Modular Synth Panels

I have a couple questions regarding the construction of the CGS65 (Tube VCA/Timbrel Gate) module as it relates to the Bridechamber panel.

1) Do I wire Input 2 to the "Audio SN" on the PCB? If not where does Input2 go?
2) For the bias switch, am I supposed to use a DPDT to create a circuit between the two optional pairs on the board?

Thanks,
Jim

ps I'm kind of a newbie as it comes to DIY/EE stuff, so please forgive my ignorance




____________________________________________________________________________________
Be a better friend, newshound, and
know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ

On 2/19/08, James Elliott put forth:

>I have a couple questions regarding the construction of the CGS65
>(Tube VCA/Timbrel Gate) module as it relates to the Bridechamber
>panel.

Since you mentioned Bridechamber, and a bias switch, I'm going to
assume this is MOTM format. Adding a bias switch to half of
Richard's panel was my request, so Scott added it, because that's the
kind of happening guy he is :)

Now, ironically, I haven't built my cgs65 yet. To save energy, I have
the heat turned off in the room off the garage where I do my
soldering. Not because I would want to prevent global warming, it's
20 degrees this morning, but to avoid a heating bill the size of a
CS-80. I also have been spending the little "gear time" I've had
doing other things.

Anyway, the first thing I would do is get some pot chiclets. This
will make the front panel wiring much easier.

http://www.wavemakers-synth.com/motm/chiclets.html

>1) Do I wire Input 2 to the "Audio SN" on the PCB? If not where does
>Input2 go?

I have no idea what "Audio SN" means, but I'm not planning on using it.

Each pot acts as a voltage divider. I always forget which pin is
which. You can use a an ohmmeter to determine that the wiper is at ground
when the knob is fully CCW.

Wire ground to the correct end of the two input pots, wire the input
1 jack to other end of the input 1 pot, wire the output jack to switched
lug on the input 2 jack, wire the input 2 jack to other end of the
input 2 pot, wire the wiper from each input pot to the audio inputs
on the PCB so that each signal goes through its own 100K resistor of
the summing amp.

Wiring the switched jack on input 2 allows you to feed the signal
back into the tube when a second input is not being used.

>2) For the bias switch, am I supposed to use a DPDT to create a
>circuit between the two optional pairs on the board?

I don't think you are "supposed" to do anything. How you want to
bias the grid is up to you. This is what it says on the CGS site:

"On the first (yellow) run of PCBs, the grid bias resistor (330k)
goes to the negative rail. In this position, it causes some
distortion of the signal. This can be a good thing. I prefer the
resistor in this position when using the unit as a timbral gate. This
is shown in BLUE on the schematic. An alternative is to bias the grid
to around 1.2V with respect to the negative rail - in other words, to
the other end of the heater/cathode."

If you were to do that, you would use a SPDT switch -- wire the
actuator to the 330K resistor going to the grid, and one end of the
switch to the ground side of the cathode, and the other end to the
negative rail. A center-off switch would add another setting.

You might want to look at a few websites that discuss grid biasing.
What makes the cgs65 somewhat unique, is that the cathode is
connected to a negative voltage (rather than using a resistor and cap
to make the cathode positive in relation to the grid). Generally,
the grid is biased negative in relation to the cathode for at least
two reasons, to prevent excessive grid current, and to offset the
input to reduce distortion.

If the input signal is too negative there is plate-current cutoff,
if it is too positive there is plate-current saturation, resulting in
distortion. Biasing the grid between these two allows for the most
gain and the least distortion.

I'm thinking since the input cap is connected to an op-amp
(something you generally don't see in tube circuits), that the 330K
resistor and input cap aren't going to behave like a typical shunt
grid-leak circuit. If grid current charges the cap, the op-amp will
swing around to compensate. I'm also thinking since the cathode is
negative, the input wouldn't need as much of an offset to produce a
symmetrical output. So I'm going to try to a few different things
and see what happens.

Anyway, asymmetrical distortion is one of the things that make tubes
sound like tubes. So by adjusting the grid bias, the characteristic
of that distortion can be adjusted.

My plan is to omit the potentially noisy diodes, and use a current
limiting resistor instead. Then by running a sine wave through the
VCA, I'm going to figure out what bias voltage results in the least
distortion, then figure out another setting that produces more
asymmetrical tube distortion.

