global negative feedback in el84 push pull - diyAudio
 global negative feedback in el84 push pull
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

 Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
 5th November 2011, 11:22 PM #1 diyAudio Member   Join Date: Dec 2010 global negative feedback in el84 push pull Hello all. I am trying to apply global negative feedback as described in the Bevois Alley amp by Morgan Jones and I can't figure out the Rl of the input stage and Rl of the concertina. How do I calculate each? Another question is in the formula needed to find rk : rk=(Rl+ra)/u+1 ... that u is the gain coefficient of the tube or the actual gain measured at one of the outputs of the concertina? The 6n1p-ev dictates a u of 35 while I measured 24.8 in my amp. Witch one should I use? My soundcard outputs 1.2V measured at full volume. What is the full voltage needed for 12.5W output? So I can adjust the gain of the amp accordingly.
diyAudio Member

Join Date: Dec 2010
here is my current setup
Attached Images
 photo2.jpg (142.1 KB, 249 views)

 6th November 2011, 10:46 AM #3 diyAudio Moderator     Join Date: Oct 2002 Location: Chicagoland Blog Entries: 2 That rk is the "looking in" impedance at the cathode. For the purposes of feedback calculations, it's effectively in parallel with the cathode resistor. The u is the mu of the tube. By Rl, do you mean the plate load resistors? Those are derived by playing with load lines on the tube's plate curves. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
diyAudio Member

Join Date: Feb 2010
Quote:
 What is the full voltage needed for 12.5W output? So I can adjust the gain of the amp accordingly.
The open loop gain of the schematic you showed is some 43 ( 32,7 dB)
assuming you have typical 8k to 8 ohm output transformer.
With 5k6 feedback resistor from the secondary to the cathode of 6N1P the GNFB is 11 dB and total gain is 21,7 dB (A=12.2). Thus the 12.5 W output (10 Vrms) requires some 0.82 Vrms input level. Feedback resistor should have small 47...100pF capacitor in parallel depending on the OPT you use.

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Kashmire Tubes / Valves 9 28th November 2012 08:46 AM sdinfo Tubes / Valves 160 26th August 2010 07:16 PM slideman82 Tubes / Valves 4 19th June 2009 03:40 PM chungtat Tubes / Valves 12 3rd November 2005 11:25 PM gingertube Tubes / Valves 14 17th May 2005 04:22 PM

 New To Site? Need Help?

All times are GMT. The time now is 04:52 AM.