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CGS65 Tube VCA issue

CGS65 Tube VCA issue

2015-02-08 by yekuku


Hi all,
I have build one of these and I am not quite sure if it works right.
The gain potentiometer works but I am getting full amplification when the pot is at half range, at the rest range there is no change in sound.
Also a light 50hz hum is introduced while adjusting the gain control, is this normal ?
My unit is unmodified and does not have the output normalled to input 2.

What is your experience with the original CGS65 Tube VCA ?
Which modification would you recommend?
Thanks


Re: CGS65 Tube VCA issue

2015-02-08 by cg_synth

The optional initial gain pot? That sort of functionality is okay, as it allows you to compensate for negative CVs on the other inputs, such as when using LFOs that are centered around 0 volts.
The hum would be related to your build. Perhaps using shielded wires on the signal input would help.

Ken

Re: CGS65 Tube VCA issue

2015-02-08 by yekuku

Hi Ken and thanks for your help.
I have build a MOTM kit provided by Bridechamber where the optional initial gain pot comes standard.
The gain control works from range 0 to 5 but from 5 to 10 it does not change the sound.
I am not quite sure if I got the calibration right.
I need to set all pots to 0 except input ( set to 1 or 2, not 10) and adjust the trimmer until no sound is coming out, right ?
About the light hum issue, I will use shielded cables and I will let you know about how it goes.
many thanks

Re: CGS65 Tube VCA issue

2015-02-08 by Richard Brewster

I think I know the problem. The schematic shows the Initial Gain pot going to +15 volts and the resistor on the PC board is 100K. The CV input resistor is also 100K, which is for a sensitivity of full gain at +5V. I would have to look, but I think for my build I substituted a 270K or 300K resistor for the 100K to the Initial Gain pot to give it the same sensitivity as the CV input. With the 100K you're going to be reaching unity gain at about 1/3 rotation of the pot. And beyond half way you'll be maxed out.

Richard Brewster
http://pugix.com

On 2/8/15 7:57 AM, ukukey@... [cgs_synth] wrote:
Hi Ken and thanks for your help.
I have build a MOTM kit provided by Bridechamber where the optional initial gain pot comes standard.
The gain control works from range 0 to 5 but from 5 to 10 it does not change the sound.
I am not quite sure if I got the calibration right.
I need to set all pots to 0 except input ( set to 1 or 2, not 10) and adjust the trimmer until no sound is coming out, right ?
About the light hum issue, I will use shielded cables and I will let you know about how it goes.
many thanks

Re: CGS65 Tube VCA issue

2015-02-08 by yekuku

Hi Richard and thanks for your input.
Your suggestion sounds logical, I will install a trimmer and I will see how it goes.

Re: CGS65 Tube VCA issue

2015-02-10 by yekuku

I have installed a trimmer and I have found out that with adding a 270Kohm resistor to the existing 100Kohm resistor (370Kohm in total) , the gain potentiometer works in full range :) thanks for pointing this out Richard :)
Ken suggested that the previous range was "normal" behavior; I have just build it and I have not spent enough time experimenting so I can not understand quite well why.
I am pretty sure Ken knows what he is talking about, since he is the "father" of our modules ;)
I am wondering what provides better functionality, leave it unmodified as Ken suggests or add 270Kohm resistor?

About the light hum I am experiencing, i have installed shielded cables to the inputs but nothing changed. the hum is audible only when increasing the gain control. Could it be that the hum is the regular noisefloor resulting by tube's operation? It is my first "tube" project , so I dont know what kind of behavior to expect.

Gentlemen, thank you all for your time and your help, I deeply appreciate it.

Re: CGS65 Tube VCA issue

2015-02-10 by Laurie Biddulph

Have you tried grounding the front panel?

