Fender "The Twin" (red knob) bias question

[Chevy]

Enzo,
OK... plugged my AC signal gen straight into the Effects Loop In , and got pretty much the same result..... just over 60 watts. .

Tested again at 400HZ, same result. Checked out the schematic's DC test points, and at the PI and PI driver stage, they were within around 5% of spec. My signal gen doesn't go down to the AC levels spec'd on the schematic; don't know how they did that (680mV on the plate of the first input tube?).

I changed the PI tube with a known good one, and same result. Watched the waveform at both the PI plates, while looking at the amp's output wave, and the PI was still going strong as the amp's output began to clip. Checked B+ again, and it's 465 no load, 445 at ouput clipping.

To measure ouput power, I'm using my Fluke meter to measure the output RMS volts (while watching the clipping level on the scope), then square that then divide by 8 ohms (set at 8 right now) to get watts. And it's clipping pretty good, not just a very teeny flat on top and bottom of waveform, but a good healthy clip. Been away from this a long while, but that seems about right to me.....?

Anyhow, dont' have good new tubes to test it with, but that's all I can conclude is causing the problem. Any other ideas? I can't seem to find anything wrong!!!!

[Enzo]

SO plug the signal generator into the power amp input jack. Set it so the desired 2.2v is at the plate of V3A. Between your generator and the three way level switch at that jack, you ought to be able to get there. Now do the 12AT7 plates show more or less the 22 and 25v? And with that input, does the output into 8 ohms look about like 12v?

SO you are now getting about 22v on the speaker load cranked?

And try this. DOn;t look at the scope. Put the meter across the load, stick a signal at the power amp input jack and run it up. Can you get about 28v RMS on your meter?

[Chevy]

If you look, you can learn somethin every day!



Like I said, Enzo, there's nothing wrong with this amp now!

I checked the test points, and all is well... 2.2V, 25V, and I got 11.2 volts out at the 8 load instead of 12V per spec. Close enough, I assume.

Then I ran it up with the sig gen into the power amp input, and got 26.2 volts or so out, which translates to 85 or so watts. What I was originally expecting from this amp. But NOT how I usually measured it; this output waveform was pretty much square wave with tilted tops and bottoms. I always "used to" measure an amps' output with the tops and bottoms flattened somewhat, but that's it. Boy, this seems too stupidly simple to pass over!!!!!

I plugged into channel one, and got pretty much the same 26.2 volts RMS, square wave out, and then into channel 2, I got about 27.6V which is about 95 Watts. More drive on channel 2.

So THAT's how you do it, huh????!!!!!!

Thanks for your time Enzo, nothing wrong with the amp, just had to reprogram the software.

[pdf64]

Power output should be checked at the onset of clipping ie turn up the input until it starts to clip on the top or bottom or both of the sine wave, then back off a little to take it out of clipping. That rms voltage across the load is the data for the wattage output calculation. Square type waves have much more heating energy ie wattage than sine type waves of the same p-p, but it's the max unclipped sine wave which is the relevent thing.
Tubes can be +/-50% of design centre specs and still be considered good, so that will be reflected in the output power capabilities of the amp they're in. So the tubes in there might not be the best, but you might buy matched tested new ones and still get the same output. Peter.

[Enzo]

When Fender calls it a 100w amp, they are not measuring it at 0% distortion.

[Chevy]

Power output should be checked at the onset of clipping ie turn up the input until it starts to clip on the top or bottom or both of the sine wave, then back off a little to take it out of clipping. That rms voltage across the load is the data for the wattage output calculation. Square type waves have much more heating energy ie wattage than sine type waves of the same p-p, but it's the max unclipped sine wave which is the relevent thing.
Tubes can be +/-50% of design centre specs and still be considered good, so that will be reflected in the output power capabilities of the amp they're in. So the tubes in there might not be the best, but you might buy matched tested new ones and still get the same output. Peter.

Hmmmmm..... well this puts me back to where I originally thought it should be.... and what you describe is close to what I used to do for max power measurement, except I would allow a little clipping on top and bottom.

So... does anyone know how Fender actually measures their power ouput? Marshall? Anyone ever run across any corporate tech info on that? ie How much distortion do they figure into the claimed power rating? Anyone ever actually get 90 or 100 clean watts out of a Twin?

[Enzo]

Guitar amps are not clean, they are not designed to be clean. You demonstrated to yourself that you get the specified numbers from the schematic. Getting a few small watts from a Marshall head before the waveform starts to lean would be a pointless measurement. SHould they start calling their 100 watt heads 12 watt heads because that is all the louder they can get before a sine wave is no longer?

Fender specs the Twin Amp at 100w into 4 at 1kHz and 5% with presence on 1. Yours is The Twin, but things ought to be about the same.

We've had this debate before.

[Chevy]

Don't want to beat this to death...... I got my answer and am happy. Thanks Enzo.

But to add a few thoughts; I'm among the first to hear the onset of distortion in a guitar amp, and know for sure they aren't designed to be clean. I wouldn't buy one if it was.

I guess I'm just looking now for a confirming point of reference that I can use in the future... the 5% distortion rating on a Fender Twin at 100W is not believable to me. The one I just finished working on must've been at 20% (just a wild guess) and had to push it to full square wave output to get 95W. I have also worked on a 100W SWR bass amp, and measuring the power out at just barely clipping, it was an honest 100W if not more. Obviously I can't apply that same criteria to a Fender tube amp, can I?

I guess I just need a little more hands-on, and will get a feel for what an amp should do, by the fact that "50 others" have done the same thing when they were working right.

[Enzo]

Get or borrow a distortion analyzer and see how much it takes to be heard. You might be surprised at how much distortion can be present and the sound is still perceived as clean.

I am not defending Fender, but the solid state amp makes clean up to the clip point. The tube amps start bending and compressing on the way up.

[Albert Kreuzer]

Had a red knob twin on the bench last week and it tested 84 W clean, 122 at full distortion (square wave output) into a 16 Ohm load (impedance switch set on 16).
Highest output reading on a 4x6L6 Fender so far was 112 clean Watts from a SF Twin Reverb.
Clean measurements were taken at the onset of visible clipping on my scope.
Guess it has to do with the Fender "20% rule"

Cheers,
Albert