6L6/807 AB2 triode-- screen drive?

Matt.B.H. · 13th October 2011, 01:49 AM

I have finally completed the breadboard of my 5B255M amps. I am getting 112WRMS from four of them in PP triode with 2% distortion. Nice, it sounds good too.

Using an IRF820 source follower for each output valve into a 2.2K load. I know its not ideal but that is the trannys I have. Increasing the secondary load to 12R, giving a reflected impedence somewhere near the ideal actualy results in more distortion and much the same power.

Anyway my question is: how about screen drive? I have searched the archives and cannot find the old threads relating to screen drive. Is it worth trying?

I may as well try it whilst I have it in the lash up stage and I guess with the followers and more than enough drive voltage, I may as well give it a go.

Cheers Matt.

SY · 13th October 2011, 01:52 AM

Screen drive is a great technique- just not with that tube. It's best with very high perveance tubes, usually TV sweep tubes. You can run them very close to class B and get great results.

Matt.B.H. · 13th October 2011, 02:03 AM

Ok I seem to remember it also had something to do with tubes with a low screen voltage rating? ie designed to be efficient in a class B setup, much like line output in TV sets. Perhaps even class C, maybe good for RF finals.

I know the 5Bs I have dont mind their screen grid being abused big time. I have run them at 500V UL and 500V anode 400V screen just to see. Nothing happened, infact it apeared to me that the 300V limit the datasheet quotes was actually the ideal as they worked best there. Mind you they sound dreadful in tetrode mode. Maybe some Schade FB would sort them out in that setup.

Cheers Matt.

13th October 2011, 01:49 AM	#1
Matt.B.H. diyAudio Member Join Date: Jul 2009 Location: Cheltenham	6L6/807 AB2 triode-- screen drive? I have finally completed the breadboard of my 5B255M amps. I am getting 112WRMS from four of them in PP triode with 2% distortion. Nice, it sounds good too. Using an IRF820 source follower for each output valve into a 2.2K load. I know its not ideal but that is the trannys I have. Increasing the secondary load to 12R, giving a reflected impedence somewhere near the ideal actualy results in more distortion and much the same power. Anyway my question is: how about screen drive? I have searched the archives and cannot find the old threads relating to screen drive. Is it worth trying? I may as well try it whilst I have it in the lash up stage and I guess with the followers and more than enough drive voltage, I may as well give it a go. Cheers Matt.

13th October 2011, 01:52 AM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Screen drive is a great technique- just not with that tube. It's best with very high perveance tubes, usually TV sweep tubes. You can run them very close to class B and get great results. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
What is grid resistance of 807/6l6 in pentode	ryuji	Tubes / Valves	9	4th May 2011 09:52 AM
EL34 in class AB2 triode mode push-pull?	ray_moth	Tubes / Valves	29	8th March 2009 07:34 PM
807/350B/6L6 PP amps PSU ?	coresta	Tubes / Valves	7	3rd April 2008 08:20 AM
807 AB2 oscillation maybe?	Craig405	Tubes / Valves	27	25th March 2008 09:01 PM
807 screen regulation	Klimon	Tubes / Valves	19	15th September 2006 05:58 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

13th October 2011, 02:03 AM	#3
Matt.B.H. diyAudio Member Join Date: Jul 2009 Location: Cheltenham	Ok I seem to remember it also had something to do with tubes with a low screen voltage rating? ie designed to be efficient in a class B setup, much like line output in TV sets. Perhaps even class C, maybe good for RF finals. I know the 5Bs I have dont mind their screen grid being abused big time. I have run them at 500V UL and 500V anode 400V screen just to see. Nothing happened, infact it apeared to me that the 300V limit the datasheet quotes was actually the ideal as they worked best there. Mind you they sound dreadful in tetrode mode. Maybe some Schade FB would sort them out in that setup. Cheers Matt.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)