Scrounged power transformer CT question

HiFiFreq

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I have a PT from some old gear that I parted out . Total of 6 leads .
Two for primary , two for secondary HT @ 500 vac and two for secondary filament @ 6.3vac . No center taps . Had it wired into a single ended circuit with a SS bridge rectifier and it got hot immediately so I shut it down . Checked the circuit and can find no mistakes . Pulled the transformer to test on my bench . On the bench with no load and AC applied to the primaries it works fine . With no AC to the primaries I ohmed out the leads with my meter . I found what I expected with low resistance across the primaries at about 12 ohms , .4 ohms across the filament secondaries and 272 ohms across the HT secondaries . But , when I check with my meter from one leg of the secondary HT to the transformer itself I get roughly half of the resistance . My question ; Is the body of the transformer considered a ground or center tap ? If so is my SS bridge rectifier with it's virtual center tap grounded causing the PT to get hot? Also forgot to mention that when the primaries are connected to AC the secondaries will measure about half when taking voltage readings from one leg to the transformer body . So the 500 volts across the secondaries measures half @250vac when measuring from one leg to the transformer body . The filament leads act this way as well .
 
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andrewRneumann

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Your analysis seems logical. 500vac into a full-wave bridge rectifier would get you some serious DC (700VDC unloaded). Where did this PT come from?

Check your rectifier and make sure you didn't fry it when you tried it.
 

HiFiFreq

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Sorry , voltage measured across both legs is 500 vac . Each leg puts out 250 vac . It is a smallish transformer that I'm pretty sure came from a webcor product . I actually have two and just tested the other one and it behaves in a similar way . Also I found a schematic on this site from a webcor reel to reel and it shows the transformer part number which is the same part number on my transformer . Schematic shows center taps but no wire color code for center taps . Using solid state rectifier bridge . 4 x 1n4007 diodes .
 

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printer2

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I came across a transformer where they had the center tap connected internally to the iron core which then must have been grounded to the chassis.
 

HiFiFreq

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Printer , your statement is the question I was attempting to ask . I would reinstall the PT now but I am afraid it might be damaged . Stinky it is . Smells like dead transformer but tests well on the bench , no load. Still good ?
 

HiFiFreq

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Andy , this was a question I was going to ask to make sure I am implementing the rectifier correctly .
I have already converted the rectifier to a half wave using two diodes ,I think .After reinstalling the PT , I removed the virtual center tap by clipping out the two diodes connected to ground leaving the two "forward" facing diodes for my B+ . In your schematic remove D2 and D3 and that's what I have now . It's giving me about 345 dcv unloaded for B+ . Make sense ?
 

andrewRneumann

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Andy , this was a question I was going to ask to make sure I am implementing the rectifier correctly .
I have already converted the rectifier to a half wave using two diodes ,I think .After reinstalling the PT , I removed the virtual center tap by clipping out the two diodes connected to ground leaving the two "forward" facing diodes for my B+ . In your schematic remove D2 and D3 and that's what I have now . It's giving me about 345 dcv unloaded for B+ . Make sense ?

Perfect sense. Technically still a "full-wave" rectifier, just not a "bridge".
 

HiFiFreq

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Thanks again to all here on the TDPRI site for your help . Have to go to work now but tomorrow should have the filament circuit wired and will be ready to start it up . I'll let you all know how that goes .
 

printer2

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Printer , your statement is the question I was attempting to ask . I would reinstall the PT now but I am afraid it might be damaged . Stinky it is . Smells like dead transformer but tests well on the bench , no load. Still good ?
I tend to stay away from bad smelling parts. You might get by if you run the transformer on a fraction of its rated power. I doubt I would want to get a questionable transformer all that warm. Mind you, I have lots of transformers.
 

HiFiFreq

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All wired up now . Seems to be working well . PT not getting hot or emitting foul smells .
Bonus , I have sound that is pretty good so far with my limited testing . I haven't wired in the 6v6 cathode resistor yet . I'm using jumper cables to switch in different values . I'd like to get some input on biasing this amp . Should I start another thread ?
 




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