slightly OT... old tube AM radios, and diy amps.

[Brian Marshall]

Ok, so i was at this antique store today, and saw an old plastic AM radio that was about the size of a half gallon carton of milk.  I picked it up and looked at it.  I saw a diagram on the bottom listing replacement tubes. Looked inside of it through the back and saw 4 tubes inside it.

Now i know that a radio like this could only have appeared in a very brief time period.  most radios old enough to have tubes in them are huge, and come in a wood case and fetch a prety penny.  most plastic radios are transistor types.

ANYWAYS...........
I was wondering if anyone has ever had any luck trying to convert one of these in to a guitar amp.  obviously it would be low power, but that would be so cool?

[Peter Snowberg]

I'm torn when it comes to things like this. If the radio works, I think the antique value outweighs the benefits of stripping it to make a small amp; even if there arn't big technical problems to doing so.

Chances are that to make a guitar amp from it you will have to gut it, replace all the caps, replace the tubes, and add a few new parts. When it comes down to it, it may almost be cheaper to buy a chassis, a couple transformers, and the rest of the parts to build a little amp like a FireFly or a Champ. You will also get much better tone from a "guitar" speaker which is not what you'll find in the radio.

I would pick it up, restore it, and use the experience to build the next thing.

It's very likely that the output stage is done with a single compound tube like the 6BM8. It's a good tube and very worthy of use in good amps, but unless you combine it with a preamp stage, it won't do much. Most (possibly all) of the other tubes will be useless. Since this is a small radio, it may use a selenium rectifier rather than a tube. You DON'T want selenium rectifiers around.

Caution: many little radios like that use a hot chassis and/or other tricks like a series heater string which made them cheaper to manufacture, but much less useful for turning into a guitar amp. If it has a hot chassis, it could actually be deadly to plug into.

That's my 2 cents.

Take care,
-Peter

[Ansil]

it can be done.  although it is usually easier to just use the parts you need to make it a amp.  .i guess is what you mean by convertin it. i took otu an old stereo console radio amp and used all the parts for a firefly  including the output transforemr

[Brian Marshall]

I'm torn when it comes to things like this. If the radio works, I think the antique value outweighs the benefits of stripping it to make a small amp; even if there arn't big technical problems to doing so.

Chances are that to make a guitar amp from it you will have to gut it, replace all the caps, replace the tubes, and add a few new parts. When it comes down to it, it may almost be cheaper to buy a chassis, a couple transformers, and the rest of the parts to build a little amp like a FireFly or a Champ. You will also get much better tone from a "guitar" speaker which is not what you'll find in the radio.

I would pick it up, restore it, and use the experience to build the next thing.

It's very likely that the output stage is done with a single compound tube like the 6BM8. It's a good tube and very worthy of use in good amps, but unless you combine it with a preamp stage, it won't do much. Most (possibly all) of the other tubes will be useless. Since this is a small radio, it may use a selenium rectifier rather than a tube. You DON'T want selenium rectifiers around.

Caution: many little radios like that use a hot chassis and/or other tricks like a series heater string which made them cheaper to manufacture, but much less useful for turning into a guitar amp. If it has a hot chassis, it could actually be deadly to plug into.

That's my 2 cents.

Take care,
-Peter

First off it was made of plastic....  it was only $30, and i know this place... you can usually get it for half.  most of the time they just have junk, and if they have something good, they'd never know it.

I know a lot of older stuff has a hot chassis.... back when everything was 2 wire instead of 3. If it were that would essentially make it useless.

the tubes were all 12ax7 sized (physically) but i dont remember the exact numbers  12b..... something or other.  never heard of them.  it had 4 tubes in it.  

I would think, that even thogh it was a radio it still has to have a preamp in it... probably have to be severely modified to work for guitar... that's fine.  

if i ever see one of these for under $10 im buying it regardless.

[Peter Snowberg]

I'm glad you know about the hot chassis problem. If it actually has an HV transformer, you can always add a grounded cord, but it may just use a voltage doubler to get B+ which would make it unsuitable.

It's very likely that all the preamplification happens in the I.F. stages, but you could always add a 12AX7 in one of the other sockets.

If you go for it, best of luck!

Take care,
-Peter

[Doug H]

It's very likely that the output stage is done with a single compound tube like the 6BM8.

The 6BM8 is a cool little tube. Some of the ax84 guys are experimenting with them. Another compound tube that is starting to get some use is the ECL86 (6GW8). The pentode section is kind of like a "mini" EL84. So with two of these you can build a little low-wattage push-pull output stage, using the triode sections for the PI. I have a couple of these and have some plans for them after my next amp build.

Doug

[Brian Marshall]

hmmmm... now i think i am just going to build an amp from scratch, but i still think it would be cool to mount it in an old radio chassis.


That would be soooooooooo cooooooollll

but what do i know about coo. i'm a geek.

[runmikeyrun]

I know a lot of older stuff has a hot chassis.... back when everything was 2 wire instead of 3. If it were that would essentially make it useless.

Not so... you can get an isolation transformer that is essentially a 1:1 transformer that prevents you from being electrocuted to death if there is a short to chassis.

the tubes were all 12ax7 sized (physically) but i dont remember the exact numbers  12b..... something or other.  never heard of them.  it had 4 tubes in it.

That's good.  If they are 12a somethings (ax7, au7, etc) you can swap out for other common tubes. 12b somethings might be power tubes, good for about 5-10 watts.  I have an old Webcor that uses them, with a 12ax7 in the preamp, and a 5Y3 as a rec tube, it sounds nice.

I would think, that even though it was a radio it still has to have a preamp in it... probably have to be severely modified to work for guitar... that's fine.  

Maybe so, maybe not.  It might really scream when you crank it up or push it hardwith a clean boost or distortion pedal.  Some of these things operate fine even though they've been sitting (although for regular use i would recommend replacing caps) and it will allow you enough time to test it out to decide if you want to mod it or not.  Another thing, these small amps can be used into a speaker sim/load attenuator to get a line level signal and then used basically as amazing distortion pedals.  If you can turn it on in the store, and it works, and you can get it for $15 or 20, BUY IT!  Even if it doesn't work if the housing is in good shape it might make a cool pedal/preamp housing.  Heck, for $15 if it has cool knobs it might be worth it to you!

if i ever see one of these for under $10 im buying it regardless.

[Ansil]

i was doing some thinkin there brian and let me tell you i was goin to make a tube amp otu of this non working tube radio i had. that was given to me to tinker with. and let me tell you. i found out the radio was worth liek 150 usd.   lets just say i sold it.

[Brian Marshall]

I think i will just by an old plastic radio.  I see them all the time for $1-5.  and build an amp inside it.