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?.

[TweeDLX]

If this amp is set up like an old BF/SF Fender with two separate channels (normal & reverb) "V1" would be the second tube from the right as you look into the back of the amp. Otherwise, it's probably the first one. You can probably follow the input leads to find out... I have no clue about that ring, could it be the remains of a bias pot?

Mike

[bluesjunior]

Thanks for the reply TweeDLX, From what I found out the 1st channel is for Bass or Keyboard and the other channel is for guitar. There is a high and low input for each channel and the limiter and presense are shared. The Response knob which works like a sort of master tone control going from a bassy sound when it is all the way to the left and trebly all the way to the right.

I think the ring is from the Reverb model. I found a schematic on the net and that's what it looks like. Is there any wording to look for on the drawing which might solve this. I can see where it says EEC83 for the pre-amp tubes and EL34 for the power tubes but no sign of anything like V1 although it will probably be called something entirely different. I found these pics on the net of an amp exactly the same as mine. I don't have a digital camera so I hope the guy forgives me for loaning his pics.

[TweeDLX]

Hi!
If you look at your schematic; follow the signal path from the second set of inputs (or whichever one you use) to the first ECC83 in the path. That would be V1. The schematic I found shows both channel sharing the first preamp tube. Each uses one side of the 12AX7 (It was a Carlsbro 60 TR Twin). Found it here:http://www.chambonino.com/carlsbro/carlwires.html

Mike

[jockman]

They are fantastic amps. The above site will give you all the details you need, as will the carlsbro forum. Carlsbro tended to design amps with mullards in mind and as long as you use your favourite ECC83/12Ax7 you will have no problems, they are a wee bit sensitive to biasing, I have found that out over the years, the transformers tend to be partridge or drake, or both.. They also made the same chassis for several different models, hence why you have a space, my 50top had 2 spaces for power valves, as used on the 100top.

[bluesjunior]

Thanks for the help guys. TweeDLX, I had that site but I was looking at the diagram for the 60 TC Twin but you are right in that it is the 60 TR Twin diagram which is the correct one. Now I understand it. There are three pre-amp tubes which are shared on both channels ie V1 a&b, V2 a&b, V3 a&b while V4 controls the Response which is only available on channel two. So according to the diagram the solitary 12ax7 above channel one is V1 then it would follow that after the empty hole it would be V2, 3 and 4.
I will open it up again tomorrow and swap the valves as I have put the made in USA valve in V2.

From one old Scotsman to another Jockman you are absolutely correct about these being great amps. I could never really use it before with my ES335 at home as even with the limiter on full it was still too clean at that low volume. However since building my partscaster with the 25db Dennis Cornell mid-boost system I find that using various combinations of the limiter and the mid-boost I can get some really nice bluesy distortion at room temperature without having to worry too much about annoying the neighbours. Thanks again for the help fellas.