amp not outputting full wattage

[johnhoef]

Alexradium, I have substituted a known good output transformer with no change...still 50watt output. I agree, not the greatest idea to run a single tube, but I thought for test purposes for a few seconds, it did prove all the tubes as well as the sockets are good, and equal. Also, nothing connected wrong or missing or burnt. This is a simple circuit. I have painstakenly traced the circuit, tested all resistors and capacitors, and all is as the schematic shows. Plate voltage is 401 VDC with no input, 377 VDC with fully driven to clipping into 8 ohms. The schematic shows 390v.

[g1]

Have you checked for AC ripple voltage on the B+ with no signal applied?
And once again, can you check the AC line voltage at the power switch or fuse holder while amp is delivering full power into load?

[Chuck H]

Have you checked for AC ripple voltage on the B+ with no signal applied?
And once again, can you check the AC line voltage at the power switch or fuse holder while amp is delivering full power into load?

I wouldn't expect AC line voltage to sag, but some kind short in the power transformer or filters may be limiting available power for the tubes.?. Ripple and low HV at full output should show this I would think.

[nosaj]

I wouldn't expect AC line voltage to sag, but some kind short in the power transformer or filters may be limiting available power for the tubes.?. Ripple and low HV at full output should show this I would think.

Well if the amp is being played on an extension cord a thin one at that you have a voltage drop through the extension. Reasoning like not running an air compressor on an extension cord.
nosaj

[g1]

I've recently dealt with a wiring fault where the voltage at the breaker box was good but was sagging at the outlet. He had mentioned some low readings at the outlet earlier, so I think it's best to monitor it while testing the amps power output.

[johnhoef]

Hi, Thank you so much for all your wisdom, suggestions, and thoughts. I continue in this new year with this Fender Bassman dilemma. Indeed, still no idea how to get this amp to output more than 50 watts. I recently installed four brand new 6L6 tubes. No change. Still 50 watts.
g1, AC ripple on the B+ line is 40 volt p-p at full output.
g1, AC line voltage at the power transformer input is 117.5v, at full power output
Indeed, I have dealt with low AC readings, but that was due to having a 2000 watt heater on the same line. Amazing how those heaters cause a sag! All my measurements are with the heaters turned off, thus the AC line is in the 117v vicinity.
I really hate to give this amp back to the customer unrepaired. But I have a feeling this is what will eventually happen. And I am extremely frustrated that I cannot solve this problem. Such a simple and classic power amp design, yet near impossible to fix!
One last help to you all, if you please!
John

[J M Fahey]

Please:

Please retest with higher and lower load impedance.
If 4 ohm nominal try 2 and 8 ohm and post results.

My point being I want to know whatīs limiting your power: lack of adequate voltage or adequate current.
.....................

All tests still with 400 Hz signal or thereabouts and just surpassing clipping on both peaks, usually one clips before the other.

Allow for a little clearly flat area on top and bottom peaks, as Enzo says , this is a Guitar amp, not an Audiophile one and even less a NASA thingie.

Also some screen capture showing these "just beyond clipping" waveforms might show someting useful.

[johnhoef]

Vertical is 10v per division, 1khz, taken at speaker output.

[Chuck H]

That roll at the front and sharp at the other end of the clip is indicative of a high frequency deficiency and probably phase error. And that's with 1k going in too. Plus the wave form doubling. That may be a scope error, but I d still say there's definitely something wrong, though I can't say exactly what. Maybe an HF instability and oscillation. Can you please report if the trace doubling is a scope error (I think you would know) and if it's not, have you scoped along the signal chain to look for weirdness? What is the phase inverter doing when the amp is clipping like this?

[Helmholtz]

Plus the wave form doubling.

What doubling? Trace jitter is probably due to hum modulation.

[Chuck H]

What doubling? Trace jitter is probably due to hum modulation.

Easy there you silver tongued devil You're probably right that's trace jitter due to modulation. But I've never seen it just on the clipped portion like that. Not that it doesn't happen, but I haven't seen it.

[Helmholtz]

Easy there you silver tongued devil You're probably right that's trace jitter due to modulation. But I've never seen it just on the clipped portion like that. Not that it doesn't happen, but I haven't seen it.

Easy to verify by reducing time deflection and watching the envelope.

[J M Fahey]

Oh, I also see supply ripple on the clipped tops ... where else?
Because there tube is saturated so practically a piece of wire between main filter cap and speaker, you have ripple on supply, you have ripple on waveform peaks.
You donīt see it in clean signal because then tube rules, but once tube saturates it loses all control.
FWIW same happens on saturated SS amps.
Through an OT to adapt impedance but that does not change the ripple influence, which to the OT is "just another Audio frequency".

[johnhoef]

Hello everyone,
It's been a while, and the amp in question is long gone back to a happy customer. I have had more of these sort of amps in my shop, and I find the same issues with them all...they do not output rated wattage. As was mentioned, this is the way it goes with the industry.
At the moment, I have a pair of powered speakers that need a new tweeter. Brand is Harbinger. After replacing the tweeter diaphragm, all went well. I decided to measure output wattage on these, and was not surprised to find they are EXTREMELY over rated! According to the literature, they are supposedly 600 watts each. I measure a solid 100 watts RMS. Obviously musical instrument amp manufacturers cannot be trusted with their specs! They are bi-amp units, so the tweeter has it's own power amp, the wattage of which can be added to the 100 watts for the woofer. I didn't measure the tweeter wattage, but I'm sure it's well under 100 watts....still, a far cry from 600!

[Chuck H]

Hello everyone,
It's been a while, and the amp in question is long gone back to a happy customer. I have had more of these sort of amps in my shop, and I find the same issues with them all...they do not output rated wattage. As was mentioned, this is the way it goes with the industry.
At the moment, I have a pair of powered speakers that need a new tweeter. Brand is Harbinger. After replacing the tweeter diaphragm, all went well. I decided to measure output wattage on these, and was not surprised to find they are EXTREMELY over rated! According to the literature, they are supposedly 600 watts each. I measure a solid 100 watts RMS. Obviously musical instrument amp manufacturers cannot be trusted with their specs! They are bi-amp units, so the tweeter has it's own power amp, the wattage of which can be added to the 100 watts for the woofer. I didn't measure the tweeter wattage, but I'm sure it's well under 100 watts....still, a far cry from 600!

There's a VERY common trend in the sound reinforcement, home audio and car stereo industries to state "peak" power rather than RMS. Peak power is the highest possible wattage demonstrated at whatever resonance is needed to get it and no matter how brief. It can be a very tiny, but very tall spike. An anomalous activity in the overall system. This is meaningless for useful power, but very good for advertising. You'll sometimes see this ploy taken to silly extremes. I once had a "5-way" speaker system for my TV. The individual satellite boxes were about 3"x3"x5" and weighed about five ounces. The woofer box was about 10"x10"x20" and weighed about three and a half pounds. The amplifier was integrated into the DVD player/interface system that then plugs into the TV. The audio aspect of this unit and it's speakers was touted as 500W. Which is laughable. I'm certain that even 10W RMS put into one of those satellite speakers or 20W RMS into the woofer would have blown them to smithereens.