Determining cathode resistor
 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
 26th April 2009, 01:19 AM #1 bereanbill   diyAudio Member   Join Date: Jan 2009 Determining cathode resistor Hi again, everyone !! I have a single-ended amp with a 6V6 a 6X5 and a 6SJ7. The 6SJ7 is grid leak biased and I would like to switch to cathode bias. How do I determine the value of the cathode resistor ???
 27th April 2009, 02:35 PM #2 Arnulf   diyAudio Member   Join Date: Feb 2009 1: Measure grid voltage (that's the tricky part, especially is Rg is very high already) and cathode current, then calculate appropriate cathode resistor (cathode resistor = grid voltage / cathode current). 2: Measure plate voltage and cathode current, refer to datasheet for approximate grid voltage at that particular operating point, calculate cathode resistor and fiddle with its value until you get to same operating point where your amplifier was operating before the change. A suggestion for option #2: take smaller resistor (smaller value than calculated, say 20% smaller) and wire a trimmer (its value at least twice the 20% you subtracted from the resistor) as rheostat in series with it so you can adjust cathode resistance without having to rewire the amplifier. I.e. if your calculation tells you to use 1K resistor, use ~800R + 470R trimmer (rheostat !) in series. Since this is a signal pentode you are using you probably don't need to concern yourself with wattage ratings of the resistor and potentiometer/trimmer.
 27th April 2009, 09:32 PM #3 costis_n   diyAudio Member   Join Date: Aug 2008 I can guess that measuring the grid voltage will be really tricky, even with a VTVM! I would not trust any meter to do this, due to their non-infinite input resistance. Maybe a scope with a 100x probe? Conclusion: Just look up the values given in the datasheet, and make an educated guess. Then adjust accordingly.
Arnulf
diyAudio Member

Join Date: Feb 2009
Quote:
 Originally posted by costis_n [B]I can guess that measuring the grid voltage will be really tricky, even with a VTVM! I would not trust any meter to do this, due to their non-infinite input resistance.
That's the tricky bit

However if meter probe impedance is relatively high (comparable to the value of Rg) one could simply remove Rg from circuit and let the meter sit there in its place.

 27th April 2009, 10:56 PM #5 bereanbill   diyAudio Member   Join Date: Jan 2009 I'm a dumba** Well...I am definitely a dumba**. Of course using a pot in place of the cathode resistor and dialing it in until the plate voltage is the same as it was before will give me the proper value. Sorry to waste everyone's time...I KNEW that, my brain just passed a bit of flatus on me....
 27th April 2009, 11:33 PM #6 tomchr   diyAudio Member     Join Date: Feb 2009 Location: Calgary Do yourself a favor and measure the grid, collector, and anode voltages first. Any digital voltmeter should have enough input impedance to do this without issues. Then do the math and figure out the appropriate cathode resistor. You can then use a fixed resistor plus a pot to find the final value if desired. If you just insert any random pot without a fixed series resistor you run the risk of biasing the tube *way* hot (much too high anode current), thereby, wrecking the tube. If you decide to go this route anyway, at least turn the pot to the max resistance (lowest current). Assuming the grid and cathode are referenced to the same supply rail (typ ground), you should be able to calculate the cathode resistor like this: Rk = Vgk/Ia, where Rk is the cathode resistor in ohms, Vgk is the grid-to-cathode voltage in volts, and Ia is the anode current (assuming zero grid current) in amperes. Tom

 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         Holton Precision Audio         CSS         Dx Classic Amplifiers         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 Burnedfingers Tubes / Valves 88 14th January 2014 06:36 PM bereanbill Tubes / Valves 7 24th May 2009 04:11 PM audiohead Tubes / Valves 3 9th December 2007 07:15 PM Klimon Tubes / Valves 12 30th August 2006 10:19 PM liberty Tubes / Valves 3 9th September 2005 01:37 AM

 New To Site? Need Help?

All times are GMT. The time now is 03:26 AM.