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Re: Help! Marshall 4203 blowing fuses, not PT

1/31/2006 5:06 PM

R.G.

Re: Help! Marshall 4203 blowing fuses, not PT quote: "I pulled the the tubes from the amp and powered it up, hit the standby and this time the fuse did not blow. This leads me to believe the problem is in my tubes somewhere. "   It's the tubes. That comes in three flavors:   1. A tube shorted internally on the operating electrodes - K to P, K to S, etc.   2. A tube shorted internally across the heater windings, putting a short on the heater winding of the PT.   3. A power tube with shorted, improper or otherwise missing negative bias on the grid.     Generally, the output tubes and perhaps the phase inverter tube is the only one that can pull enough current to kill a PT because of the power supply arrangements.     If you can afford one fuse per test, do it this way.     With all tubes pulled out, turn it on. Fuse holds (i.e. the power indicator stays lit)? If yes, plug in one power tube. Does the fuse pop immediately? Yes, that's (one of) the bad tube(s). No, let it heat up. Fuse pop? Yes, the bias is probably bad.   No, put in the other output tube. Pop immediately? *That one* is the bad tube. No - let it warm up and see if the fuse pops when it's warm or if the plates on either one get cherry red. If the fuse holds and the tubes don't red place, put in the PI tube. Keep checking until you find the tube(s) that make fuses pop. Then you'll have a much smaller place to look.   quote: "Is it possible that a tube could have been responsible for my power tranny blowing in the first place? "   Yes. Shorted or otherwise overcurrent tubes will blow a power transformer especially if they're just a little over. Big overcurrents pop the mains fuse.     The worst thing you can get in a tube amp is a tube that's shorted across the heater voltage. That will eat huge currents from the heater winding and generally not blow the line fuse until the winding melts. One bad tube can kill the power tube, the single most expensive part in the amp.

 

Replies:

John L. Awesome! Thanks for the help. I wil... -- 1/31/2006 10:44 PM John L. Hi,OK Heres what happened. ... -- 2/1/2006 2:59 PM