1485 blowing fuses

[hossymandias]

Hello gurus,

I need help diagnosing a Silvertone 1485. I replaced the output transformers so we can run the amp on 8 ohm speaker loads. When I powered up the amp, the 6fq7 powering the reverb started crackling and arcing. I quickly powered down and removed the tube (I own a 1484 and removed both the 6fq7 that powers the reverb and the 12ax7 that powers the tremolo and the amp works fine without both.

When I powered the amp back on, it seemed to work fine for 30 seconds, then the fuse blew. Under inspection, I noticed R44 (which grounds pins 8 and 3 from the faulty 6fq7) was fried.

Can anyone diagnose the problem for me? Why would R44 fry? Did removing the 6fq7 pop the fuse?

Thanks for all your help!

[hossymandias]

Here's the schematic:

http://www.silvertoneworld.net/schemati ... e_1485.pdf

[Stevem]

That was a shorted tube and it burned up r44 which is the cathode resistor. Now that tube socket may be arced over and blowing the fuse even with the tube out!
Disconnect the wires from 1 and 6, make sure they will not short on anything and fire the back up and your new fuse should hold.

The all of the amps red cased power supply filters and the ones in the bias circuit been replaced yet?

[hossymandias]

Thanks Steve.

When you say "fire the backup" do you mean replace the tube (6fq7)? Can I just leave the tube out?

Also, no the caps are all stock. I was waiting to hear what the amp does before telling my buddy he needs to invest more! We already bought two new transformers.

[hossymandias]

Sorry Steve, I got it.

I'll fire the AMP back up after disconnecting wires.

Thanks again.

[Stevem]

Yup, sorry !

[hossymandias]

Okay, I cut 6 and 1, but the fuse still pops. D'oh! What do you advise as the next step?

[hossymandias]

I removed the 6l6's and was going to power up to see if the fuse still blew. But before I do that, I want to make sure it will not melt down my amp. Can anyone give me a safe go-ahead?

Do I remove just the 6l6's, and leave the preamp tubes/rectifier?

[Stevem]

The 1485 does not have a rectifier tube! But yes pull out all the output tubes to see if the fuse still blows, if it holds then replace the one by one until the fuse blows again.

[hossymandias]

Okay, I'll see what happens tomorrow when I take out the 6L6's. So what is the other 6FQ7 doing? Another preamp tube?

[Teleguy61]

Why not build a light bulb limiter so you don't burn anything up?
Just a thought.

[Stevem]

One 6fq7 is the reverb driver tube and the amp will play just fine without that tube even being in the amp, and the other 6fq7 is the phase splitter to drive the output tubes and if not in the amp and working you will have no sound to the speakers.

Note that these amps are famous for have the output transformers go bad so keep that in mind.

If you find that the fuse still blows with all of the output tubes out then take all the tubes out of the amp and drop down to a fuse of half the rated amperage that is needed, this will let work with out a bulb liniter as was posted yet let you test the basic power supply but not burn up the power transformer!
This is assuming that the power transformer is not already bad, but we will get to that as we step you thru the trouble shooting format!

[hossymandias]

Thanks Steve, will follow your directions tonight or tomorrow. I did replace both output transformers already, and I really don't want to do anything that will put those at risk. Will all of the above troubleshooting keep my output transformers safe?

Also, do I need to replace the wires to pins 1 and 6 on the reverb tube?

[Stevem]

The transformers will be fine as long as you do not pump sound through the amp with no speaker hooked up or if the output tubes have little or no bias to pin 5 and they red plate for too long!

For total protection you can install a inline 160 ma fuse to each of the OTs center tap lead and also wire a 250 ohm 10 watt resistor across each speaker output jack or what have you.

Yes you can leave those 1 and 6 wires off for now until you cure the fuse blowing issue .

[hossymandias]

Okay, I took out all the output tubes and the fuse held. I started replacing them, but it looks like every time I put one back in it starts red-plating. Is that telling?

I won't pretend that I know much about red-plating or tube malfunction so I could be mistaken. But I saw blue glow as the tube warmed up, then the vertical filaments began to glow red. I turned the amp off before the fuse blew.

I tried this with a tube in the first socket, then removed that tube and tried a different tube in the second socket with the same results.

I measured the voltage on pin 4 where the schematic says I SHOULD read 370. I'm getting 420. Could that be the culprit?

Thanks for everything!