Hi Alex
1.- Does the hum come on as soon as you switch on or after a few seconds as the valves warm up? If on start up I doubt it is a valve and I suggest you get it to Graeme at
www.ampregen.com who will sort it for you and is relatively local. If it builds up then here's a plan of action:
2.- Keep the power amp volume down so you won't damage the speakers and perform each test just for long enough to get a result. As 337alant says it is just possible there could be a quite high voltage coming out of the pre and so use a modest power amp 'in case' and don't touch the pre-out leads.
3.- Don't remove the two PSU valves at any point. These are the 85A2 (Pretty sure this is what is in there, not an 5A2 which is a diode...) and the 12BH7a.
4.-
Unplug from the mains (not just switch off) between each operation and leave for a few minutes for the capacitors to discharge.
5.- Check for any burnt or blown looking components in the pre - they will be pretty obvious ie. blackened or leaking gunk. If so, Graeme again. If not
6.- Note down where all the valves go.
7.- Starting from one side unplug
only one ECC83 from the pre. Power up and check if the hum has gone. If yes you might have a culprit. Note which it is and put it back in the circuit for now and continue. If the hum remained with the valve out continue.
8.- Unplug the next ECC83, power up and try again. Any reduction in hum? If yes, write down what happens and progress to 10.- below, if no, replace the valve and continue.
9.- Repeat for the other two ECC83 valves one at a time, noting any changes, replacing and proceeding as above.
10.- At this point you have checked either two (if coming from point 8.- ) or all four signal valves. If any are malfuntioning you might have found that removing just one of them eliminated the hum. If so this is the valve to replace. You may have found that removal of just two of the valves had no effect on the hum, if so these valves are probably good. If removal of more than one of the valves eliminated or changed the hum then these need further investigation, see below.
11.- Make a cup of tea. Tea is good.
12.- Leave the two known good valves in circuit. Power down and wait. Remove the two suspect valves and swap their positions, so both remain in the pre. Power up. There should be hum but it may be different? No matter, power down, take notes, wait, and continue.
13.- Remove one of the two swapped suspects and power up again. Is the hum still there, same or reduced? If so power down, note and continue.
14.- Replace the removed suspect valve to its new position, remove the existing suspect valve and power up. Any hum?
15.- At this point you should have found that either the hum reduced or was eliminated by one of your actions. If, with both valves in and swapped the hum was reduced you will probably find that with only one or other of the valves in place it will have been eliminated. If so the problem is the valve that was not in circuit when the hum was not there.
16.- There are other permutations but at this point you have enough chance to get the culprit that we can wait to see what you find. Assuming it is one valve then personally I'd buy a matched pair. My reasoning is that there are probably one GD and one Tungsram in each channel, and for channel balance and sonics replacing the pair will help. They don't have to be the same make as Glen's orignals however, try something different if you feel like it. Then say for example it was a GD valve that was faulty, I'd replace both GD tubes, keep the known good one as a spare, chuck the known bad one, and enjoy the sounds.