Best Regards
Laurie Biddulph
Mobile: 0400 257 645
Phone: +61 (0)2 4340 0938
Web: www.elby-designs.com
Skype: widgetoz

Elby Designs
9 Follan Close
Kariong
NSW 2250
Australia
Show quoted textHide quoted text
On 11/02/2015 7:33 AM, ukukey@... [cgs_synth] wrote:
>
>
> I have installed a trimmer and I have found out that with adding a
> 270Kohm resistor to the existing 100Kohm resistor (370Kohm in total) ,
> the gain potentiometer works in full range :) thanks for pointing this
> out Richard :)
> Ken suggested that the previous range was "normal" behavior; I have just
> build it and I have not spent enough time experimenting so I can not
> understand quite well why.
> I am pretty sure Ken knows what he is talking about, since he is the
> "father" of our modules ;)
> I am wondering what provides better functionality, leave it unmodified
> as Ken suggests or add 270Kohm resistor?
>
> About the light hum I am experiencing, i have installed shielded cables
> to the inputs but nothing changed. the hum is audible only when
> increasing the gain control. Could it be that the hum is the regular
> noisefloor resulting by tube's operation? It is my first "tube" project
> , so I dont know what kind of behavior to ex pect.
>
> Gentlemen, thank you all for your time and your help, I deeply
> appreciate it.
>
>

Re: CGS65 Tube VCA issue

2015-02-10 by yekuku

I have just checked and although the panels dye is non conductive, the panel seems properly grounded 'through the potentiometers.

Re: CGS65 Tube VCA issue

2015-02-10 by Laurie Biddulph

If the back of the panel is not exposed metal then you should scrape at
the back of the panel around the point where the pot body touches the
metalwork to ensure a good electrical connection. Normal pot bodies are
not connected to any of the pins so you may well find that there is no
ground connection there anyway.

If you have multimeter then measure the resistance between the front
panel and 0V in your systems power supply.

I always supply an earth tag with my Panther kits which is used to
provide a separate (i.e. not relying on any component bodies to do the
job) grounding point and am not aware of anyone having hum issues
through touching a panel component.

Best Regards
Laurie Biddulph
Mobile: 0400 257 645
Phone: +61 (0)2 4340 0938
Web: www.elby-designs.com
Skype: widgetoz

Elby Designs
9 Follan Close
Kariong
NSW 2250
Australia
Show quoted textHide quoted text
On 11/02/2015 9:38 AM, ukukey@... [cgs_synth] wrote:
>
>
> I have just checked and although the panels dye is non conductive, the
> panel seems properly grounded 'through the potentiometers.
>
>

Re: CGS65 Tube VCA issue

2015-02-11 by yekuku

Thanks Laurie for sharing your wisdom ;)
Scraping the back of the panel sounds like a good idea, I will do this on all my modules from now on.

Re: CGS65 Tube VCA issue

2015-02-11 by Laurie Biddulph

The important thing is to get a good electrical contact between the
metal and 'grounding part'. Just relying on contact between say, a pot
and the panel, is not guaranteed to provide an electrical path.
Aluminium is a problem because it oxidises very quickly so it needs to
be cleaned immediately before assembly.
The only point not touched on here is what is used for a ground rail.
The Oakley system provides for a dedicated ground which is used to
ground all metal parts (rack rails, housings and front panels) but it
only works in strictly Oakley populated systems. As soon as you add a
module from a different manufacturer the 'ground' becomes shorted to the
signal ground.

The euro-rack systems don't provide for a panel ground so we just use
the signal ground. I have never had an issue with this and it seems to
work well.

I provide a dedicated panel_gnd pad on my pcbs allowing a tag to be
soldered to the board (by a short piece of wire). The tag can then be
slid over a pot shaft, jack body or whatever is convenient and then gets
sandwiched between the component and the front panel. Our front panels
have a special finish which improves the long-term electrical
conductivity of the panels so a ground connection should remain good for
a long time.

Best Regards
Laurie Biddulph
Mobile: 0400 257 645
Phone: +61 (0)2 4340 0938
Web: www.elby-designs.com
Skype: widgetoz

Elby Designs
9 Follan Close
Kariong
NSW 2250
Australia
Show quoted textHide quoted text
On 11/02/2015 9:41 PM, ukukey@... [cgs_synth] wrote:
>
>
> Thanks Laurie for sharing your wisdom ;)
> Scraping the back of the panel sounds like a good idea, I will do this
> on all my modules from now on.
>
>

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