I think Thoglette gave you some good tips earlier.
When no schematic can be found, you have to play Sherlock Holmes, and deduce the schematic yourself, or at least, the important parts of it.
For instance, I see what look like op-amp chips on your board. Look up the part numbers, that will tell you which pins are input, which are output, which are power supply pins. You can then use this information to (a) identify the power supply traces on the board, and (b) listen for noise (using an audio test probe) at each op-amp output.
In the same way, you know the pinout of the valve (12AX7?) used on the board. That means you know where the audio signal enters each triode stage, and where it leaves. You also know which PCB trace carries the B+ voltage.
Test the triode control grids and anodes for noise with your audio test probe. Use the knowledge of which trace carries valve B+, and which carry the op-amp power supply voltages, to entirely eliminate sections of the PCB (i.e. isolate the sections that are parts of the power supply, and not parts of the audio chain.)
Of course, you also know where the signal pin of the input audio jack is, and which pins of the output XLR and 1/4" jacks carry the output signal. This means you can follow those PCB traces back to whatever active device (op-amp, vacuum triode, transistor) they come from.
The audio test probe need be nothing more than a small audio amplifier with a reasonably high input impedance, a small output power (so that it never gets loud enough to startle you or damage your hearing), and a small capacitor connected in series with the input to block DC. Touch it to the op-amp input and output pins, listen for hiss.
Simple tests like this will lead you to at least a block diagram of the amp. You will be able to tell where the signal comes in, which active devices it goes through, and in what order. The audio test probe will tell you if there is a sudden increase of hiss at one stage; when you find that stage, you know the trouble is between the last non-hissy stage you probed, and this hissy one.
At that point, get more detailed in that area. Put your audio probe on the power supplies to that active device - is there hiss on the power supply rails? Look closely for signs of visual damage - are there capacitors that have bulged or broken, which are no longer filtering out hiss? Use your nose: smell that area of the PCB, are there any components that smell as though they overheated or burned?
Another thing to consider is, what failure modes can generate more hiss than before? I've seen transistors go bad, become leaky, and hiss. Zener diodes always hiss, and the hiss is usually suppressed by a capacitor or two wired across it; if those capacitors fail, you'll have hiss on your hands. So look for zener diodes on the board, then find and check the capacitors wired across them.
In the end, this is just an audio preamp, so the circuit can't be super-complex (compared to, say, a 32-band graphic EQ, or a compander, or a digital audio processor using DSP.) It should be possible to get a decent handle on it just using some of the simple steps outlined here, and in Thoglette's earlier posts on this thread.
Good luck!
-Gnobuddy
I think you may have to end up taking Gnobuddys advice.But looking at the pictures you posted it won`t be an easy or quick task to correctly trace the circuit.I know its less than rigously scientific but after 53 years of experience my money is still on the tube.As tubes age they become more and more headed to space charge limits which causes a huge increase in white noise when used in high gain apps.Test (as suggested) for noise in at 1st grid and then noise out at the second anode then isolate 1st grid (cut track or remove ccrt component to isolate) and repeat the test. If the tube is innocent Well then you are just going to have to work through the hard way.BTW the guys on this site are great (even if we do go off-topic) If any us had a cct diag I am certain we would share it to help you.
Good luck
Good luck
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