Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: angelodp on September 28, 2016, 05:05:14 pm
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Hi, helping a buddy, an older AA763 non-reverb has now blown 2 PT. Second PT was not brought up on bulb limiter. Caps in doghouse are original as are the board caps. Rectifier is known to be good as are the tubes. Looking for recommend on procedure.
How's this
1. Clear the old Tranny out
2. Check for any damage to other components
3. Recap doghouse and other large hi voltage caps
4. Install new PT.
5. Bring up with bill limiter then populate tubes one a t time w/ limiter test. If clean then do voltage readings
Any suggestions
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an older AA763 non-reverb has now blown 2 PT. Second PT was not brought up on bulb limiter.
How did you come to the conclusion that the original PT is blown?
So, you installed the new PT? What brand, model?
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I will say that it is very, very, very rare to blow a PT. OK, an original may fail, 50 year old part. *Still* rare. You throw in another and there's a fault. I would suspect you'd have 5-10 seconds of stinky-smoky-smelly (which should definitely blow the fuse, but let's say there's a 10 amp NO-BLO fuse in there) and if you didn't get to the on/off switch in that time you *could* burn out the new PT. But if you caught it after only 1-2 seconds of stink/smoke, most of the time they will survive that.
Must employ a light bulb limiter/variac, that's too costly a part to blow off. Oh, and be sure the right fuse is in there, but don't blow off 5 fuses one after another. They too, cost money nowadays.
This is an unusual circumstance. You really cannot detect many of the faults that would plausibly cause this just measuring the tranny, as the DC ohms on the 5 volt winding and 6.3 winding look darn near a short. However, I would take the bad one or ones and compare the ohms readings on the various windings to the new one. That could give you a clue as to what part of the amp circuitry is shorted. Next time, at minimum power up the tranny before you even mount it in the chassis, with no load, with your meter leads clipped to the wire ends *before* you power it up. Take readings. Unplug. Go to another winding.
Try the amp with a different rectifier tube, that's definitely a valid idea. And NO rectifier tube. Got a 5Y3? If so, that's a valid temp substitute that will lower your B+ by 35-40 volts relative to a GZ34. That cannot hurt you and a junky old 5Y3 is a cheap tube. Maybe you have a shorted GZ34...in fact...that possibility hereby jumps to the top of my possibility list.
He's not plugging it into a 220 VAC outlet?
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Hi, helping a buddy, an older AA763 non-reverb has now blown 2 PT.
What does that mean? Be specific. That PT should have a primary winding and three secondary windings. Which winding has failed? Is it shorted or open?
What are the circumstances of the second 'blown' transformer. Did it blow immediately after installing and powering on? Did it work for a week and then blow?
The answers to these questions will provide clues that will lead toward a problem that is causing the PT to blow.
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suggest you follow Paul Ruby's guide for startup read the info in the link
http://paulrubyamplifiers.com/info.html (http://paulrubyamplifiers.com/info.html)
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suggest you follow Paul Ruby's guide for startup
This may solve your issues as well.
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suggest you follow Paul Ruby's guide for startup
This may solve your issues as well.
Thank you Steve, very thoughtful of you to consider my welfare on another persons post. Much appreciated. thumbs up !
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Sour grapes!