Maximum Input Voltage?

[PepeSilvia]

I'm trouble shooting an old Peavey Butcher. The output has a constant hum, regardless of volume, and hardly any sound comes out, and when it does, it has a bit of a warble to it. I've put new power tubes in, and swapped out the pre-amp tubes. I want to isolate the problem to either the preamp or poweramp stage, so I was planning to run the "Booster" output (preamp out) to another amp. It's a 1V RMS signal, my question is, is that too much signal for the input of the other amp I'm using? My research tells me that a pickup maxes out around 800mV RMS, so I'm assuming I'll be fine doing that, I just want to confirm.

Thanks!

[52 Bill]

You are only using this set-up for testing the output of the preamp, so there should be no problem with the signal being too hot for the input of the second amp. Just keep the volume down on both amps and listen to hear if the signal is clean or not.

Have you checked the filter caps on the Butcher?

[PepeSilvia]

Visually, one of them does look a bit suspect, it's got a bulge in one end. I used an ohmeter to see if they were shorted, the resistance slowly built up to ~200kohms. I'm guessing this is because they meter is supply a small amount of current and charging the capacitor? Are there any other tests you suggest? I'm 95% sure these are the original caps, and the amp is probably pushing 30 years old, so it's probably about time for a cap job anyways.

[52 Bill]

Read the amount of ac voltage that is on the dc high voltage buss.

When a filter cap goes bad, it will often show up as a ripple voltage on the dc supply. Just meter the dc buss with your meter set to ac volts and see how much ripple is present.

[Enzo]

The filter caps won't measure shorted unless they are shorted. And a shorted cap across teh B+ will blow a fuse, not just result in some hum.

And what Bill said.

AC volts on the DC supply is ripple. Check the B+, and really should find no more than a very few volts of ripple. Also check across the bias supply C26,C27. C26 ought to have about zero AC volts.

And while the tubes all run on 6VAC for their heaters, that AC voltage is elevated from ground by about +25vDC. Set your meter to DC volts and verify that the heaters are indeed up off ground that far. And does turning the hum balance control do anything?

Look again at the Booster Out. It is NOT a preamp out signal, though it is more or less a line level signal. Note it is derived from a voltage divider off the feedback line - which is simply a connection to the speaker terminals. SO the booster out comes right off the speaker and is padded.

You wanna check power amp? Pull V7, the PI tube. Does the hum remain? If it goes away, the powr tubes are clear.