I understand that mains valve radios are not to be messed with because of the high voltages but are battery valve radios ok for converting to guitar amps. Some guidance would be helpful before I fry myself!
Valve radio circuits are not the same as a valve guitar amp. Some of the valves in a radio (tubes, for those reading west of the Atlantic) are designated as signal detectors (radio waves), while others are there to invert the signal to a usable wavelength for continuing through the circuit. Of course there are valves present to boost the signal (pre-amp), and at least one valve to drive the speaker.
It may be possible to convert the radio. But unless you have a schematic for the radio circuit, you will be working blind.
Perhaps if you had the schematic and posted it here, more help could be offered.
I build guitar valve amplifiers. And personally, I would not take on converting a valve radio to an amp. But that's only my take on the idea.
If you're looking for something small, like a 1 or 2 watt valve amp, there are online resources to help you out too.
I've done numerous high voltage tube radio amps to guitar amps. The easy way to do this is to tap into the volume pot with the hot lead from your jack, your ground goes to a good chassis ground. Use test leads or a built input jack & leads, Install your ground, strum your guitar and touch volume pot leads till you hear your guitar. Test again to see if you have control of the volume and the correct contact and you should be good to go.
Hope this helps.
I realize this is a slightly dated thread. But after reading your reply I noticed something that should be mentioned regarding safety. You said to "tap into the volume pot with the hot lead from your jack"...no worries there.
But you then said "your ground goes to a good chassis ground".
This would seem a normal process if the power cord on old valve radios were 3-prong grounded ones, with the green ground wire soldered/star grounded to the chassis. But old valve radios have 2-prong units without the larger neutral prong to ensure the plug is inserted correctly.
So what can happen is that the plug can be inserted into an electrical outlet with the "hot" mains current running either through the direct circuit path as it should. Or if the cord is inserted in reverse (which can happen), the current can go to the chassis.
If this potential situation is not averted and a guitar is plugged into the converted amp, the moment the player touches a string/bridge, etc, smoke and sparks...
Hey Scott, I meant the ground for the 1/4" guitar jack. I mention the jack in the thread, so I assume the reader would understand there are 2 connections for the guitar jack. (+ & -)
Yes, I saw that. But power cords on vintage tube radios can be plugged in right side up or down, which could result with the ac wall power going to the chassis, and send the 120v ac through the guitar jack negative, through the patch cord negative and up to the strings, if bridge is grounded, or through the ground on guitar pots, resonator metal, license plates.
Was only concerned about safety.
I appreciate your input guys but I have a battery powered valve radio that I fancy converting to a guitar / CBG amp by tapping into the amplifier circuit just as if it were a battery transistor radio. I think I understand the safety issues associated with mains valve radios but I have no idea if there are similar issues with the battery variety.
Oh, and I do have the schematic Farmer Ted!