Hi Guys,
I have an early eighties Peavey Classic VT amp which I've had for many years. When it's switched on with no guitar connected, all volume controls set to zero, I have excessive noise which can be heard here:
https://soundcloud.com/user-676608456 I took it to a local guy to sort out who I have used before and is very good. He has serviced the amp and replaced the valves. When I tested it before I brought it home I pointed out that it was still noisy. He suggested that it is due to the age of the amp and general electrical noise introduced by the components into the signal chain. Having tried it in my quieter home environment, I'm just not happy about how noisy it is. I'm going to take it back to him, but thought I'd see if anybody here could offer any thoughts on this.
I believe that the amp still has the original capacitors - could it be that they are the source of the white noise? Why is there noise even when no instrument is connected? The level of the noise level steps up for a few seconds, then cuts back down again, could this point to a specific component that has failed or is on its last legs?
The amp has a solid state pre-amp and tube power amp (2 x 6L6GC). The noise appeared some time back, but as I had another amp to use, I didn't bother getting it sorted out. I've now fallen in love with the amp again, and would love to re-instate it to it's former glory.
Any help would be much appreciated!
Cheers,
Paul.
Comments
The electrolytic filter caps won't be the source of the noise either, although it is possible (or likely) that they need changing anyway - they usually cause hum rather than hiss.
The noise sounds more like a failing carbon-comp resistor to me, although it could be a variety of things including one of the many ICs… tracking it down will be the difficult bit.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Just because I don't care, doesn't mean I don't understand." - Homer Simpson
The alternative could be for me to run through the circuit board with a multi meter to check each individual component (usual safety caveat regarding high voltages associated with tube amps would apply!!!). Whether or not my man had carried out this procedure, I'll have to ask. Thanks for the heads up on the resistor - I only hope that it's something that simple!!
Thanks for taking the time to respond.
Paul.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Just because I don't care, doesn't mean I don't understand." - Homer Simpson
Perhaps best left to the professional amp repairer!
I have the schematics for the amp, so if it is a resistor failure I'm guessing that the culprit could be identified fairly quickly by taking a reading across each resistor?
I'm now concerned about the static issue, as some years back I took it to be serviced by another amp "repairer" for some pot replacements. note the "" marks! He didn't instill much confidence, so could have done more damage than good!
You won't be able to identify a faulty resistor by measurement usually. Normally the best way to locate something like this is to go through the signal path muting it (short to ground) until you can find the point where the noise is getting in - but be very careful not to short the wrong bit or you can blow things.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Just because I don't care, doesn't mean I don't understand." - Homer Simpson
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:
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 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.