Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Changing a EL81 from class B to AB?
#1
Hi guys, i'm changing an old Philips tube amp into a guitar amp. That amp has a PP amp with 2 EL81 tubes in class B. Plate voltage 400V. Screen V around 200V. Fixed bias -24V. 
Sounds fine but not when overdriven.. 
Is it possible to bias the tubes more into AB? I don't think so...given the available voltages but maybe someone has a good idea?
What else can I do? Large grid resistors? Now its 1K, maybe go to even 100K? To avoid grid current as much as possible? Something else?

I appreciate it if you guys can think along with me! Thanks for response.
Here is the schematic:
https://www.pa3esy.nl/Philips/versterker...0_sch1.pdf

Btw I don't want to change to other output tubes.
Maybe change the cathodyne phase inverter to a 12au7 instead the 12at7 cause the au can handle grid current better?
Reply
#2
You could try adding diodes or LEDs in series with R22 & R23.  I think you just need to force the two tubes to conduct at the same time, then you can play around with the cross over point by varying the transition from A to B.  https://www.electronics-tutorials.ws/amp...asses.html
Reply
#3
You could convert the output stage from fixed bias to cathode bias.  The Kay 720 bass amp is set up this way.  Kay 720 Schematic You would need to look up the specs on the EL81 to set the cathode resistor correctly and probably want a bypass cap so you don't lose the bottom end.  The Kay 720 uses a 250 ohm shared cathode resistor. I added 100uF cap.  You will lose power but might love the new sound.
Reply
#4
Thanks both for your input!
Reply


Forum Jump:


Users browsing this thread: 1 Guest(s)