Home Tube DIY Asylum

Do It Yourself (DIY) paradise for tube and SET project builders.

RE: SE 2A3 beta follower : spectrum graph @ 1W , finals biasing




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'
Have Fun and Enjoy the Music
"Still Working the Problem"


This post is made possible by the generous support of people like you and our sponsors:
  Signature Sound   [ Signature Sound Lounge ]


Follow Ups Full Thread
Follow Ups

FAQ

Post a Message!

Forgot Password?
Moniker (Username):
Password (Optional):
  Remember my Moniker & Password  (What's this?)    Eat Me
E-Mail (Optional):
Subject:
Message:   (Posts are subject to Content Rules)
Optional Link URL:
Optional Link Title:
Optional Image URL:
Upload Image:
E-mail Replies:  Automagically notify you when someone responds.