Hi guys,
I have a SSE I built 5 or so years ago that has been going strong until recently. I'm not in front of it right now but it is built with:
- Edcor CXSE25-8-5K OTs
- Edcor XPWR033 PT
- Motor run cap
- Choke
- 560Ohm R17/R27
- CL90 on PT Primary and center tap
- EH KT88
- JJ 5AR4
- No FREDs on board
- UL/Feedback switches
All was well until I blew up C12/C22 a couple months ago. My chassis is pretty tight so there's not a ton of heat loss. Despite using 105C rated caps I figured they probably just dried up. I replaced them and while I was at it I replaced the power tubes and rectifier. All seemed well - bias/dissipation was good (I don't recall what it was at this point) and it sounded as it did prior to popping the caps. The next oddity was the left channel having low output. I traced it down to the 12AT7. I had another on hand and all was good again for maybe 12 hours of use. Then fuses started popping immediately. I checked the PT, choke and OTs for dead shorts to ground and high resistance through the coils - everything checked out. I had some filter caps on hand so I replaced them - no dice. Finally disconnected the PT from the board and jigged it up with no switch (directly wired to the fused IEC socket). It turns out the PT is bad.
I don't mind replacing it but I'm a bit stumped what took it out. I'd hate to replace it just to fry another one. Any idea what the smoking gun is here or what I should test next? Could a bad rectifier arc take out the PT? It is possible that my wife turned the SSE off/on quickly - I wasn't there for the first fuse popping incident.
Any help is much appreciated!
Josh
I have a SSE I built 5 or so years ago that has been going strong until recently. I'm not in front of it right now but it is built with:
- Edcor CXSE25-8-5K OTs
- Edcor XPWR033 PT
- Motor run cap
- Choke
- 560Ohm R17/R27
- CL90 on PT Primary and center tap
- EH KT88
- JJ 5AR4
- No FREDs on board
- UL/Feedback switches
All was well until I blew up C12/C22 a couple months ago. My chassis is pretty tight so there's not a ton of heat loss. Despite using 105C rated caps I figured they probably just dried up. I replaced them and while I was at it I replaced the power tubes and rectifier. All seemed well - bias/dissipation was good (I don't recall what it was at this point) and it sounded as it did prior to popping the caps. The next oddity was the left channel having low output. I traced it down to the 12AT7. I had another on hand and all was good again for maybe 12 hours of use. Then fuses started popping immediately. I checked the PT, choke and OTs for dead shorts to ground and high resistance through the coils - everything checked out. I had some filter caps on hand so I replaced them - no dice. Finally disconnected the PT from the board and jigged it up with no switch (directly wired to the fused IEC socket). It turns out the PT is bad.
I don't mind replacing it but I'm a bit stumped what took it out. I'd hate to replace it just to fry another one. Any idea what the smoking gun is here or what I should test next? Could a bad rectifier arc take out the PT? It is possible that my wife turned the SSE off/on quickly - I wasn't there for the first fuse popping incident.
Any help is much appreciated!
Josh
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Excuse my ignorance as I am an electronics neophyte but can you tell me what the CL90 is? I asking because a many of the components you specified are part of my build too.
My plan is to use tubes I already have for my VTA ST-120 amp which are Sovtek KT-88's, a 12AU7 (I have a few different brands) and perhaps a Weber SS rectifier I can plug into the rectifier socket.
I'm sorry I can't offer a possible solution.
My plan is to use tubes I already have for my VTA ST-120 amp which are Sovtek KT-88's, a 12AU7 (I have a few different brands) and perhaps a Weber SS rectifier I can plug into the rectifier socket.
I'm sorry I can't offer a possible solution.
See here for an explanation -
http://www.diyaudio.com/forums/tubelab/283196-tubelab-sse-inrush-optimal-setup.html
It is a thermistor. When cold it's resistance is 120 ohms. As it warms up it's resistance drops to near 1 ohm. It is rated to do this with up to 2 amperes of current.
Well, new PT showed up today. Got it wired up and everything checks out with no tubes... I think I'll go back to my old rectifier unless someone else can point me in a direction. Certainly a better outcome than cooking the brand new PT. Then again I can't be certain that something isn't drawing more than it should.
I wired the pt directly to my mains fused IEC plug (bypassing the power switch). The secondaries were safely brought out to a barrier strip where I could connect my voltmeter while power was off. I then switched power on using a power strip. The fuses would pop immediately indicating to me that there must be an internal dead short (despite not being able to detect it with my ohm meter).
Everything has been working well with my old tube rectifier running in triode mode with no cathode feedback and the new PT (same spec). It made some intermittent pops when running in cathode feedback so I guess I'll avoid that for now until I want to spend time trying to figure out what's happening there...
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