Ampeg V4B Rev A HUMMMM

[mhuss]

This amp is on my bench and it has a really loud, low hum. The filter caps all measure OK (and it looks like they have all been replaced), there's only about 15v p-p sawtooth ripple on the main supply and the bias supply is clean. The hum balance works as you'd expect, but the hum gets REALLY loud just a little bit off the optimum position. Even the optimum position is too loud. The power tubes are new-ish JJ 7027s, and they're not red-plating. I haven't tried to measure the plate current (don't have a plug-in bias rig). The preamp tubes might be original (the 6K11 certainly is :-\). The 'polarity' switch was disconnected at some point. Any suggestions?

Schematic here: http://www.vintageampeg.com/wp-conte...014/07/V4B.jpg

Thanks

[DrGonz78]

Definitely check the output bias. They don't need to be red plating to be biased a bit hot especially those amps. Not saying that is the cause of the hum but should be checked. I remember working on a V4 that was biased too hot and at idle it made it hum loudly.

[g1]

I'd also suggest verifying the hum balance pot with a meter, even if it seems to be doing it's job. Bad hum just off the optimum setting seems real suspicious.

[nevetslab]

This amp is on my bench and it has a really loud, low hum. The filter caps all measure OK (and it looks like they have all been replaced), there's only about 15v p-p sawtooth ripple on the main supply and the bias supply is clean. The hum balance works as you'd expect, but the hum gets REALLY loud just a little bit off the optimum position. Even the optimum position is too loud. The power tubes are new-ish JJ 7027s, and they're not red-plating. I haven't tried to measure the plate current (don't have a plug-in bias rig). The preamp tubes might be original (the 6K11 certainly is :-\). The 'polarity' switch was disconnected at some point. Any suggestions?

Schematic here: http://www.vintageampeg.com/wp-conte...014/07/V4B.jpg

Thanks

You can measure plate current by first (with AC mains disconnected, supply caps fully discharged) measuring the DC Resistance of both plate windings from the centertap. If your meter has a Relative function, that will null out the test lead's resistance. If not, measure your test leads resistance first, then measure both plate windings, and subtract the test leads resistance. Next, with one pair of power tubes installed at a time (upper half, lower half), power up the amp, warmed up, out of Standby, and record the voltage drop across the two plate windings relative to the center tap. Swap power tubes so you can get the individual tubes' voltage drop across the plate windings. Then, divide that DCV reading by the winding resistance, and that will give you the plate current per tube.

[mhuss]

Oddly, I re-did the output tube ground wiring (the hum balance is factory connected to an output tube cathode pin) and the hum went away.

Thanks for the help and suggestions.