Also, others have reported a high-pass effect, so I'm going to try
increasing the input and output caps, and a cathode capacitor, and
see what happens.

Ok, CGS65 #1 is complete! The other 3 are all assembled component wise - they just need to be mounted.

Now to address my own questions...

1) For some reason I thought the output point was the input point, and vice versa - hence thinking there was only one input point, thus the reason for asking the first question. For those out there who may wonder about what the Audio SN & CV SN points are, they are summing nodes. You can add more audio and cv inputs to their respective SN points by connecting each additional input to their SN input by using 100k resistors... (Thanks for the info Michael Ford on the CGS_SYNTH group for providing this info)

2) Mark, I don't know how you will get the Bias switch hooked up with only a SPDT switch and get this to work. You have two pairs of items you are switching between the two options, a jumper & a 330k resistor. I used a DPDT switch and it worked fine.

----- Original Message ----
From: Mark <yahoogroups@...>
To: ModularSynthPanels@yahoogroups.com
Cc: James Elliott <johans121@...>
Sent: Wednesday, February 20, 2008 10:05:45 AM
Subject: Re: [ModularSynthPanels] CGS65 Questions....

On 2/19/08, James Elliott put forth:

>I have a couple questions regarding the construction of the CGS65
>(Tube VCA/Timbrel Gate) module as it relates to the Bridechamber
>panel.

Since you mentioned Bridechamber, and a bias switch, I'm going to
assume this is MOTM format. Adding a bias switch to half of
Richard's panel was my request, so Scott added it, because that's the
kind of happening guy he is :)

Now, ironically, I haven't built my cgs65 yet. To save energy, I have
the heat turned off in the room off the garage where I do my
soldering. Not because I would want to prevent global warming, it's
20 degrees this morning, but to avoid a heating bill the size of a
CS-80. I also have been spending the little "gear time" I've had
doing other things.

Anyway, the first thing I would do is get some pot chiclets. This
will make the front panel wiring much easier.

http://www.wavemakers-synth.com/motm/chiclets.html

>1) Do I wire Input 2 to the "Audio SN" on the PCB? If not where does
>Input2 go?

I have no idea what "Audio SN" means, but I'm not planning on using it.

Each pot acts as a voltage divider. I always forget which pin is
which. You can use a an ohmmeter to determine that the wiper is at ground
when the knob is fully CCW.

Wire ground to the correct end of the two input pots, wire the input
1 jack to other end of the input 1 pot, wire the output jack to switched
lug on the input 2 jack, wire the input 2 jack to other end of the
input 2 pot, wire the wiper from each input pot to the audio inputs
on the PCB so that each signal goes through its own 100K resistor of
the summing amp.

Wiring the switched jack on input 2 allows you to feed the signal
back into the tube when a second input is not being used.

>2) For the bias switch, am I supposed to use a DPDT to create a
>circuit between the two optional pairs on the board?

I don't think you are "supposed" to do anything. How you want to
bias the grid is up to you. This is what it says on the CGS site:

"On the first (yellow) run of PCBs, the grid bias resistor (330k)
goes to the negative rail. In this position, it causes some
distortion of the signal. This can be a good thing. I prefer the
resistor in this position when using the unit as a timbral gate. This
is shown in BLUE on the schematic. An alternative is to bias the grid
to around 1.2V with respect to the negative rail - in other words, to
the other end of the heater/cathode."

If you were to do that, you would use a SPDT switch -- wire the
actuator to the 330K resistor going to the grid, and one end of the
switch to the ground side of the cathode, and the other end to the
negative rail. A center-off switch would add another setting.

You might want to look at a few websites that discuss grid biasing.
What makes the cgs65 somewhat unique, is that the cathode is
connected to a negative voltage (rather than using a resistor and cap
to make the cathode positive in relation to the grid). Generally,
the grid is biased negative in relation to the cathode for at least
two reasons, to prevent excessive grid current, and to offset the
input to reduce distortion.

If the input signal is too negative there is plate-current cutoff,
if it is too positive there is plate-current saturation, resulting in
distortion. Biasing the grid between these two allows for the most
gain and the least distortion.

