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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
CGS65 -- strange behavior...
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forbin



Joined: Jan 29, 2009
Posts: 117
Location: Fremantle, Australia

PostPosted: Tue May 04, 2010 9:26 pm    Post subject: CGS65 -- strange behavior... Reply with quote  Mark this post and the followings unread

Hi Chaps

I built a CGS65 a while ago and didn't have an enormous amount of success so I have decided to revisit it and was wondering if anybody has seen the behavior that I am experiencing. When I inject a sine wave of about 5V p-p And I adjust the control voltage to let the signal through I get what amounts to a rectified signal -- i.e. just the positive pulses of my sine wave. They look pretty clean. It seems to be that the grid signal is not biased enough? Has anybody seen this behavior? I have tried the other "option" and get basically the same but more distorted rectified signal. I am considering changing the 330k to be driven from a potential divider gnd->-15V and see if that gives me the ability to move the bias point into a more useful position?

Any suggestions would be much appreciated...

Cheers
Forbin
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TekniK



Joined: Aug 10, 2008
Posts: 1062

PostPosted: Wed May 05, 2010 1:02 am    Post subject: Re: CGS65 -- strange behavior... Reply with quote  Mark this post and the followings unread

forbin wrote:

Any suggestions would be much appreciated...

Cheers
Forbin


well,like you probably know,all those stone circuits have something weird,a strange behavior or in some cases simply not working (ASR!),so
u have to consider it as 'normal' and not as strange.
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andrewF



Joined: Dec 29, 2006
Posts: 1174
Location: australia
Audio files: 4

PostPosted: Wed May 05, 2010 3:59 am    Post subject: Reply with quote  Mark this post and the followings unread

not sure if these help
but, what goes into the tube vca is not necessarily what comes out

on these shots, the upper waveform is the input
lower is the output (it would be very interesting if the reverse were true)

Posted Image, might have been reduced in size. Click Image to view fullscreen.
Posted Image, might have been reduced in size. Click Image to view fullscreen.
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forbin



Joined: Jan 29, 2009
Posts: 117
Location: Fremantle, Australia

PostPosted: Thu May 06, 2010 5:31 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks very much for the Photo's Andrew... Spent more time after work having a bit of a prod and changing the valve over... Still not too exciting... Second valve is exactly the same as the first... It may be a manufacturer thing? All the other signals from the opamps seem correct. I have found a couple of other sites with designs that use the 1ZH24B and I may try doing a quick breadboard of them just to see. There was also an intersting tip about initially running the heaters with no other current flowing for 10 hours to increase the life of the valves!?!
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otherunicorn



Joined: May 11, 2008
Posts: 136
Location: Australia

PostPosted: Tue May 18, 2010 4:55 am    Post subject: Re: CGS65 -- strange behavior... Reply with quote  Mark this post and the followings unread

TekniK wrote:

well,like you probably know,all those stone circuits have something weird,a strange behavior or in some cases simply not working (ASR!),so
u have to consider it as 'normal' and not as strange.


Would you get off my case? The ASR works fine. Many have been successfully built. I personally have built at least a dozen of them.

_________________
http://www.cgs.synth.net/

Last edited by otherunicorn on Sat May 22, 2010 9:46 pm; edited 2 times in total
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otherunicorn



Joined: May 11, 2008
Posts: 136
Location: Australia

PostPosted: Tue May 18, 2010 5:19 am    Post subject: Reply with quote  Mark this post and the followings unread

When the input signal is too high, the grid begins to act like a plate. This adds a load to the input, distorting the signal there, as well as having an affect on the signal seen at the plate, which is now being starved of electrons.
This could account for your "rectification".

You could also try changing the biasing by adding a trimmer between +ve and the grid, or (Better) between +VE and -VE, and moving the grid pulldown to the wiper.

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