Triple Checking Calcs: metering bias with mA meter
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki 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
 14th September 2010, 11:08 AM #1 DrewP diyAudio Member   Join Date: Jan 2002 Location: Canberra, Australia Triple Checking Calcs: metering bias with mA meter Okay, deal is as follows... My PP EL84 amp wants output tubes biased in the ballpark of 35mA standing current. For measurement purposes I have a 10R in the cathodes so there should be 350mV from the cathode to ground. I have a panel meter which reads 1mA full scale deflection and has a 200R coil. If I want my chosen bias point to be half of FSD, then I want 0.5mA flowing with 350mV applied. This being the case, If I put 500R in series with my meter winding so the total is 700R I should read 50% FSD at 350mV. (and 100% FSD at 700mV or 70mA) Correct? (I've done the sums about 5 times but I still don't trust myself) I plan on having 4 momentary switches, one from each bias resistor so the meter is usually not in the cct at all, but a quick dab of the toggle enables me to read the draw on any of the output tubes.
 14th September 2010, 12:42 PM #2 Huntingdon 361   diyAudio Member   Join Date: Apr 2010 Location: Oxford You’re pretty close with 500Ω. The actual series resistor needed is 490Ω, because the series resistor + meter resistance appears in parallel with the 10Ω sense resistor, and thus the voltage dropped by 35mA will be slightly less than 350mV. However, bearing in mind the tolerances on all the resistance values given, the tolerance on the meter movement sensitivity, the accuracy within which the meter can be read, and the acceptable variation around your 35mA target current, I would say go ahead with 500Ω or any convenient value close to this.
 14th September 2010, 01:12 PM #3 jackinnj   diyAudio Member     Join Date: Apr 2002 Location: Llanddewi Brefi, NJ
 14th September 2010, 01:27 PM #4 TheGimp   diyAudio Member     Join Date: Aug 2009 Location: Johnson City, TN The other issue is that you will be placing a 700 ohm resistance meter in parallel with your 10 ohm sense resistor. You will add another 2% error. Use a 470R resistor plus a 100R 10 turn trimpot and adjust the trimpot for the exact reading. Calibrate it with a good high input impedance VOM.
 14th September 2010, 03:55 PM #5 DF96   diyAudio Member   Join Date: May 2007 A 1.5% error (700 vs. 10) is irrelevant for two reasons: 1. valve quiescent current is not that critical 2. moving coil meters are only a few % accurate anyway, and they can vary across the scale Huntingdon 361 is correct.
 14th September 2010, 10:41 PM #6 DrewP diyAudio Member   Join Date: Jan 2002 Location: Canberra, Australia Thanks all, nice to know that I'm not having a total brain fade...

 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     Car Audio     diyAudio.com Articles     Music     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         Aussie Amplifiers         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 east electronics Solid State 3 19th March 2010 08:57 AM rtsang Tubes / Valves 11 29th January 2009 03:53 PM FrankRoss Parts 4 9th November 2006 01:54 AM audiobot Tubes / Valves 14 15th March 2004 11:13 PM jackinnj Swap Meet 0 27th January 2003 07:18 PM

 New To Site? Need Help?

All times are GMT. The time now is 11:22 PM.