I'm thinking since the input cap is connected to an op-amp
(something you generally don't see in tube circuits), that the 330K
resistor and input cap aren't going to behave like a typical shunt
grid-leak circuit. If grid current charges the cap, the op-amp will
swing around to compensate. I'm also thinking since the cathode is
negative, the input wouldn't need as much of an offset to produce a
symmetrical output. So I'm going to try to a few different things
and see what happens.

Anyway, asymmetrical distortion is one of the things that make tubes
sound like tubes. So by adjusting the grid bias, the characteristic
of that distortion can be adjusted.

My plan is to omit the potentially noisy diodes, and use a current
limiting resistor instead. Then by running a sine wave through the
VCA, I'm going to figure out what bias voltage results in the least
distortion, then figure out another setting that produces more
asymmetrical tube distortion.

Also, others have reported a high-pass effect, so I'm going to try
increasing the input and output caps, and a cathode capacitor, and
see what happens.





____________________________________________________________________________________
Be a better friend, newshound, and
know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ

On 3/1/08, James Elliott put forth:

>2) Mark, I don't know how you will get the Bias switch hooked up
>with only a SPDT switch and get this to work. You have two pairs of
>items you are switching between the two options, a jumper & a 330k
>resistor. I used a DPDT switch and it worked fine.

How did you use a DPDT switch?? Are you switching the suppression
grid as well?? I'm thinking a SPDT switch could work in two ways:

1)
--- ground side of cathode
grid -- 330K----
--- negative rail

2)
--- resistor -- some voltage point (eg. ground side of cathode)
grid ---
--- resistor -- some other voltage point (eg. negative rail)


Another option would be a SPDT switch with one end unconnected, or a
SPST switch, to switch between a bias and leaving the grid floating
(or always connected with a large resistor so the switch would put
another resistor in parallel connected to the same point).

Anyway, I'm not planning on not using the diodes to reduce noise and
current draw, and might add a bypass cap to the cathode (in theory,
it could be an issue at low frequencies), so in my version, the
"ground side of the cathode" might not be the best point. Ideally,
a trimpot would be the best way to set the bias. So what I plan on
doing, is using a pot to figure out two values -- a clean VCA
setting, and a distortion setting, then building fixed voltage
dividers as necessary, and setting the switch between them.

If you don't mind me asking, with no signal, what is the voltage at
the grid at each of your switch settings??

>>If you were to do that, you would use a SPDT switch -- wire the
>>actuator to the 330K resistor going to the grid, and one end of the
>>switch to the ground side of the cathode, and the other end to the
>>negative rail. A center-off switch would add another setting.

Mark, I haven't forgotten your request. It's going to take some time before I can answer your questions.

Why? Well because I have no EE experience other than the little I've gained building full KITS from MOTM. I've just recently started working from "scratch". ie buy a PCB and then source all the parts and troubleshoot issues on your own because of little to no documentation.... I spent about an hour and a half reading about vacuum tubes yesterday so I could get a little perspective on how they work before crafting a response to your message.

With all of that being said, on the latest version of Ken's CGS65 PCB, Ken has implemented the "bias option" on the board itself. In order to choose which option you want to implement you have to set two different components: 1) A jumper between one of two points 2) AND a resistor between one of two points. With the DPDT switch, one pole switches the jumper position, the second pole switches the resistor position. Now, it very well may be possible to use a SPDT switch to do this, however upon a novice's first (or second, third, fourth) glance it seems that the jumper switches between to pins on the tube - so I don't know that a SPDT will work without some other circuit voodoo magic. I'm writing this at work so I can't necessarily verify what I've just said at this moment....

I'll try to do some more research sometime this week so I can tell you EXACTLY how everything is connected (unless someone beats me to it). I'll also try to get that measurement for you once I figure out what you were asking me to measure :)

-Jim


----- Original Message ----
From: Mark <yahoogroups@...>
To: ModularSynthPanels@yahoogroups.com
Cc: James Elliott <johans121@...>
Sent: Sunday, March 2, 2008 9:23:16 AM
Subject: Re: [ModularSynthPanels] CGS65 Questions....

On 3/1/08, James Elliott put forth:

>2) Mark, I don't know how you will get the Bias switch hooked up
>with only a SPDT switch and get this to work. You have two pairs of
>items you are switching between the two options, a jumper & a 330k
>resistor. I used a DPDT switch and it worked fine.

