Peavey Nashville 400

[mrm0to]

Anyone have any experience with these? They're *very* clean amps geared towards pedal steel players. Some friends of me brought one that had scratchy pots, pops, sqeaks, etc. Visually all the components look fine, so I squirted some cleaner in the pots and all the intermittent noises left.  But there's a hissing/white noise still present. Is that normal for these amps? They're huge - 200+ watts, all solid state. VERY loud.

The noise is there w/ the guitar unplugged and is affected by all the controls - notably the preamp gain.

[mrm0to]

I found a schematic. Here's the input -> pre knob.  What're those diodes doing? I can't see how any current would pass through them and, if they failed, wouldn't they dump a bias offset onto the OpAmp?

Is there a way to isolate the OpAmp w/out desoldering? I can't just jumper it, can I?

[PRR]

> What're those diodes doing?

If you plug this to a LOUDspeaker OUTput, the diodes prevent your 30V swings from exceeding the opamp's rails. Or not by much.

The excess over 15.6V is dissipated in the 22K resistor. This resistor will smoke at like 75V drop (90V input), long before the diodes are strained. Below that point, it will make awful loud soundz but nothing is smoked. For a stage amp, for systems that use the same plug for 20mV pickups and 30V speakers, stupid things happen and "no permanent damage" is wise design.

> if they failed, wouldn't they dump....

Why would the diodes fail? In normal use they have tiny voltage. In gross abuse, even 1N914 will stand 60V reverse, which is more than speakers carry. Well, yeah, they can fail for no reason; the diagnosis is easy, the input is stuck to the rail and snipping the diode restores operation.

> there's a hissing/white noise

Have you tried it WITH a guitar? Any high-gain amp will hiss. And 33K/0.47= 1:70 is a lot of gain for a +/-15V preamp.... you probably are NOT going to turn that PRE pot anywhere near max in normal use. Set it up to play pretty loud, then damp the strings.

That said: there are now "better" opamps than 4558. This thing is from the 1980s? However, e-gitar developed around the hiss of 12AX7. 4558 and TL072 hiss just about the same. A lower-noise chip, like NE5532, may test quieter, but the AX7 4558 '072 devices will be quiet enough on stage.

There is a remote chance the 4558 -has- gone sour. High hiss can be a sign of moisture leakage, which will kill the chip eventually.

First clean the board. Whatever crapped-up the pots may be laying on the traces and oozing stray current. Not enough to measure, but maybe enough to hiss.

Then replace with a TL072. Drop-in replacement, really a better device. Use a socket.

[mrm0to]

Have you tried it WITH a guitar?

Yes and no. My only electric is a very hummy Les Paul Jr. So I used that to make sure the random junk was gone after I'd cleaned the pots. The white noise, though, is with nothing plugged in.


there are now "better" opamps than 4558. This thing is from the 1980s?...High hiss can be a sign of moisture leakage..

Yeah, 1980 I think. I was thinking it might just be that there's better quality stuff around now and that's what I'm used to hearing. Looking at the pedal steel guitar fourms, it seems a lot of people do swap out the op amps.

The guys who dropped it off heard it, thought it was great as is w/ the noise and took it away, so it must not be that bad. They use it for a vocal amp at a small club they play but had retired it for a year or so due to the pops and squeaks. I don't know for sure, but I bet it sat in an uninsulated garage through at least 1 Chicago winter.