Removing any of the 3 AX7's stops the noise. ... i wanted ideas on how to find it ...
Very valid point.
If you remove a preamp tube of a given channel, you break the signal at the point of that preamp tube. If the signal path is broken, but the noise persists, then the noise has to be happening after the removed tube but before the speaker.
The general technique is to remove the phase inverter tube first (or whatever is the halfway point of the circuit). If the noise stops, it is before the break,
and you just ruled out the other half of the amp, greatly saving time.
If this were a very big, complicated circuit, you'd find the halfway point of the remaining-suspect section, and break the signal there; moving to half-of-half-of-half zeros in on the problem in the fastest manner.
However, guitar amps are generally simple enough that this is needlessly complicated. If the noise stops when the phase inverter is removed,
replace the phase inverter (some folks overlook that!) then pull the next tube, working back towards the input.
I am
assuming you did the pull-replace-pull properly, so that would indicate the buzz stopped when the input tube was the only tube pulled from the chassis. Since this is probably a dual-triode in your case, the buzz originates from somewhere between your guitar and the plate circuit of the latter-half of the input tube.
Now, you could proceed without the quick-n-easy tube-pulling exercise (which is really to help you zero-in before you even remove the chassis from the cabinet), and pick a halfway-point in what you have left, where you will unsolder something to break the circuit. Again, assuming the input tube has 2 cascaded gain stages in your amp's circuit, unsolder one leg of the coupling cap passing signal from the frist stage plate to the second stage grid. Noise stopped? Then it is originating between the guitar and the plate of the 1st tube stage. Noise still there? Then the problem is in the 2nd gain stage.
Again, you could re-solder that coupling cap and unsolder the wire from the input jack to the 1st tube grid to break the circuit in half again. Hopefully, you see the wisdom of cutting the circuit in half to check for problems, then in half again, and so forth so you don't have to work through checking every single element of the amp's circuit.
ALTERNATIVE METHOD: Use an existing small amp and make a "probe" to create the Listening Amp (bottom of
this page). You use this in the same manner as breaking the circuit (half of half of half), but you listen for where in the amp the noise does not exist (or in some scenarios, where it does exist). When you find 2 adjacent circuit points where the noise is in one but not in the preceding one, you've localized the area of the problem.
For both these methods, you are finding
where in the amp the noise is happening. Once you know where, it's up to your electronic/troubleshooting knowledge to figure out why there's noise and how to fix it.
Note that in some modern amps, there are built-in flaws that mean you'll never be able to get rid of the noise. Such a "flaw" may even be too much gain, which is the amp's character, so there's no way to reduce noise without reducing gain and changing what the amp is. Be aware that silly hi-gain and low background noise in recordings can be obtained through the use of much outboard recording gear and/or heavy editing in a digital environment. You likely can't duplicate some of these results in person, and there was probably tons of noise present during the recording.
Topic: How 2 determine origin of buzz? (Read 7808 times)