KT88 grid stopper
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 1st November 2010, 04:10 PM #1 overdrajv   diyAudio Member   Join Date: Dec 2007 KT88 grid stopper I just calculated the value for the KT88(tetrode connected) grid stopper for 20kHz roll off, and it seems very high (usually its from 5k-10k in guitar amps). Here is the math: http://www.drtube.com/datasheets/kt88-mov74.pdf Gm@g2=250V = 11,5 mA/V rp = 12k mu = Gm*rp = 138 I use 5k primary on a SE transformer so Av = mu*Rl/(Rl+rp) = 138*5k/(5k+12k)=v 40,6 and the Miller capacitance is Cg1(all less anode) + Cg1a*(Av+1)= = 16pF+1.2pF*(40,6+1) = =66pF so the so 20kHz the R in this internal RC filter should be R= 1 / 2*pi*f*C = 1 / 2*pi*20kHz*66pF= =121k Where did I go wrong? Thanks, Miha
 1st November 2010, 04:20 PM #2 kevinkr   diyAudio Moderator     Join Date: Sep 2004 Location: Boston, Massachusetts Why would you want to impose a 20kHz roll-off with a grid stopper resistor? The grid stopper is there to prevent parasitic HF/VHF oscillations and for no other reason. I didn't look at your math but the numbers seem ok, use a 1 - 10K grid stopper and if you need to introduce a roll off for feedback stability you can do it in the driver stage. Bandwidth in excess of 20kHz is a good thing if it doesn't come at the expense of something else like stability. Most of my amps are designed to be flat to something considerably greater than 20kHz if possible. __________________ "To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine Last edited by kevinkr; 1st November 2010 at 04:30 PM.
 1st November 2010, 04:27 PM #3 kruesi   diyAudio Member     Join Date: Oct 2009 Location: Chicago - Gritty, Grey and Windy I don't think you did go wrong- I think you've got it right. 12.1 k Ohms would put the response down 3dB at 200 kHz, which is usually plenty low enough. You're not trying to set the upper frequency open loop response with the stopper resistor- only trying to prevent oscillation at (usually) several MHz by reducing the gain above the intended passband. Indeed, 5-10 kOhms seems about right based on your calculation.
 1st November 2010, 08:28 PM #4 cjkpkg   diyAudio Member   Join Date: Aug 2008 Location: Roswell GA I have not got into the math...however...I have typically used a 100ohm to 1kohm grid stopper on 3 amps that I have built and it seems to work nicely. 120k seems high. I am currently using 100ohm on a SimpleSE running KT88s and it sounds fabulous.
 1st November 2010, 09:10 PM #5 DF96   diyAudio Member   Join Date: May 2007 High value grid stoppers are often used in guitar amps, partly to cope with poor layout involving long leads, and partly to limit the HF to less than the full audio range. In my view this is poor engineering. It makes them very sensitive to details of valve capacitance, and so encourages tube rolling as a crude way to adjust capacitance. In hi-fi amps smaller grid stoppers are used simply to avoid VHF oscillation. Tone controls can be added separately if required.

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