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In Reply to: RE: Need some opinions and maybe a lot of help with 300B design. posted by gavinhaley on March 14, 2016 at 10:38:13
A reasonable design, but you won't get a lot of power - maybe 3-4 watts. To get more, raise the 300B plate voltage or reduce the output transformer impedance (and raise the plate current). Or do both.Here is the WE table of 300B operating points:
p-k Bias Current Load Power 2nd 3rd
Volts Volts mA Ohms Watts db db
200 -42 30 2000 3.0 20 31
200 -39 40 2500 2.6 26 38
200 -37 50 2500 2.5 30 45
250 -55 30 2000 4.9 18 27
250 -55 30 4500 3.2 27 40
250 -52 40 3000 4.0 26 36
250 -50 50 2500 4.4 26 39
250 -48 60 2000 4.7 26 38
250 -48 60 2700 4.1 30 45
250 -45 80 1500 5.0 26 41
300 -65 40 2500 6.7 20 30
300 -63 50 2000 7.2 21 29
300 -63 50 3000 6.1 26 37
300 -61 60 2400 6.6 26 37
300 -61 60 3400 5.6 30 44
300 -58 80 1700 7.5 26 37
350 -76 50 3600 7.8 26 38
350 -76 50 5000 6.2 30 45
350 -74 60 2000 10.2 21 30
350 -74 60 3000 8.3 26 38
350 -74 60 4000 7.0 30 44
350 -71 80 2200 9.6 26 39
400 -91 40 5000 8.4 26 37
400 -89 50 3000 11.5 21 31
400 -89 50 4000 9.4 25 38
400 -87 60 3500 10.5 26 38
400 -87 60 5000 8.3 30 46
400 -84 80 2500 12.5 25 37
450 -104 40 6000 9.5 26 38
450 -102 50 5000 10.7 27 39
450 -102 50 6500 9.0 30 45
450 -100 60 4000 12.5 26 38
450 -100 60 5500 10.1 30 44
450 -97 80 2000 17.8 21 30
450 -97 80 3000 14.6 26 37
450 -97 80 4500 11.5 31 45When the second harmonic distortion at full power is 26dB down, that's 5%. That's a good middle-of-the-road value IMHO.
Edits: 03/14/16Follow Ups:
Hi Paul. I guess I'm not getting it. I was aiming for 350 on the Plate, -74 on the grid which with a 5000 ohm load should give me around 6.5 watts or so. Am I looking at this wrong? I can increase the cathode voltage to -76 easily enough. That should give me at least 6 watts right? Thank you for taking the time to look at it by the way.Gavin
Edits: 03/14/16 03/14/16
The schematic you posted indicates cathode bias. That means the anode-cathode differential will be less than what the PS provides relative to ground.
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Buy Chinese. Bury freedom.
Exactly right!
T_K knows this but for others who might read this post and scratch their heads I'll note that it's a classic mistake, probably everyone here has fallen for it at some point: There is no such thing as "plate voltage"; there are only voltage differences, e.g. plate to cathode in the table and on data sheets, plate to ground, etc.
Here is my attempt at a power supply for the above. Yes I know the 5AR4 is not drawn correctly but the liitle sketch-it web app @ Digikey is kinda limited. Best I could do.Gavin
Edits: 03/14/16
This should give you much more than 375 volts, closer to 500 volts. With that, you can run around 60mA (using a 1500 ohm cathode resistor) and drive 8+ watts through the Robin Hood 5K output transformer.
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