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Old 9th April 2011, 04:53 PM   #1
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Default Correct power rating for Chicago OT - opinions

I just acquired an old Chicago BO-12 PP OT and was looking for a good amp to build with it.

The only online catalog I could find lists this unit as a 10W rating suitable for 807s. I think this must be a typo based on tube type and the size of the transformer : 4 X 4.5, X 5.5, 9lbs. 807s could be ran class A triode at 6W but every other operating condition is 15W to 80+W.

So the question is: If it's not 10W, what is the accurate rating? My first guess would be 40W based on size relative to their other 40W iron. Still, 40W seems kind of a narrow application window for 807s, and I certainly don't want to burn up this transformer.

TDSL Tube data [807]

Does anyone have another Chicago catalog to verify this? Any guesses as to the true rating or opinions as to what power limits I should keep?

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Old 9th April 2011, 06:05 PM   #2
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Location: Taxland, New Jersey
Your information is correct. This is a scan from a 1966 Radio Master.
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Old 9th April 2011, 07:07 PM   #3
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Thanks for the scan HollowState. Officially the biggest 10W OT I've ever seen.
Comparing it to the BO-13 - same basic Z taps except BO-12 has the 600r winding. BO-13 is 20W and a smaller case size. Wonder if the 600r winding accounts for that. BO-9 is 15W w/600r tap and still smaller case size but less wire for the primary. Difference must be in the pri current ratings - 100ma for the BO-12 and only 75ma for the 15W BO-9. Looks like it might be good for a nice class A 807 triode strapped amp.

Anyone have another single BO-12 they want to get rid of?

Last edited by lassoharp; 9th April 2011 at 07:10 PM. Reason: add info
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