Hello! I’ve recently had success in turning a small transistor radios into a portable guitar amplifiers and as such, I decided to step it up and try the same thing with a big ol’ 1950’s tube radio. It’s a Blaupunkt Palma 2435 from 1957 or something. It has seven tubes, and in addition to FM/AM radio it also has tape deck and phono inputs. I connected a 1/4” jack to the phono input and managed to get decent clean sound out of it (didn’t get it to break up as excpected, but it makes sound alright).

However, after a bit of googling, I stumbled on a bunch of threads pointing out the major safety concerns with tampering with unearthed old appliances in this way. So in order to make sure I don’t electrocute myself I thought I’d check with you guys. So if I understand things correctly, as long as the radio has an isolated power transformer, the approach described above should be safe right? Since I’m having a hart time to tell for myself, I took a print screen of the part of the schematic containing the 230v input, and I’ll provide a picture of the insides as well in addition to the full manual. Hoping someone with more knowledge within circuitry will be able to help.

What do you guys think? Is it safe? Why/why not?

Cheers,
Halloy

The first thing you need to check for is voltage on the ground/chassis of the radio. Since it's not earth grounded and it's really old there could easily be full mains voltage on the ground. That means if you touched the amplifier (or anything connected to the amplifier, like your guitar) and something that IS earth grounded at the same time, it could kill you. To test measure AC voltage between ground in the amp, and ground coming out of the wall, being careful not to touch the amp while you do it.

A lot of old radios had capacitors going to the chassis from the AC line. This was done to prevent RF interference. Sometime (actually a lot of the time) the caps shorted and put the full AC mains voltage on the chassis. Samwinston123 had a very good suggestion. Always good to check for AC voltages on the chassis before working on the equipment.

danger will robinson....

Old vacuum tube radios were not intended to safely interface with humans.

There is no simple way to protect the input like with a transformer because of the high impedance guitars pickups need.

I would pursue a different project.

JR

You have to look at the tubes. If you have the heaters in series, it's not a suitable DIY for someone who hasn't played with power circuits and isolation before (look up "all american five" if you're curious...).



-tINY

I wouldn't be playing about with this one, unless you can find a way to make it safer and add in a ground.

so much hand wringing.

the schematic clearly shows a power transformer.
the circuit common is isolated from the ac line.
it is not a "all american five", the tube heaters are in parallel.
no death capacitors.
this thing looks safer than an unmodified Ampex 351.

as suggested, prudent to check for AC leakage on the chassis as aged power transformers can break down internally.

install a grounded ac cord and you should be good to go.

Fwiw, I've done a couple of these. A 5c1 is a good circuit to implement. Here's the last one I did.

Solid advice here. I didn't see the whole schematic - but there's at least one or two tubes that use a 6.3v heater in there... maybe all are galvanically isolated from mains power.


-tINY

Thanks for all your replies guys! Will definitely check for ac leakage from the chassis. Regarding installing a grounded ac cord, is it just a matter of replacing the old one and soldering the ground wire to some convenient location on the chassis?

That's the usual procedure. Make sure that any "death cap" is removed and the fuse is on the hot wire (after the power switch).



-tINY

1. drill hole, suitable for M4 or 8-32
2. insert bolt/screw
3. external tooth lock-washer, aka in some parts as "star washer"
4. nut, tighten securely
5. flat washer, not essential but proper
6. carefully crimp or solder RING terminal on green or green/yellow ground wire
7. place ring terminal on bolt/screw
8. another flat washer
9. lock-washer
10. nut, tighten securely.