push pull EL84 amplifier cathode bias question
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Mark Forums Read

 Instruments and Amps Everything that makes music, Especially including instrument amps.

 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
 30th November 2020, 08:33 PM #1 cggs diyAudio Member   Join Date: Oct 2011 push pull EL84 amplifier cathode bias question I am reviving an old WEM guitar amplifier which has push pull EL84s in what is rated (optimistically I thought) as an 18watt effort. It is a conventional setup where the EL84 anodes are directly connected to the OP transformer while the screen grids run from HT via a 1.5K resistor. The EL84 cathodes are commoned to ground via a single 100R bypassed by 50uF. To avoid having to use matched EL84s I would like to institute separate cathode biasing. My question is should I merely duplicate the existing for each valve or should resistor values be changed? I am afraid I do not really know how to calculate this and would be grateful if it could be explained. Thank you.
 1st December 2020, 02:01 AM #2 Printer2   diyAudio Member   Join Date: Apr 2010 The bias voltage is to remain the same for both tubes as it is now. But with the existing resistor you have twice as much current with two bias currents. So the calculation uses Ohms Law. Ohm's Law can be calculated as Voltage /Current = Resistance. Or it can be arranged as Current x Resistance = Voltage. Since we want the same voltage and we are going to use half the current through the resistor, to have the same voltage the resistance has to double. You can get 220 ohm resistors for each cathode.
 1st December 2020, 04:33 AM #3 Enzo   diyAudio Member   Join Date: Sep 2004 Location: Lansing, Michigan Can I vote to not bother? You don't NEED to use matched pairs, certainly the original amp maker never did. ANy small differences in tube current won't hurt the amp. Remember, it is just a guitar amp, not a hifi. So balancing the power tubes is more of a hifi thing, and unless everything else about the power amp stage is "balanced" doing separate tubes won't accomplish that. Unless you plan to make those cathode resistors adjustable, making them separate won't balance the amp, you'd still "need" to use matched pairs of tubes.
 1st December 2020, 02:50 PM #4 Printer2   diyAudio Member   Join Date: Apr 2010 You can make them adjustable by using a pair of 330 ohm resistors and a 1K pot across them and the wiper to ground. That would give 200 ohms on each side if balanced, might better to use a 1.5k though and it giving 230 ohm. Last edited by Printer2; 1st December 2020 at 02:54 PM.
 1st December 2020, 08:15 PM #5 mike567   diyAudio Member   Join Date: May 2011 Location: here If you balance the output tubes you will also lose some of your nice second order harmonics which some players want.

 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 Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Site     Site Announcements     Forum Problems Amplifiers     Solid State     Pass Labs     Tubes / Valves     Chip Amps     Class D     Power Supplies     Headphone Systems Source & Line     Analogue Source     Analog Line Level     Digital Source     Digital Line Level     PC Based Loudspeakers     Multi-Way     Full Range     Subwoofers     Planars & Exotics Live Sound     PA Systems     Instruments and Amps Design & Build     Parts     Equipment & Tools     Construction Tips     Software Tools General Interest     Room Acoustics & Mods     Music     diyAudio.com Articles     Car Audio     Everything Else Member Areas     Introductions     The Lounge     Clubs & Events     In Memoriam The Moving Image Commercial Sector     Swap Meet     Group Buys     The diyAudio Store     Vendor Forums         Vendor's Bazaar         Sonic Craft         Apex Jr         Audio Sector         Acoustic Fun         Chipamp         DIY HiFi Supply         Elekit         Elektor         Mains Cables R Us         Parts Connexion         Planet 10 hifi         Quanghao Audio Design         Siliconray Online Electronics Store         Tubelab     Manufacturers         AKSA         Audio Poutine         Musicaltech         Holton Precision Audio         CSS         exaDevices         Feastrex         GedLee         Head 'n' HiFi - Walter         Heatsink USA         miniDSP         SITO Audio         Twin Audio         Twisted Pear         Wild Burro Audio

 Similar Threads Thread Thread Starter Forum Replies Last Post kanurys Tubes / Valves 10 30th October 2019 04:43 PM tiefbassuebertr Solid State 70 15th August 2012 09:08 PM rsumperl Tubes / Valves 8 10th July 2012 06:34 PM chungtat Tubes / Valves 12 4th November 2005 12:25 AM jraraujo Tubes / Valves 19 19th October 2002 01:23 AM

 New To Site? Need Help?

All times are GMT. The time now is 03:27 PM.