SWR Bass 750 bias question

[glebert]

Got a Bass 750 head that I bought as a fixer, no output. Replacing the preamp tube and cleaning some contacts and the amp sounds fine. I'm looking to sell it off now, as it really isn't my favorite sound. I didn't touch the output section, but feel like I should check the bias anyway. The bias procedure for these is a bit complicated, requiring a 2 ohm power load (which I don't have), and I have heard of guys frying working amps trying to do this test (probably did more output than the test procedure specs). .The procedure also seems a bit vague saying "turn until you are just passed when the crossover notch disappears." Something like voltages across ballast resistors to me is a much simpler and reliable method, but I am assuming there is a reason they didn't just require that. So in this situation do you do the bias procedure, something simpler, or not do anything at all if there are no problems?

[Enzo]

Something like voltages across ballast resistors to me is a much simpler and reliable method, but I am assuming there is a reason they didn't just require that.

Seems like it, doesn't it? But it seems like it because it tells you a number to adjust something to, rather than adjusting the actual performance. It is more convenient, not more accurate. The whole reason we adjust bias is to eliminate crossover distortion but not overheat the amp.

There is nothing vague about adjusting until the crossover notch disappears. You feed a sine wave through the amp, scope the output, and look at the crossover notch in the waveform. Then adjust the bias hotter until that distortion just disappears.

the amp sounds fine.

Then stop fixing it.

[nevetslab]

I had to pull up my Service Notes on the SWR 750X amps in our rental inventory to see how I handled them, going thru a Bass 750 and three 750X. What Enzo said above^^^^^ is completely legitimate. I used the Power Analyzer following my Variac for overall current reference, and used a DMM across the 0.1ohm 5W emitter resistors during biasing. While I had the scope and generator there on the bench, I didn't confirm crossover distortion. I had adjusted the output current thru stage xstrs for 30mA average, being 3mV across the 0.1 ohm resistors.

One thing I did find, is there's no bleeder resistors across the power supply caps, so when you shut the amp off, the front end turns off, turning the output stage off, so there's no drain on the supply caps. During testing, there's often substantial voltage across the supply caps (+/-40V on one of the units I was adjusting), so watch out for that. One of the amps had it's bias pot turning opposite that of the others...must have been the Bass 750, so turning CCW would INCREASE bias voltage instead of CW for increasing it.

Ah...I just saw you added the Service manual on your post.........yup....follows what Enzo said.

[glebert]

Then stop fixing it.

Done! It's funny that I either seem to be cutting too many corners, or trying to make work where there doesn't need to be any, without hitting a good balance.

I totally understand how the bias procedure is to get to minimum bias without crossover distortion, I still think it would be good if there was an idle current max given, just as a way to allow a quick sanity check.

Thanks for the input!

[Enzo]

I forget who now, I know Carvin did it, but someone else...could even been a Fender model, they had determined that when the amp was biased like they wanted, the MAINS current draw was some amount - call it 150 watts, whatever. SO you monitored mains draw and adjusted until that spec was made. No crossover notch, no output device current, just the whole damned amp current.

[Jazz P Bass]

I saw that procedure on a Crate Blue Voodoo.

[glebert]

It kind of makes sense if you believe that whole "the amp is the thing in between the outlet and the speaker" bit.

[Enzo]

Well, if we get rid of the whole bias is some super accurate precise critical adjustment notion, then it just becomes a more or less seat of the pants ball park adjustment.

[Axtman]

I just did this bias adjust procedure last night to an SWR SM-400. This is a two amps in one (stereo) unit. So there are two power amp sections that have to be adjusted.

The procedure was easy to do. I used a 4 ohm wire 100 ohm adjustable wound resistor instead of a 2 ohm one. The crossover notch is very small and it took me some fiddling around to "zoom" in to see it. Also be sure that your input signal is very small or you will get more than 2 volts on the output.

The bias procedure is here: Bias Procedure.pdf

[Enzo]

Crossover notch will be more or less centered on the wave form. We don't car about the rest of the waveform, so get the tiny notch in view then turn up the scope vertical gain so the waveform widens off the screen. That will make the notch part larger and easier to see.