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Output Transformer Question

5/9/2004 6:29 PM
billebob Output Transformer Question
How do you determine the correct primary impedance for an O.T.?  
I am rebuilding an amp that uses 2 6BM8 output tubes (p-p). The original transformer is rated for 16k center-tapped. The Tube data sheet indicates a load resistance of 5,200 ohms.  
1. Should I double this number for push-pull config.?  
2. How does using a transformer of higher or lower impedance effect things?  
As I recall from my old text book, the maximum power transfer occurs when the output load is equal to the source. Is that correct?  
Thanks in advance. This website has inspired me to take up an old hobby or two!  
5/9/2004 7:39 PM
Mark Lavelle
"How do you determine the correct primary impedance for an O.T.?"
You can sometimes find a recommended value in the tube's datasheet(s) or – if you're lucky enough to find any – in application notes.  
If you can only find data sheets without any P-P Zout recomnmendation (not quite the same as load resistance, BTW), you need to find specs for similar circuits or use the loadline method. For info on that, read Steve Bench's articles at  
In this case, however, you're lucky! There's an app note from Svetlana (sorry, I don't have a link, but I could email a PDF to you) that shows a P-P circiut with the OT labelled as "Hammond 1620 or similar," which would be 6.6k ohms.
5/10/2004 2:03 PM

See if this link helps any....  
Good luck,  
5/10/2004 5:03 PM

From my experience with the 6bm8 (3/5) they are happy with a 6.8K Z xfmr, or more.  
I ran some into a princeton opt and loved them, although they weren't super durable.  
5/10/2004 6:00 PM
bob p
nice post! :)
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