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Shorted primary?

10/25/2004 5:15 PM
Mark Lavelle
Shorted primary?
I think the 270EX in my Verberator may have a short in the primary. My secondaries are *way* low even with all the tubes removed (e.g., under 80 VAC across the red leads), and I've already been through a couple of fuses on the primary side after not finding any shorts on the secondary side of the circuit.  
Does anyone out there have a 270EX they can measure? I'm getting less than 3 ohms across the primaries (a 270DAX I've got measures 5.2 ohms).  
Any other suggestions are welcome, too, of course...
10/25/2004 10:01 PM

Disconnect the secondaries altogether so only the primary is connected to the mains. Put an ammeter in place of the fuse, or leave the fuse and connect the meter across the power switch so you can measure draw. The unloaded tranny should not draw more than a tiny bit of current. if it draws moderate or heavy current, then there is trouble inside it.  
You may have removed the tubes, but with the secondaries connected, you still are applying voltage to filter caps and rectifiers, heater related parts, and so on.  
Make sure to check the tranny cold for shorts between windings and from each winding to frame.  
And don't forget to remove any surge suppressor MOVs from the mains input and any death caps. I usually disconnect the primary and connect it to the mains via test leads for this stuff.
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10/29/2004 10:05 AM
Mark Lavelle

Thanks for pushing me in the right direction, Enzo.  
I'm not sure what I was thinking the first time I looked at it, but last night the ammeter & the good old light bulb limiter proved to me that the short is downstream from the PT (i.e., everything is cool with the B+ secondaries disconnected).  
Since I'm not reading a short anywhere on the B+ out by the tubes, does that point to the PSU diodes & R/C filters?
10/30/2004 12:05 AM

When I see a tube amp blowing fuses I always suspect the power tubes first. If I pull them and it still blows fuses I look for shorted rectifiers, shorted flyback diodes off the output tube plates, and output tranny shorts.  
SHorted filters could cause this, but that doesn't seem to happen all that much. Also, if it is not the first or second filter, it won't blow fuses anyway. Once the B+ goes through a 10k resistor to get to the preamp supply, shorting said supply only puts a 10k resistor across B+, and that won't blow fuses.  
If there is no short to ground on the B+, then check those things. Disconnect the CT of the OT. If the fuse now holds, it is tranny or flybacks.  
You can check rectifier diodes in seconds, so why not start with that.  
Of course there are other odd things it could be - a little mylar cap shorted somewhere say.  
And don't forget the 6VAC lines. A bad pilot lamp socket can kill you. If it is coming loose and the two terminals rotate and touch, there is a dead short across the 6V winding and that will surely blow fuses. Might as well look into the bias supply too.
11/6/2004 4:24 PM
Mark Lavelle

"You can check rectifier diodes in seconds, so why not start with that."
That was it! The only trick was finding a few seconds in a row to spend on it... ;)  
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