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Jean Hiraga 12AX7 amp
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=6218
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Author:  dudleyperkins [ 14 May 2018, 02:19 ]
Post subject:  Jean Hiraga 12AX7 amp

There is a very well publicised circuit by Jean Hiraga using a 12AX7 input tube driving a KT88, in single ended mode.
My question concerns the small variable capacitor from the KT88 anode HT supply to earth, I have never seen any other circuit employ this, how do you know it's adjusted correctly?

Author:  Geek [ 14 May 2018, 10:00 ]
Post subject:  Re: Jean Hiraga 12AX7 amp

I believe the attached is the one you speak of?

It's to null ringing in the OPT by tuning it off resonance. Transformer specific as well. Limiting frequency response with a pole-zero in the driver eliminates the need for it.

1-1.5nF mica variable capacitors aren't cheap! As a low frequency radio experimenter, I know :cold:

Cheers!

Author:  dudleyperkins [ 15 May 2018, 11:34 ]
Post subject:  Re: Jean Hiraga 12AX7 amp

Thanks, I can't find any 1-1.5nF variable capacitors for sale.
I had no idea they are expensive.
What do you mean by "pole-zero"?

Author:  Geek [ 16 May 2018, 00:23 ]
Post subject:  Re: Jean Hiraga 12AX7 amp

Pole Zero or PZ, is basically setting a HF cutoff point so ultrasonic audio content doesn't get into mischief. Attached is a way Dynaco does it.

As for trimmer capacitors in that range, here's a couple options for you:
https://www.rfparts.com/312m.html
https://www.rfparts.com/311m.html

Cheers!

Author:  dudleyperkins [ 16 May 2018, 03:48 ]
Post subject:  Re: Jean Hiraga 12AX7 amp

Thank you for your help.

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