Aikido - calculating Rk for new tubes
 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
 20th September 2010, 02:41 PM #1 Tfan69   diyAudio Member     Join Date: May 2009 Aikido - calculating Rk for new tubes hello how can i do my own math to calculate Rk?? i have a couple of tubes i wish to try (e80cc & e182cc) thanks
 20th September 2010, 03:08 PM #2 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland It's best to do it graphically. Decide on the operating point (voltage and current). Then B+ is twice the operating voltage. Now, for that current, go to the tube curves on the datasheet, draw a vertical line at the operating voltage. Then draw a horizontal line at the desired current. If they intersect on a drawn line for a specific grid voltage, great. If it falls between, estimate the needed grid voltage. Call that Vg. The the cathode resistor is then calculated from Ohm's Law, Vg divided by the current. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 20th September 2010, 03:09 PM #3 DF96   diyAudio Member   Join Date: May 2007 You decide what operating point you want then choose the grid bias. Ohm's law then gives you the resistor value. How do you choose operating point? Look at the characteristic graphs in the valve data sheet. The only maths involved is the final part for Ohm's law.
Tfan69
diyAudio Member

Join Date: May 2009
already did that way, i tought it was obvious but i was wrong, my results are quite different from jb's manual values...
let's take 12bh7 tube at 250v 10mA as example: JB gives 383ohm
i do my own math: from the tube datasheet Vg is 11,25v
now i take my Vg/A that results is 11,2v/.010A=1120ohm
so something is wrong...
Quote:
 The cathode resistor and plate voltage set the idle current for the triode: the larger the value of the cathode resistor, the less current; the higher the plate voltage, the more current. In general, high-mu triodes require high-value cathode resistors (680-1K) and low-mu triodes require low-valued cathode resistors (100-840). The formula for setting the Iq is an easy one: Iq = B+/2(rp + [mu + 1]Rk) So, for example, a 6CG7 in an Aikido circuit with a B+ voltage of +300V and 1k cathode resistors will draw 300/2(8k + [2 + 1]1k) amperes of current, or 2.6mA.
i fact he answer me the opposite of what i've asked, and don't want bother him again..
for sure i'm missing something.. but what?
thanks

 20th September 2010, 08:25 PM #5 DF96   diyAudio Member   Join Date: May 2007 The formula you have been given is an approximation, which relates the DC anode resistance to the AC anode resistance. It is roughly true for a typical triode when used at a typical bias point, so only useful if you intend to use a typical bias point! By the way, B+ in the formula should be Va (or for the Aikido you could assume Va=B+/2, so the formula should start with B+/4 instead of B+/2). Of course, it doesn't tell you what current to choose.
SY
On Hiatus

Join Date: Oct 2002
Location: Chicagoland
Quote:
 Originally Posted by Tfan69 already did that way, i tought it was obvious but i was wrong, my results are quite different from jb's manual values... let's take 12bh7 tube at 250v 10mA as example...
Whoa! You have a 500 volt supply? Let me assume for a minute that's not correct and you have a 250V supply. That means 125V across the tube (since they're stacked). For 10mA, that's roughly 3.5V of cathode to grid, eyeballing the curves. Doing the Ohm's Law calc, the value is pretty close to what Broskie predicts.
__________________
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."

 20th September 2010, 09:47 PM #7 Tfan69   diyAudio Member     Join Date: May 2009 ok.. got it... you've been very helpful,thanks SY... and DF96 i completely forgot to divide b+/2 since the triodes stands one on top the other.... please forgive me.. i'm just learning
 20th September 2010, 09:52 PM #8 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland That's OK, this is the reason we're here. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
piero7
diyAudio Member

Join Date: Apr 2009
Location: Matera
Quote:
 Originally Posted by Tfan69 ok.. got it... you've been very helpful,thanks SY... and DF96 i completely forgot to divide b+/2 since the triodes stands one on top the other.... please forgive me.. i'm just learning

 21st September 2010, 07:27 AM #10 Tfan69   diyAudio Member     Join Date: May 2009 hi Piero

 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         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 jrebman Tubes / Valves 4 25th March 2008 08:47 PM 2KW Tubes / Valves 47 13th July 2007 12:16 AM cbutterworth Tubes / Valves 11 27th January 2007 10:15 PM jayme Tubes / Valves 8 31st December 2006 03:03 AM Beelzebub Tubes / Valves 24 29th August 2006 10:54 AM

 New To Site? Need Help?

All times are GMT. The time now is 08:22 AM.