Silvertone 1481. New filter caps. Blown resistor.

[windupsmiles]

Hello. I'm rebuilding a Silvertone 1481. I replaced the OT and the filter caps, but when I plugged it in and flipped it on, the 4.7k resistor that connects the 6x4 rectifier (pin 7) and the 10mf/450v filter cap burned up after only a split second of speaker hum. My caps are all negative-to-ground and wired exactly to the schematic's instructions. What could be causing this? Did i F-up the caps somehow?

Paul

http://www.freeinfosociety.com/elect...ertone1481.pdf

[ParthaD]

Hi Paul... this is a classic symptom that I've seen a couple of times before. In a properly functioning tube amp with a tube rect you will never hear any sound from the speaker until the rect has had a chance to warm up. The fact that you heard the hum as soon as you turned it on a split second before the resistor blew up and its location is the clue. What has most likely happened is that the new OT has a short between the primary taps and its body which is obviously bolted to the chassis so your high voltage immediately went to ground pulling a large current. Since the step where this happened is before the filter caps what you heard was the rectified ripple from the tube. Easy enough to confirm with a continuity test between the primary taps and the chassis.

Good luck!

[windupsmiles]

Suggestions?

I have the 4.7mf/450V filter cap grounded on the mounting lug of the OT... would that be why?

Paul

[ThermionicScott]

You mentioned in the other thread that you replaced all of the resistors and caps (for some reason). I can't access the freeinfosociety website at work, but the hand-drawn 1481 schematics floating around on the Internet have mistakes. Did you rebuild it using them?

- Scott

[ParthaD]

This hi-rez pic might help...

There is no 4.7mfd cap in the circuit (there's a 5/10/20 uf can), there is a 4.7k resistor that blew. That resistor connects from the 6x4 tube to the 10uf cap. In the schematic it looks like it is between the 10uf and 20uf caps but that is not how it is soldered in the amp - effectively it is the same thing though.

Here is a pic that might help. This particular one was taken of the amp in its original state, i.e. before 3 prong cord installation. Hopefully this will clear things up.

[windupsmiles]

Closer still...

I replaced the caps and resistors because most of them had drifted in value and I wanted to start fresh. After talking to a couple people at a local guitar shop, it seems that I need a tougher 4.7K resistor (a 2watt). It also seems that I put in a couple other inappropriately rated resistors that haven't blown up (yet). I'm ordering new filter caps (again) and two other 100k resistors that should be 2 watt. According to the guitar shop guys (techs, not customers), that should do the trick.

Also, the schematic on freeinfo is legit. The problem is me... a newbie with three books, an iron, a DMM, and a calculator. Haha.

Thanks to everyone for their input.

Paul

[ThermionicScott]

The schematic has no grid-leak resistor on the second stage, and a dubious 220k resistor to ground from the first stage's plate. If the 1meg resistor that functions as the tone control is truly connected to ground, then there's no point to the other 220k resistor to ground, except maybe to reduce overall gain. Maybe your amp was legit when you got it, but the schematic certainly isn't!

-Scott

[Lee_ranaldo]

I haven'0t checked the schematic
but my first though was

"some tube rectifiers have a certain capacitance limit" it means not any filter cap will work nicely with some tube rectifier models: Check the tube rectifier datasheet and make sure the total capacitance the tube rectifier is "seeing"