Which pre-amp valve would be V1 in my Carlsbro 60 Twin?.

[bluesjunior]

I have recently taken this old non master volume 1973 amp out of my store room where it had been gathering dust the last few years. As I had been getting a slight intermittent crackling type sound when turning up the volume knob I decided to give it a service.
So today I stripped it all down and took the amp out of the cabinet. I discharged the caps to earth and had a good look at it and it was surprisingly clean inside. Nothing obvious to the naked eye. The only thing I saw which looked unusual to me anyway was that with the amp on its side, the pots etc at the top and the circuit board facing me with the capacitors on the right hand side. I noticed that just to the left of the caps there was a sort of metal ring, it looked a bit like the track inside a potentiometer. There was a build up of what looked like carbon dust on it. I carefully cleaned it off with an old toothbrush. As all the solder joints looked good I just gave all the pots, jack sockets and the valve housings a good clean with some switch cleaner and after inserting an old EL34 and ECC83 a couple of times in each socket to clean the contacts I left it to dry. I am finishing off some repair work on the cabinet tomorrow and I shall put it all back together and try it out.

Does anyone know what the piece I mentioned above with the black carbon type powder on it is and if this buildup was normal after years of use or is it something I should know about that needs changed renewed.

On this amp the pre-amp tubes are 4 12AX7's. One I could see was connected to the volume and tone controls for channel one. Next to that socket there is an empty hole and then the three remaining 12AX7's in a line going to the volume and tone controls for channel two. I am using the three Groove Tube 12AX7's which came with my Fender BJr' as I put in a whole set of JJ's into the Fender when I bought it as well as a made in the USA 12AX7 which was already in the Carlsbro. I figure the US made one is the best of the 4 and would like to put it in V1 but am not sure which is V1. Is it the one furthest to the left or right of the three together or is it the solitary one going to channel 1?.