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Aikido Cathode Follower Input Impedance
Aikido Cathode Follower Input Impedance
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Old 9th November 2017, 11:19 AM   #1
moley is offline moley
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Join Date: Sep 2010
Default Aikido Cathode Follower Input Impedance

Hello folks,


I have a Glassware PS1 regulated PSU kit and a ACF 9 pin Cathode follower kit on its way to me and I am doing some planning on the other parts for this build.

I have seen the chart detailing output impedances of the various valve options but I am a little unsure what sort of input impedances I would be looking at with the cathode follower kit.
I haven't quite decided on the valves but 6CG7 or 6DJ8 is probably what I will go for.

I am looking at options for potentiometers and will probably use a shunt stepped attenuator. Would a 100K series resistor based shunt POT be a good bet in the ACF 9 pin with 6CG7 and 6DJ8?

One of my sources has a relatively high output impedance at around 3.5K.

Many thanks in advance!
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Old 9th November 2017, 12:40 PM   #2
DF96 is offline DF96  England
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Join Date: May 2007
Ordinary cathode followers have high input impedance, which can be very high if the grid bias resistor is bootstrapped. I have no idea about the "Aikido Cathode Follower". Show us the circuit? Or provide a link.
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Old 9th November 2017, 01:01 PM   #3
moley is offline moley
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Join Date: Sep 2010
Hi and many thanks for the reply!

I guess it would defeat the object of this being a buffer if it did not offer a high input impedance.

It does state the following in the instructions that I found on the glassware audio website.

"This unity-gain buffer, using a modified cathode follower, offers a high input
impedance and a low output impedance and low distortion and great PSRR."


I have attached the schematic and also the table with the part values for the different valves that can be used in this circuit.
Attached Images
File Type: jpg Aikido 9 Pin CF Schematic.jpg (196.2 KB, 264 views)
File Type: jpg Aikido 9 Pin CF Valve options.jpg (461.0 KB, 263 views)
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Old 9th November 2017, 01:30 PM   #4
DF96 is offline DF96  England
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Input impedance will be approximately given by R2, R5 and R10 in parallel - so 333k if they are all 1M.
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Old 9th November 2017, 01:32 PM   #5
moley is offline moley
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Thank you that's great!
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Old 9th November 2017, 06:28 PM   #6
Steve Morley is offline Steve Morley  Canada
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In my experience with the Aikido circuits: 6CG7 = warm and tubey, 6DJ8 = much more incisive (depending on tube chosen). Russian reissue E88CC Genalex is a decent sounding tube. Avoid the ECG Philips (and others of similar USA manufacture). The sound is dreadful.
6DJ8 will have a lower output impedance than a 6CG7, see Zo on the chart. Also you can run 6DJ at 150 to 200 volts where the 6CG usually runs 250 to 350, if that makes any different to you.

Steve With no annoying tag line. Oops.
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Old 9th November 2017, 09:09 PM   #7
kodabmx is online now kodabmx  Canada
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If you build it point to point like I do or you get an adaptor a 5670/6N3P is great with a 220R R3/R6 250V.
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Old 9th November 2017, 09:29 PM   #8
jameshillj is offline jameshillj  Australia
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Is there anything to be gained by using a grid choke in place of R7 in this circuit?
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Old 9th November 2017, 09:42 PM   #9
kodabmx is online now kodabmx  Canada
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My opinion, no. Others might disagree but I've never understood the point of using a choke where there is (almost) no current flow.
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Old 9th November 2017, 10:39 PM   #10
jameshillj is offline jameshillj  Australia
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Yeah, after I bought the things I got to wondering the same thing but haven't got around to try them out - maybe I'd better just do that and see for some piece of mind!
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