Hey All,
I have a tube guitar amp that the power transformer just broke on. The primary winding has a short (I determined this by using the 'light bulb' in series trick. When putting a 60W light bulb in series w/ the power transformer the light bulb lights up since there is no load across the primary windings). I'm trying to determine the cause of the problem, and I thought some of you might have some ideas. Here's what I know so far:
1) The transformer has one primary winding (115V) and three secondary windings (320-80-0-320, 17-0-17, and 6.3V).
2) There was a 3.15A fuse on the primary winding and a 1A fuse on the HT when it blew.
3) The amp had previously had a screen resistor blown out on one of the output tubes. I replaced the broken resistor with one of the same value/wattage and checked all of the leads on the PCB near the blowout to make sure no traces were damaged . It was my assumption that this blow out was caused by a faulty power tube, so I replace all of the tubes as well.
4) After replacing the broken screen resistor and the tubes the amp worked great for about 6 hrs total playing time.
5) The primary winding broke while in standby mode, after playing it for about an hour at moderate volumes. (Actually it was the primary fuse which blew, leading me to discover that the primary winding had a short).
6) The amp is configured in such a was that when in standby, there is nothing connected between the high voltage secondary windings (no caps, resistors, fuses, etc). Therefore, the only things drawing current while the amp was in standby were the tube heaters, the channel switching power supply, and the output tube bias power supply.
7) I've pulled the PT out of the amp and checked most of the connections in the output section (primarily the tube heaters, bias circuit, etc).
I can replace the PT, no problem, I'm just trying to figure out how/why the PT might have broken in the first place. I would hate to spend the money to buy a new transformer, only to have it break too.
What confounds me is that it blew while in standby, not during the hour long high volume session before it. Even so, shouldn't the fuse on the primary side have protected the wire from conducting too much current? Could something else have caused the problem (Power surge, overheating, bad tube, etc)?
Thanks everyone!
I have a tube guitar amp that the power transformer just broke on. The primary winding has a short (I determined this by using the 'light bulb' in series trick. When putting a 60W light bulb in series w/ the power transformer the light bulb lights up since there is no load across the primary windings). I'm trying to determine the cause of the problem, and I thought some of you might have some ideas. Here's what I know so far:
1) The transformer has one primary winding (115V) and three secondary windings (320-80-0-320, 17-0-17, and 6.3V).
2) There was a 3.15A fuse on the primary winding and a 1A fuse on the HT when it blew.
3) The amp had previously had a screen resistor blown out on one of the output tubes. I replaced the broken resistor with one of the same value/wattage and checked all of the leads on the PCB near the blowout to make sure no traces were damaged . It was my assumption that this blow out was caused by a faulty power tube, so I replace all of the tubes as well.
4) After replacing the broken screen resistor and the tubes the amp worked great for about 6 hrs total playing time.
5) The primary winding broke while in standby mode, after playing it for about an hour at moderate volumes. (Actually it was the primary fuse which blew, leading me to discover that the primary winding had a short).
6) The amp is configured in such a was that when in standby, there is nothing connected between the high voltage secondary windings (no caps, resistors, fuses, etc). Therefore, the only things drawing current while the amp was in standby were the tube heaters, the channel switching power supply, and the output tube bias power supply.
7) I've pulled the PT out of the amp and checked most of the connections in the output section (primarily the tube heaters, bias circuit, etc).
I can replace the PT, no problem, I'm just trying to figure out how/why the PT might have broken in the first place. I would hate to spend the money to buy a new transformer, only to have it break too.
What confounds me is that it blew while in standby, not during the hour long high volume session before it. Even so, shouldn't the fuse on the primary side have protected the wire from conducting too much current? Could something else have caused the problem (Power surge, overheating, bad tube, etc)?
Thanks everyone!
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