In Reply to: SE 2A3 beta follower : spectrum graph @ 1W , finals biasing posted by deathtube 667 on March 27, 2014 at 15:15:46:
Using Paul Joppa's output tube operating point rule of thumb, 43ma at 325 volts into a 5.6k plate load would give normal power and distortion specs.
If you're talking about 43ma at 250 volts into 2.5k or so....then I'm not surprised.
The operating point is not right for the tube.
When you lower the current, you have to increase the load impedance to rotate the load line more horizontal to keep the tube from operating in the non-linear cutoff region as seen in the lower right half of the plate curve graph where the grid lines all bunch up.
Looking at the plate curves, if the idle current point is simply dropped from 60ma to 43ma (without adjusting the load impedance to rotate the load line towards the horizontal) the load line just drops smack into the non-linear cutoff region and distortion goes up.
That is to say, the plate voltage still decreases by the same amount for the positive half of the input grid signal but (because of the non-linearity of the tube in the cutoff region that the load line is now trying to operate in) the plate voltage no longer can increase by the same amount for the negative half of the input grid signal.
Any plate voltage movement, that is not symmetrical and linear, + vs. -, is what we call harmonic distortion.
Tre'
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Follow Ups
- RE: SE 2A3 beta follower : spectrum graph @ 1W , finals biasing - Tre' 19:22:16 03/27/14 (9)
- RE: SE 2A3 beta follower : spectrum graph @ 1W , finals biasing - deathtube 667 20:06:50 03/27/14 (8)
- RE: SE 2A3 beta follower : spectrum graph @ 1W , finals biasing - Tre' 12:26:43 03/28/14 (0)
- RE: SE 2A3 beta follower : spectrum graph @ 1W , finals biasing - smart845 00:41:12 03/28/14 (5)
- SE 2A3 beta follower : 5K primary THD measurements ... - deathtube 667 16:45:44 03/28/14 (4)
- RE: SE 2A3 beta follower : 5K primary THD measurements ... - Tre' 18:13:04 03/28/14 (3)
- RE: external AF generator vs. internal generator... - deathtube 667 07:19:21 03/29/14 (1)
- RE: external AF generator vs. internal generator... - Tre' 07:47:06 03/29/14 (0)
- RE: SE 2A3 beta follower : 5K primary THD measurements ... - dave slagle 20:38:31 03/28/14 (0)
- RE: SE 2A3 beta follower : spectrum graph @ 1W , finals biasing - Caucasian Blackplate 20:26:39 03/27/14 (0)