Peavey Classic VT - Noise help

[Enzo]

I agree the probability of a new set having the exact same issues as the old set is low, but it is the sort of thing you can check very easily and quickly, just to rule it out.


You have two power tubes. Have you tried removing just one, then putting it back and removing the other? Each time running on one tube. Is the noise equal either way? Or does one side seem to be the offender?

The last thing I am is a preacher of replace all the caps, but in this series of amps, they used a ton of little electrolytics in the signal path. There are a number of 2uf/35v ones plus some larger ones like the 25uf/25v ones in the phase inverter. I would plan on replacing all of them.

As to your immediate problem, I agree with Chuck it could be a noisy semiconductor in the phase inverter. Likely one of the transistors.

[Barnsleyboy]

I've just tried the tube removal tip, and doesn't seem to make a great deal of difference. I'm going to return the amp to get it checked out.
I'd prefer to keep the original caps rather than getting them replaced. Apart from the cost, I'd be worried about loosing the tone of the amp.
I'll let you guys know how it ends up.

Once again, thanks to you all for your help and advice.

[Barnsleyboy]

Hi Guys,

A quick update on my noise problem:

My repair man spent 4 hours trying to track down where it was coming from. The following seems to have fixed the background noise

Replaced output valve bases with ceramic type
Remove main board and replace driver transistors

It is now nice and quiet. However when I engage the reverb I get noise. As I increase the master reverb volume it increases, when I switch the reverb off,
it stops. The sounds along with my original clip are here:

https://soundcloud.com/user-67660845...t-reverb-noise
https://soundcloud.com/user-67660845...verb-switching


I can't remember if engaging the reverb originally brought the noise on. I seem to recall that it made
no difference.

So it is either the reverb tank, which is a relatively simple job for me to replace as it's only 2 phono
plugs into the main board, Or it's possibly component(s) in the signal path either before or after the
reverb. One will cost me the price of a new tank which I can fit myself, the other will increase the repair bill by however
much the repair man puts on in labour. If I were to bypass the reverb by connecting the input phono
to the output phono it would still send the signal through any components pre or post reverb so I could
eliminate that from the equation. This would give me a definitive yes or no on replacing the reverb tank.
My question to you Jedi gods of all things electronic is: Will this diagnostic test work? I understand that I
would need to match the input/ output impedence with a replacement tank, so would just temporarily bridging
the gap with a phono to phono connection damage the circuitry?

Any help and advice as always would be very much appreciated!

Cheers,

Paul.

[Chuck H]

I hate to tell ya, but that's probably normal. It just sounds like more gain. The reverb circuit amplifies a very low level signal and some additional hiss and often some hum too are pretty common. I can hear that it's not THAT loud because I can hear the mechanical click of the switch very clearly for scale. It's possible that U6 is noisier than it needs to be, but I think it's just the extra noise floor of the circuit that you're hearing.

Do not plug the tank send into the tank return. The tank requires considerable power to drive and the return circuit is designed to amplify a very low level signal a lot. In other words, the in/out with the tank in between isn't designed to be a 'through' circuit without the tank. You might even cause damage. The reverb circuit is in parallel to the dry signal. If you want to test the tank for noise simply unplug it, but do not connect anything together. The main amp signal will still be operational. and if the noise is something other than the tank it should still be present. I wouldn't bother though. Reverb tanks don't hiss when they fail.

I assume the extra noise is relative to the reverb knob setting? So, at what level was it set for the test? full up? At what level do you usually use the reverb? How objectionable is the noise level at that setting?

[Barnsleyboy]

Thanks Chuck.

Unplugging the tank just generates lots of noise like the guitar isn't connected and I've left an open lead.
But here's the thing, I tried disconnecting and re-connecting a couple of times, and it seems to have reduced the
noise quite significantly. I've got a hunch that the phono connectors might have been a bit oxidized and dirty. Me
fiddling with them seems to have improved the connection.Could the noise be related to a bad connection? If so,
my messing around may have inadvertently fixed it!

Point taken about amplifying any unwanted noise through the reverb tank, and thanks for the advice about bridging the
connection. The last thing I want to do is damage the amp now it's back to it's former glory!

Cheers fella!

Paul.

[Enzo]

Yes, of course dirty jacks can cause noise. You just demonstrated that to yourself. I find rotating the plug in the jack back and forth cleans it fairly well. I also get out a wire wheel for my Dremel to do the job.

[cjenrick]

you can hardwire the coax cables to the reverb tank, snip off the RCA, stick the wire thru the hole and solder it to the jack plate terms. if the coax fits that is, otherwise get out the drill and "borrow" a couple of the shock grommets,

and a lot of those PV's used socketed opamps, so easy to upgrade, order a bag of 4558's

might be some 072's in there also

stick some old burr brown chips in there , maybe some expensive AD stuff,

have you witnessed the Music Man opamp adapters now that is some slick stuff there,
course this is a PV, don't wanna get off topic, might get quoted and chastized,

chips, dips, chains and whips, it's all good,

[Barnsleyboy]

Yep - point taken!

Before I removed the phonos and re-connected, the amp sounded like this:

https://soundcloud.com/user-67660845...t-reverb-noise

at the 10, 30 and 40 second marks, there are spikes in the noise level. I can understand that maybe
the low level noise would be due to a bad connection, but could I attribute the sudden increases
to this same dirty connection?

[Barnsleyboy]

you can hardwire the coax cables to the reverb tank, snip off the RCA, stick the wire thru the hole and solder it to the jack plate terms. if the coax fits that is, otherwise get out the drill and "borrow" a couple of the shock grommets,

and a lot of those PV's used socketed opamps, so easy to upgrade, order a bag of 4558's

might be some 072's in there also

stick some old burr brown chips in there , maybe some expensive AD stuff,

have you witnessed the Music Man opamp adapters now that is some slick stuff there,
course this is a PV, don't wanna get off topic, might get quoted and chastized,

chips, dips, chains and whips, it's all good,

..... Probably a bit outside my comfort zone!!!

[cjenrick]

that almost sounds like RF noise, do you live near a radio station of ham radio operator?

to find out if that is it simply take it to a friends house and see if it still does it,

if you can get the amp to be noisy all the time, take a .1 cap at 400 V just to be safe and put an alligator clip lead on the chassis and the other end on the cap, then go around all the opamps and put the cap lead on the output of each opamp, pin 1, pin 7 etc, see if you can isolate the noise, you are simply shorting out the noise signal at each opamp if it is there.

an O Scope would be real handy in a situation like this,

[Enzo]

Still sounds like a noisy semi to me, they often are a random noise source.

If plugging a guitar into the power amp in jack and zeroing its control does not kill the noise, then the noise is in the phase inverter.

You mention reverb, zero the reverb control, if the noise stays, then it can't be the reverb pan connections.

Your op amps are socketed, yes? U15 is the ultimate preamp out IC, pull it, does the noise stop?

U14 isn't an op amp, but it feeds the output of the two preamp channels into the master, pull it, does the noise stop?