Hi...once again thanks to all of you who have so freely offered your knowledge to this noob..I sucessfully built my first amp from scratch. Couldn't have done it without all your help !
Here's my question: I have and OT that I pulled from an Allied phono/mic amp a while back. I believe it is from the 1940s as there are no IEC codes or any other markings on it. I foolishly did not label the leads, as I hadn't planned on using it, but now I am going to give it a go. I know...lack of foresight on my part. The pimary is easy...2 plates and a CT...but the secondary has 7 taps including the common. I know they are: 4, 8, 16 ohm, and I believe 250 ohm and 70 volt? I know which is the common...the only black lead, but how do I deceipher the others? Physically, the 4 ohm would be the first off the bobbin no ? Then 8 and 16 ohm respectively? I just don't want to hook up a speaker to the other wierd taps and cook it. Any help would be greatly appreciated !!! 😀
P.S. I guess that I should add that this OT was for a pair of 6V6s in cathode bias, with a 5Y3 rectifier.
Here's my question: I have and OT that I pulled from an Allied phono/mic amp a while back. I believe it is from the 1940s as there are no IEC codes or any other markings on it. I foolishly did not label the leads, as I hadn't planned on using it, but now I am going to give it a go. I know...lack of foresight on my part. The pimary is easy...2 plates and a CT...but the secondary has 7 taps including the common. I know they are: 4, 8, 16 ohm, and I believe 250 ohm and 70 volt? I know which is the common...the only black lead, but how do I deceipher the others? Physically, the 4 ohm would be the first off the bobbin no ? Then 8 and 16 ohm respectively? I just don't want to hook up a speaker to the other wierd taps and cook it. Any help would be greatly appreciated !!! 😀
P.S. I guess that I should add that this OT was for a pair of 6V6s in cathode bias, with a 5Y3 rectifier.
Multimeter on Ohms will tell the tail.
More turns on the secondary means more winding resistance means higher impedance tap.
So measure from the common.
Wire with lowest resistance to common will be 4 Ohm tap.
Next highest resistance will be 8 Ohm etc.
Cheers,
Ian
More turns on the secondary means more winding resistance means higher impedance tap.
So measure from the common.
Wire with lowest resistance to common will be 4 Ohm tap.
Next highest resistance will be 8 Ohm etc.
Cheers,
Ian
Assuming the secondary is a single tapped winding, not multiple windings, you could use an ohmmeter to measure incremental increases in the DC resistance, You will need a good meter, preferably digital, and one that uses a constant current source like the better HP's or Keithley's. Also one that is comfortable in the sub ohm range. Many cheap battery operated meters are difficult to interpret at very low ohms, but they can serve if you are familiar with it. Good tight connections are necessary.
Lacking this, you could connect the primary to a low level AC voltage source and then measure the incremental secondary voltage. If you use a low voltage for the primary, you will need a good AC level meter that is comfortable with fractional voltage. If you have access to an oscilloscope, that will show low levels quite well.
Since the transformer can withstand higher voltage on the primary, you can go as high as 120 volts to get the secondary output to a more convenient level. Use an isolation transformer if you chose this method and be carful with the connections.
Lacking this, you could connect the primary to a low level AC voltage source and then measure the incremental secondary voltage. If you use a low voltage for the primary, you will need a good AC level meter that is comfortable with fractional voltage. If you have access to an oscilloscope, that will show low levels quite well.
Since the transformer can withstand higher voltage on the primary, you can go as high as 120 volts to get the secondary output to a more convenient level. Use an isolation transformer if you chose this method and be carful with the connections.
Gingertube: Thanks...I kind of figured I could use that type of deduction to deceipher the OT. I just wondered how to identify the wierd taps...i.e. 70v and that. I did use my multimeter and two of the taps read exponentially higher than the rest. Can I assume these are the 250 ohm and 70v taps? Also, where should I find continuity between taps ? I guess all of them since they share common windings, right?
Hollowstate: So you're saying I can hook the primary to say a variac set low and mesure voltage differences between the taps...from common to individual taps? I would hook the two plate leads to the AC source? Then I would need to be careful of the CT , huh?
Hollowstate: So you're saying I can hook the primary to say a variac set low and mesure voltage differences between the taps...from common to individual taps? I would hook the two plate leads to the AC source? Then I would need to be careful of the CT , huh?
bereanbill said:
Hollowstate: So you're saying I can hook the primary to say a variac set low and mesure voltage differences between the taps...from common to individual taps? I would hook the two plate leads to the AC source? Then I would need to be careful of the CT , huh?
You got it!
Here's some more reading. http://www.angelfire.com/electronic/funwithtubes/Amp-Transformers.html
It's long but for more of what you are looking for, scroll down to the bottom of the page. Cheers
It's long but for more of what you are looking for, scroll down to the bottom of the page. Cheers
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