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Old 27th December 2007, 07:37 PM   #1
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Default Tosted PW tranformer

I finally was able to do a bench test of a Tubelab SimpleSE that I've been working on for quite a while. In any case it sounds very nice! All for 10 minutes then I decided to flip the standby switch. The 5A fuse in my variac blew and likewise the 3A fuse in the amp's chassis. I replaced the fuses and this time I used a killawatt meter in front of the variac to measure current. The Hammond 274BX was buzzing loudly and drawing about 100watts at only 40 volts ac on the variac. I think the resistance on the 274's primaries was less than an ohm I think.

I think I shorted the primary my PW transformer.

Aside from making a doorstop out of the 274 is there anything I can do? Repair? Core exchange?
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Old 27th December 2007, 08:36 PM   #2
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Strange that the transformer started buzzing after you threw the standby switch...

Are you using solid state or tube rectifiers? With the power off and tube rectifier removed, use an ohm meter to check the wiring on the standby circuit and make sure you're not accidentally shorting a secondary to ground.

-- josť k.
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Old 27th December 2007, 08:40 PM   #3
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Post a schematic of how you wired your power supply, and double check to make sure you really wired it that way.

Next disconnect everything except the primary and check to see whether or not the transformer is actually bad or you have a shorted component elsewhere..

How about rectification - tube or ss?

Snubber network on secondary of pt, switch or ?

Does your standby switch the transformer CT or something else?

Conventional switches have been known to short to chassis when used on the HV dc side of power supplies - particularly as the secondary winding creates an inductive kick when suddenly unloaded - that energy wants to go somewhere and usually what happens is the voltage rises as the field collapses until it arcs over somewhere that is unless you have made provisions for this issue.
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Old 27th December 2007, 08:41 PM   #4
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Disconnect everything from the secondary side and leave them floating. Then repower the transformer and see if it still draws this much power. If not, check for a short further out in the circuit by reconnecting first the filaments and then the high voltage.

Victor
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Old 27th December 2007, 10:34 PM   #5
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Note that although the primary winding resistance is relatively low, your measurement does not indicated that the primary is shorted. (DCR is usually low on primaries.) Most likely you have a wiring or design error as I mentioned above. You may get lucky or not.. (Dead short somewhere?)
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Old 27th December 2007, 10:45 PM   #6
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Thanks everyone. I pulled the rectifier tube out and still the same problem. A lot of current. Then I disconnected the high voltage secondary and tried again. The transformer was pulling about 25 watts and the tube heaters were glowing happily. No buzzing. I'm embarrased I made a quick assumption about the PW tranny.

There appears to be a short somewhere on the PCB. I do have the rectifier diodes on the PCB although I'm not using them. Nothing appears to be charred or melted.

I'll try to get a photo posted soon.
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Old 27th December 2007, 11:05 PM   #7
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Here the innards:

http://mysite.verizon.net/oldamps/simplebot.JPG

In the photo I've detached one end of the power transformer's primary.

In hindsight I would have put the fuse inbetween the power socket and power switch.

The wiring is for tube rectification, and triode output with no feedback. The center tap of the secondary is the standby, but I'm undone that by now. Simple is better.
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Old 27th December 2007, 11:06 PM   #8
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Here's another photo

Click the image to open in full size.
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Old 27th December 2007, 11:09 PM   #9
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The big motor run cap is not actually attached yet, but I included it in the photo so I wouldn't have to take photos again.

I have a cage for the top too. Kidproofing and clumsy me proofing.
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Old 27th December 2007, 11:10 PM   #10
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Quote:
Originally posted by whitelabrat

There appears to be a short somewhere on the PCB. I do have the rectifier diodes on the PCB although I'm not using them. Nothing appears to be charred or melted.
Please check the assembly instructions for the board. Are you allowed to have both the tube and diode rectifiers installed simultaneously?

-- josť k.
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