Silvertone 1474 Twin Twelve Rectifier Tube Smoking

[tomcurrie]

I powered up my old Silvertone 1474 tonight and the 5u4 rectifier tube socket started smoking and the tube was arcing. I looked underneath to find the 220 ohm resistor off pin 3 of the rectifier tube was cooked. Along with the 39 ohm 5W resistor that attached to it. The fuse also blew.

Thinking my rectifier tube had gone bad...I put in a good 5u4, replaced the 220 ohm resistor and 39 ohm 5W resistor.

Powered it on again and saw smoke coming from the socket again. Didn't take out the resistors. and the fuse didn't go.

Could someone check out a schematic and maybe give me some help?

 

[ACETECH]

If you can replace the tube socket. Sometimes they carbonize between pins and short out or become resistive. If you want to isolate the circuit just disconnect one end of the filter choke and turn on, then only C31 will have high voltage. So be very careful measure everything before touching.
Paul

 

[tomcurrie]

I replaced the tube socket, correctly wired it, turned it on and I'm still having the same issues.

Any other ideas?

 

[WaltC]

well, standard troubleshooting procedures apply... and FWIW I don't understand the "I looked underneath to find the 220 ohm resistor off pin 3 of the rectifier tube was cooked. Along with the 39 ohm 5W resistor that attached to it. The fuse also blew" comment. There shouldn't be anything hooked to pin 3 of the Rectifier. B+ AC connections are to pins 4 and 6 and the heater is connected to pins 2 and 8 and B+ DC is taken from pin 8 (usually).

What happens if you fire up the amp with NO tubes in it? Smoke? Blown Fuse? If so, then there is probably something wrong with the power transformer. If not, then just install the 5U4 ONLY and see what happens (this is where a VARIAC comes in real handy, or a light bulb current limiter - cheap and easy to build - as a minimum). If the problem is still present, then there is a good chance that one or more of the main filter caps is shorted to ground and needs to be replaced.

try those (while being very careful, avoiding the hundreds of volts and amp(s) of current present and trying not to get killed), or..... take the amp to a good tech and let him revive it. It's a good amp and well worth the expense and trouble.