converting old radio for guitar use

[milothicus]

i've seen a lot of small desktop tube radios in antique shops for less than $20, and would like to just add an input to be able to use it for guitar. i don't expect a lot of volume, but something to play with anyway.

i did a search and found one guy who was looking to convert an old portable radio, and decided that particular radio was not a good candidate for the conversion, mainly because of its unconverted value.

what features would be desirable on an old radio for the conversion?

i'm looking at one that has a phono input, which was mentioned in the other radio conversion thread as a good thing...

that one thread is the only mention of this that i've seen. does that mean it's not worth doing? or people just haven't tried it?

[PRNDL]

There are several issues:

Most old radios are directly coupled to AC, which means adding an isolation transformer.
Few have mono inputs.
The main reason they are inexpensive is they were cheap to begin with, which means low quality components and unusual tubes.
Most are point-to-point nightmares

I recently purchased a very old Fanon mono amp and ran into a host of problems. I ended up scrapping the amp and built a novel single-ended 1 watt amp in a new chassis. The flimsy cabinet had a lot of buzzing, which made it useless. I spent about 3 months working on it, and ended up scrapping everything. On the other hand, the 1 watt amp is absolutely fantastic.

Here's a link to the old radio
http://www.naturdoctor.com/Chapters/Amps/Fanton.html

And here's the single-ended 1 watt amp
http://www.naturdoctor.com/Chapters/Amps/MiniChamp.html

[Dingleberry]

Hi.

Mys first "amp build" was actually a tube radio coverted to cuitar use.
Simply just put a jack socket in place of phono input.
That was about eight years ago. Sound level wasn't earth shattering (because there is usually just one triode stage before ouput tube in radios), but overall it sounded nice and was perfect for home playing, and I was able to use it as a radio also. I was hooked in tubes right that moment.
During the millenium it was easy to find tube radios here in Finland from flea markets under 20€.
I bought a new radio and started studying tube theory from old radiobooks and learnt also by trial and error method (the best way of learning I think).
I learnt to read schematics and realized that if I want to get the most of out tube radio you I to cannibalize the radio section and rewire the preamp.
That I did. Worked. Built several small amps for my guitar player friends and by building learnt more and more. Easy and economic way to learn and get hooked and be able to produce good sounds from the past.

If you are after a radio, go for one from fifties or sixties. Make sure it has power transformer. They usually have tubes that are available nowadays and with E-series tubes (6,3V heaters). Here in Europe the most common output tube in the radios was EL84 (maybe 6V6 in U.S.A?). Percect for a small amp. If you already know how tubes and how tube guitar amps work just take the transformers and build a five-watter that suits you. Champ, ax84, there are plenty of choices.

My last build was actually a sort of Marshall18W/Tweed Deluxe hybrid.
I got both transformers and plenty of mustard caps from a big tube radio which had p-p EL84 output stage.
Haven't seen those often.
Paid 30€ for it...
Amp uses now 6V6 power stage and paralleled triode preamp and long tailed p.i.
Sound is amazing and loud!

You can also find plenty of tube radio schematics from internet. They will help you. And I know here must be plenty of other people who have built amps from radios.
Hope this helps.

-T

[milothicus]

Thanks for the replies.

i guess the most important thing is a radio that has a power transformer. having to buy and add an isolation transformer is not what i want to be doing with this.

i just want something to play with. i already have a decent full-size tube amp with 2 EL84 (6BQ5) output tubes, and i'm trying to get an old 1955 open reel recorder to work.

again, thanks for the replies.