CGS65 -- strange behavior...

forbin · Posted: Tue May 04, 2010 9:26 pm Post subject: CGS65 -- strange behavior...

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

TekniK · Joined: Aug 10, 2008 Posts: 1059

andrewF · Posted: Wed May 05, 2010 3:59 am Post subject:

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.

forbin · Posted: Thu May 06, 2010 5:31 am Post subject:

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!?!

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

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

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.