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You should be able to connect to the volume pot just the same as on a transistor radio. The important thing is to keep things safe. There are high voltages on valve equipment so don't work on it live. Use a multimeter to test there are no high voltages on your jack socket after you have wired it. It might be a good idea to put a capaictor in line with it too to block any stray DC. I tried it on an olod radio like this but the volume was poor but the radio wasn't in good condition so it mabye had a fault on it. Best of luck.
i've done a few, but mostly i get 'em with the boxes too trashed to use, so complete gut and rebuild to guitar amp circuit using the existing valves, sockets, transformers, knobs and whatever else is usable. resisters and caps i replace.
like this one.
without gutting, i would do the following...
1. identify each valve. the big one closest to the power transformer is probably the rectifier, the next big one is probably the power tube. we are after the preamp tube, which is usually smaller and plain looking.
the radio tubes you can usually tell by their build. they often have strange looking screens inside and connectors on top.
if the tubes still have readable markings on them look them up on a tube database on the net. the database will tell you the pin layout and what the valve is designed to do.
2. once you identify the preamp tube and have the pin-out for it, look inside the chassis and trace back from the GRID pin on the valve base. if that pin connects via a resister to a volume pot, and the pot runs to earth, then you can connect on the other side (input) of the volume pot.
use a switched jack, that earths the input line when you're not plugged in, and pull the radio valves out.
3. if the volume put is not connected to the grid (via a resister), the input switching probably is.
if thats the case, have a look at the way a standard guitar amp is wired and do it like that.
double check the chassis is earthed to your power lead, and rewire if it is not. old US valve stuff without an earth pin on the plug is extremely dangerous IMHO.
and of course, inside of valve amps can carry charges in excess of 400v even when not plugged in. check that your filter caps are discharged before working in there, and read up to clearly understand what you're doing. nottrying to scare anyone off - this stuff is not all that hard to do - but make sure you understand the safety issues and are careful.
oh, and NEVER do that hacking trick from transistor radios of probing around with a signal inside a live valve amp EVER.