ST35 Input network

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ST35 Input network

Postby adinoto » Thu Apr 10, 2014 3:11 am

Hi folks

I will start building ST35 revE soon.
One question about the input network before V1 : seems there is filter network of C18 (33pF) /R2 (47.5k). If my calculation correct its low pass filter with F around 100kHz
Can we omit this filter ?
Also I have 100k Pot that I want to use as volume control. Can I left out the 475k grid resistor. Would 100k OK vs original 475k value?

Thanks in advance
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Re: ST35 Input network

Postby Geek » Thu Apr 10, 2014 3:38 am

Hi, welcome!

IME working with ST35 (or any Dynaco for that matter) OPT's, a filter is a must.... especially when working into modern crossovers within speaker boxes or worse - fullranges!

Cheers!
-= Gregg =-
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Re: ST35 Input network

Postby Shannon Parks » Thu Apr 10, 2014 4:40 pm

adinoto wrote:Hi folks

I will start building ST35 revE soon.
One question about the input network before V1 : seems there is filter network of C18 (33pF) /R2 (47.5k). If my calculation correct its low pass filter with F around 100kHz
Can we omit this filter ?
Also I have 100k Pot that I want to use as volume control. Can I left out the 475k grid resistor. Would 100k OK vs original 475k value?

Thanks in advance


I have left the filter off and it doesn't affect the performance as the closed loop bandwidth is lower (-3dB at 60kHz maybe?), and it's outside of the loop. Probably just to keep RFI out of the amp. I haven't looked at a 7247 datasheet but possibly it has less typical Cgp than a 12AX7.

You can swap the grid resistor for the 100k pot, but make sure the ground return is near V1.

Shannon
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Re: ST35 Input network

Postby adinoto » Wed Apr 16, 2014 2:34 pm

Thanks Shannon, I'll wired the input direct then
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