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Aikido cathode resistors
Aikido cathode resistors
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Old 11th January 2019, 09:38 PM   #21
Fully Articulated is offline Fully Articulated  New Zealand
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Join Date: Jul 2018
Location: Wellington
So, after finding a nice resource showing how to extract values from the plate curve diagrams, I decided to largely ignore the charts in the Aikido Noval manual and work from the tube datasheets themselves.


Initially, I've decided to go with 6DJ8/6922s in both sections. I don't need that much gain, but I do have a small stockpile of these types so may as well use them.


If you'll indulge me, here's the values I've come up with after drawing the lines and doing the equations. Using Broskie's PS-21 board to produce 210V B+, which should drop to 180V through a 2K resistor on the Noval board input R12, putting 90v on each plate.


I decided to run the input section at 6mA and the output at 9mA ending up in the end with Rk of 330R and 200R respectively.


R9 & R10 are 100K and 88K (well, 68K and 20K in series). This is the voltage divider referenced by Steve above? Sorry, I don't have the manual on me.


If I've done a semi-competent job with all this, it should be possible to sub the 6CG7 into the input section without changing any part values. It just won't be optimised for it.


Anybody spot any glaring issues?
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Old 12th January 2019, 02:34 PM   #22
Steve Morley is offline Steve Morley  Canada
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Join Date: Jan 2015
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Default Looks like a good starting point to me.

Don't be surprised if the current flow is a little less than you calculate (~10 -20%). It all depends on the tubes and their health. The resistors you've chosen for the first stage should be fairly close if you want to sub in a 6CG7.

You might want to try this trick with your resistors in case you want to change them: (see picture).

Here's a trick you can try to see how well the sections of each tube match: Measure the voltage from cathode to plate on each section of a particular tube. Start with the input tube first (I'll explain why in a minute). Swap different tubes in and out (power off when swapping of course), until you get matching voltages between the "top" and "bottom" sections. A match of about a 5% or better should be possible if you have decent tubes.

Once the input tubes are "matched" do the same with the output tubes. The reason you do the inputs first is that an unbalanced input tube can make a well balanced output tube seem not so.

Steve
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