, practically square waves? Is there still something wrong with this amp????

[johnhoef]

By the way, thought I would mention, there is a 12AX7 for the phase inverter. The schematic calls for a 12AT7. My understanding is that the 12AX7 has more gain, so this should not be a factor?