Silvertone 1474 Quit Working (Help)

[Thompson]

I was playing my Silvertone 1474 and it sounded awesome. The tremolo and even the reverb worked great. Then the sound faded. When I got around to the back I saw a little smoke that looked like it was coming from an output transformer, but definitely smelled burned plastic. The number 6 pin on one of the 6L6's is singed. Any ideas on what caused this and how to fix it?


the socket

[Chuck H]

Well I don't know a lot about these amps (some here do) but it looks to me like that's pin three. That would be the tube plate and is a very high voltage pin. That would explain why it arched to the chassis. The socket could have become carbonized with age or grime and flux splatter may have formed a carbon trace from the pin to near enough to the chassis that it arched. Whatever actually happened I'd say that socket needs replacing and you should test the tube and the OT to see if they're damaged. I would also replace the screen grid resistors just because the ones in there look discolored from excessive heat. It also looks like you may still have a two prong power cord. That should always be upgraded to a three prong power cord to ground the chassis. Your filter caps are also old enough that they probably need to be replaced. They have a life of ten to twenty years whether the amp is used or not.

[Gtr_tech]

There's no connection to pin 6 on a 6L6, so its pin 3 (plate). You'll need that socket replaced...actually its a good idea to replace them both....the contacts look bad to me. Also would be a good time to have all the pwr supply capacitors replaced as well and any other maintenance needed to make it sound/run the best it can.

[Gtr_tech]

Also.....I bet you had it running into a 16 ohm load when it happened.....which caused it. Don't do that. All that energy needs to have a place to go and if the load is too high and the current can't be completely transferred to it, it will find a place to go. IE, arc to the nearest path to gnd.

There.....thats my "One after 909" post.........

[Thompson]

Thanks guys. I'm still new to this. Ok, so it's pin 3 not 6. I should replace the socket and tube?

Also, how can I tell if the OT is good?

Finally, does anyone know where a good diagram is to show how to properly install a 3 prong cord? Like I said, I'm still new to this.

[Thompson]

I'm not sure about the 16 ohm load. A speaker wire may have come loose, but it has a jensen p12q and one like it. I think these are set up with two separate OTs going to their own speaker. Then it looks like each speaker has a choke in parallel.

[Chuck H]

Learn how to test the transformer (and a bunch of other stuff) here:

http://www.geofex.com/tubeampfaq/tube_amp.htm#Section8

The three prong cord is done for safety reasons. It's hooked up the same as the two prong cord but the third wire, the green one, is physically connected to the chassis with a bolt or solder and you'll disconnect the mains (other two wires) from the polarity switch nullifying it's function.

All the orange capacitors (and the smaller tan one) should be replaced with like value and voltage specs. These caps are "polarized" meaning they have a - end and a + end. Be sure to observe correct polarity.

You could repair the amp by replacing only the bad socket. There's a slim chance the other sockets may be prone to a similar fate but do as you see fit. replace all four if your feeling ambitious.

Do replace those screen grid resistors. They appear to be little half watt units. Use three watt instead.

It occurred to me that the arc may have happened partly because the tube pin wasn't making contact with the socket. Dirty or loose tube sockets can cause all kinds of problems and are very common in older amps. Clean and retension any sockets you don't replace.

[Thompson]

Ok. Thanks again Chuck. So do I leave the polarity switch in and working when I put in the 3 prong cord?

[Chuck H]

You may have read my post before I edited it. You will want to disconnect the polarity switch. You could just snip out the capacitor that's soldered to it, and that would work fine but then if the switch ever shorted in some odd way you could have a problem.

[Thompson]

Ok. I will. BTW, Chuck thanks for the link and info, but it looks like testing the OT is above my level and requires equipment I don't have access to. Is is safe to assume that since it looked like it was smoking it probably needs to be replaced or can I replace everything else and see without it blowing out a cap or resistor?

[Chuck H]

I'm not at all assuming that the OT was damaged during the failure or related to the problem. At least test for a dead short between the primary and secondary or either winding to the frame. If there is no short I would leave it in there and see how it goes. If it is bad it may take out a component but hey, you roll the dice, you take your chances. I'd rather replace a cap or resistor than the OT. Buying an OT is expensive insurance for a couple of board components.

[Thompson]

Ok. That makes sense. I really appreciate the help/advice.