silvertone 4790 tube radio conversion

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silvertone 4790 tube radio conversion

Postby bamatim on Thu Mar 07, 2013 8:52 pm

picked this up for $25 and it mostly works.loud constant hum and a hole in the speaker.but it's in decent enough shape for an 80 year old.tuned in some a.m. talk radio and besides the hum all seems well.cool tuning eye on top,10 tubes total with 6v6g output tubes.saw that it should be around 10watts which is plenty for the bedroom.where should i start?i'm not very electrically inclined but i fully understand the dangers in fooling with this old of an amp.and i can solder pretty well and work on all my own stuff.the speaker is a 12" electro dynamic ls with a transformer on it's side.how much of this unit can i convert?i would like to keep as much as i can original but pulling everything not needed for a guitar amp.i also want to keep the eye in the circuit if possible.i'm in over my head but i can handle it with good guidance.so anyone have an opinion?should i just start unsoldering everything and start with a big beefy bare 80 year old chassis?i think this has to be done in order to properly clean it.man it's dirty.plus clean slate start has to be easier than working backwards.,right?
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Re: silvertone 4790 tube radio conversion

Postby gusfinley on Sat Mar 09, 2013 4:26 am

Don't touch it until you can find a schematic!

It's likely that this doesn't have a power transformer for isolation from the main line. It was quite common to stack up the tube heater voltages to equal 110 - 120VAC to eliminate the need for a transformer in the radio days.

Also, I repaired an old tube radio that had the chassis at a -170VDC potential! I fried the mixer I was using to feed a signal into it because I assumed the chassis was ground, but it was HOT.

Guitar amps and tube radios are different beasts, so be sure you know what you are doing before you start tinkering!

Remember, with a guitar amp you need the safety of an isolation transformer because the strings are grounded!
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Re: silvertone 4790 tube radio conversion

Postby gusfinley on Sat Mar 09, 2013 4:42 am

Looking at some specs here: http://www.radiomuseum.org/r/sears_roeb_4790.html

This looks like a guitar amp candidate. Only 6V tubes (and 5V rectifier) means this one should have an isolation transformer.

It is still hard to say what to do without a schematic....

I'd start by injecting a signal into the audio driver section and see if it's got enough amplifaction to drive the power amp at a decent volume.

I've enjoyed putting an "Audio in" jack on old tube equipment and running MP3s through them.

Enjoy the project!
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Re: silvertone 4790 tube radio conversion

Postby bamatim on Sat Mar 09, 2013 7:15 pm

the chassis is now mostly bare except tube sockets and pots.i plan on using this chassis and build up a simple silvertone 1481 into it.i hated to gut it but it looked pretty crappy underneath and was just to old to play with.i have a cheap eminence made fender blues jr 12" speaker coming.this is my first tube amp,man i think it's going to be a cool one.
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Re: silvertone 4790 tube radio conversion

Postby gusfinley on Sun Mar 10, 2013 4:10 am

I know how you feel. I was given 5 old tube radios to tinker with and so far the two I looked at won't be worth fixing up, just fancy paper weights. I was hoping to add audio line in jacks to some of them and use the power amps, but these thinks are just to dangerous!

I may recycle some 50L6s and some 12xxx tubes to build something though...
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Re: silvertone 4790 tube radio conversion

Postby Tman on Fri Mar 15, 2013 5:30 pm

I would think SAMS would have a sheet on that. I see a few possible listings. Check these numbers against the serial number on the unit.

https://www.samswebsite.com/en/photofac ... mit=Search
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Re: silvertone 4790 tube radio conversion

Postby bamatim on Sat Mar 16, 2013 7:09 am

here is the schematic http://www.nostalgiaair.org/PagesByMode ... 017229.pdf sadly mine is before the changes that added phono.can someone look at those and tell me if i can use this transformer or not?i need to be able to convert the amp to 3 pin power.speaker came today.mounted it in the 4790 and tried it out with a roland cube 15xl :jam: and a jackson dinky.sounded decent enough.sold the roland on the bay to help fund this project.just giving it a proper send off lol.probably going to go a different direction than a 1481,don't know what yet.
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Re: silvertone 4790 tube radio conversion

Postby bamatim on Mon Mar 18, 2013 12:58 am

cleaned the inside of the cab,applied new cloth to the baffle,sanded and painted the amp chassis.
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Re: silvertone 4790 tube radio conversion

Postby bamatim on Fri Mar 22, 2013 5:03 am

won a silvertone organ amp to be a donor.i think it has tremelo.3 12ax7,6aq5,5y3.i'll use 2 12ax7 and the pt ot from it for my project if i can.and use the rest for another.also my mom gave me a little 30's i think tube radio that was real screwed up.good knobs,pots,sockets.but it had a sweet little 5" or so 4 ohm full range speaker.i plan on using the 6aq5 on it lol.maybe,who knows.
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