The frequency of noise.

[C Moore]

I have 2 questions.
I have a "plexi" clone that I built with a mish-mash of parts. It has always worked well and been pretty much "noise' free. I built 2 of them actually. I sold one a year ago and would like to try and sell the other one. It has been in storage for several months. So I thought I would check the bias and make sure everything was OK. To my surprise there is a buzzing noise that it never had before. It is not the low, warm hum of heater type noise, more of a buzzing. I thought I might be able to measure the frequency of the sound at the speaker, but the Hertz function on my DMM reads Zero. If I stick it in the wall power it reads 60. Am I naive for thinking I would be able to measure noise like that.?
Anyway....The noise is there with no instrument plugged in. If I plug in a guitar, the noise is the same. I have replaced the pre tubes one at a time, and that had no effect. I replaced the power tubes, and that also had no effect. The only thing I can do that has any effect is to turn the Mid knob. There is no scratchiness when turning up the Mids, just an increase of the buzzing. I do not know what component in the circuit would be causing this. I have played with lead dress, but I cannot get that to make it better or worse. Anybody have a suggestion.?
Thank You
http://www.webphix.com/schematic%20heav ... w_1986.pdf

[Cliff Schecht]

Noise is broadband and exists throughout the frequency spectrum. It will pretty much follow the shape of your overall sustenance frequency response.

What you are experiencing sounds like a grounding issue to me, maybe between the input jacks and the pots.

[Super_Reverb]

If ishe worked well at one time, then it sounds like a bad or intermittant connection to me. Try rotating all your pots several times throughout full rotation, reinsert tubes and insure all your jacks are snug and verify ground lug connection to chassis as Cliff suggested.

If you have access to a scope and you can't fix it with a quick pass, look at output waveform with no signal. You can tell alot quickly with a scope. I always check with trigger input to line also, which uses the 60Hz line to trigger or sync the horizontal sweep. You get good insight into the degree to which 60Hz noise has invaded your system this way.

With a scope, you can walk through the signal path from input to speaker out and figure out where the signal is going south.

If you don't have one, a used Tek scope can be had for the price of a good transformer.

cheers,

rob

[C Moore]

If ishe worked well at one time, then it sounds like a bad or intermittant connection to me. Try rotating all your pots several times throughout full rotation, reinsert tubes and insure all your jacks are snug and verify ground lug connection to chassis as Cliff suggested.

If you have access to a scope and you can't fix it with a quick pass, look at output waveform with no signal. You can tell alot quickly with a scope. I always check with trigger input to line also, which uses the 60Hz line to trigger or sync the horizontal sweep. You get good insight into the degree to which 60Hz noise has invaded your system this way.

With a scope, you can walk through the signal path from input to speaker out and figure out where the signal is going south.

If you don't have one, a used Tek scope can be had for the price of a good transformer.

cheers,

rob

Yes, I have access to a scope. It is a cheap BK 30 Meg, dual trace; but it is very new and works as far as I know. Do not have much experience with a scope. So, with no input at all, I ground the scope to the amp and use the other probe to look at the signal chain from the input jack to the speaker jack.?
I will give that a shot and see what happens. On an otherwise healthy amp, with no input, would you expect to see a "normal" sign wave throughout the amp.? I have never done this before. I guess the amplitude will not be very high.? In the milli volt range I guess.
Thank You

[tubeswell]

You need a signal generator to see a 'normal' sine wave with a scope

[Cliff Schecht]

Also make sure that wherever you are probing that you stay within the max voltage your probe can handle. It's usually 400V max on a 10x probe for Tektroniks stuff.