Before we go further, take a look at Amperex's 845 tube datasheet! It actually calls out that a 16K load resistance for the 845 output transformer, and it has higher power output compared to 9K load resistance at the same 845 plate dissipation at 65W. I didn't invent that. It has been there for over 60-70 years. Take a look at a snippet of the Amperex 845 tube datasheet below.
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| Amperex 845 operating point |
One thing that wasn't shown clearly is the distortion % for 845 tube at various load resistance. It states only 5%, but you all know that higher load resistance provides better linearity. So, let's do some drawing again to see for ourselves.
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| Amperex 845 Tube Curve |
Green - 9000 at Amperex tube datasheet operating point
Red - 16,000 at Amperex tube datasheet operating point
Dark blue - 16,000 at higher plate dissipation
Light blue - 20,000 at higher plate dissipation
Green: At 9000 load resistance for 845 output transformer, you can see that there are severe compression starting at -250V to -350V grid voltage. I wouldn't use this if I were designing my 845 tube amplifier. It would have premature distortion at very early stage.
Red: At 16,000 load resistance for 845 output transformer, things are much better than 9000 load resistance where severe compression starts only after -300V grid voltage. Still, from -350V to -450V, you can see that the V-swing narrows significantly.
Dark blue: still at 16,000 load resistance for 845, the operating current is raised higher. Things started to look better now. Severe compression starts only after -350V grid voltage. This is something I would consider doing.
Light blue: if I can design a 20,000 primary impedance output transformer for 845 tube that still have considerable bandwidth compared to a 16,000 primary impedance output transformer, I would definitely use a 20K output transformer. Same theory applies. I would try a 20K primary one day, when we have more free time. It would be something worth looking forward to.
As a conclusion, the magic number -> 16,000 Ohm primary impedance for 845 tube output transformer is still recommended. 9K, 10K, or 12K are still not ideal although 845 tube Rp is much lower compared to 211.
Don't follow blindly on the recommended loadline or load impedance for your tube. Think and look at the datasheet before you choose the output transformer for your design. Your design or your amplifier could be under-performing (lower power, higher distortion, worse sounding that it is supposed to be!) due to this and deserves better!
Till then.
Postscripts:
- We can make very decent 10K, 12K, 15K (sample 1, sample 2)/ 16K / 20K OPTs for pre or power output.
- Due to the large grid swing and grid current needed, 211 drive 845 or 211 drive 211 might be needed to fully utilize the high primary impedance. Ordinary drivers will not be able to manage such swings.
J&K Audio Design
2/15/2015
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