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 Post subject: Jean Hiraga 12AX7 amp
PostPosted: 14 May 2018, 02:19 
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Joined: 04 Feb 2013, 06:52
Posts: 37
Location: North Wales
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?


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PostPosted: 14 May 2018, 10:00 
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Joined: 08 Aug 2009, 03:11
Posts: 2225
Location: Chilliwack, BC
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!


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_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 15 May 2018, 11:34 
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Joined: 04 Feb 2013, 06:52
Posts: 37
Location: North Wales
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"?


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PostPosted: 16 May 2018, 00:23 
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Joined: 08 Aug 2009, 03:11
Posts: 2225
Location: Chilliwack, BC
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!


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_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 16 May 2018, 03:48 
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Joined: 04 Feb 2013, 06:52
Posts: 37
Location: North Wales
Thank you for your help.


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