How did you use a DPDT switch?? Are you switching the suppression
grid as well?? I'm thinking a SPDT switch could work in two ways:

1)
--- ground side of cathode
grid -- 330K----
--- negative rail

2)
--- resistor -- some voltage point (eg. ground side of cathode)
grid ---
--- resistor -- some other voltage point (eg. negative rail)


Another option would be a SPDT switch with one end unconnected, or a
SPST switch, to switch between a bias and leaving the grid floating
(or always connected with a large resistor so the switch would put
another resistor in parallel connected to the same point).

Anyway, I'm not planning on not using the diodes to reduce noise and
current draw, and might add a bypass cap to the cathode (in theory,
it could be an issue at low frequencies), so in my version, the
"ground side of the cathode" might not be the best point. Ideally,
a trimpot would be the best way to set the bias. So what I plan on
doing, is using a pot to figure out two values -- a clean VCA
setting, and a distortion setting, then building fixed voltage
dividers as necessary, and setting the switch between them.

If you don't mind me asking, with no signal, what is the voltage at
the grid at each of your switch settings??

>>If you were to do that, you would use a SPDT switch -- wire the
>>actuator to the 330K resistor going to the grid, and one end of the
>>switch to the ground side of the cathode, and the other end to the
>>negative rail. A center-off switch would add another setting.

____________________________________________________________________________________
Be a better friend, newshound, and
know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ

On 3/3/08, James Elliott put forth:

>Mark, I haven't forgotten your request. It's going to take some time
>before I can answer your questions.

No problem :) I still haven't built mine. At this point, I'm
waiting for the weather to get warmer. Then I'll build it, and JH's
Tau Phaser pedal. I'm also interested in envelope followers and
twin-T filters, which I will probably end up building on perfboard
and putting in project boxes, since I can't seem to find satisfactory
PCB's/panels.

>Why? Well because I have no EE experience other than the little I've
>gained building full KITS from MOTM. I've just recently started
>working from "scratch". ie buy a PCB and then source all the parts
>and troubleshoot issues on your own because of little to no
>documentation.... I spent about an hour and a half reading about
>vacuum tubes yesterday so I could get a little perspective on how
>they work before crafting a response to your message.

Imho, reading the "theory of operation" in the MOTM manuals is a
great way to learn how things work. The kits also provide very good
examples of grounding, shielding, parts selection, etc.

>With all of that being said, on the latest version of Ken's CGS65
>PCB, Ken has implemented the "bias option" on the board itself.

I just noticed that. I also have version 1.1 of the PCB. Since it's
a single-sided board, it's fairly easy to hold it in front of a light
and see where the traces go.

>In order to choose which option you want to implement you have to
>set two different components: 1) A jumper between one of two points
>2) AND a resistor between one of two points. With the DPDT switch,
>one pole switches the jumper position, the second pole switches the
>resistor position. Now, it very well may be possible to use a SPDT
>switch to do this, however upon a novice's first (or second, third,
>fourth) glance it seems that the jumper switches between to pins on
>the tube - so I don't know that a SPDT will work without some other
>circuit voodoo magic. I'm writing this at work so I can't
>necessarily verify what I've just said at this moment....

The way to do it is to not install the 330K resistor on the board,
and solder it directly to the switch. The best way is to use heat
shrink tubing, which I guess is sort of like magic :)

This way the switch either connects the grid through a 330K bias
resistor to one of two volatge points on the board. If one of those
points ends up being ground, or if I can find a good way of doing it,
I would use shielded cable to help keep noise off the grid.

Notice where the board can be cut in half, there are extra holes for
things such as ground, and the negative and positive power supply
rails.

>I'll try to do some more research sometime this week so I can tell
>you EXACTLY how everything is connected (unless someone beats me to
>it). I'll also try to get that measurement for you once I figure out
>what you were asking me to measure :)

OK :) What I waould like to know, is what is the voltage on the
signal input grid, when there is no input signal, for each switch
position. The grid is pin 6 on the tube.

Also, where did you connect the suppression grid?? While you are at
it, could you measure its voltage?? That's pin 4 on the tube. On
the CGS website, it says "it may be appropriate to connect pin 4 of
the tube to pin 1 of the tube instead of pin 2 as well, if making
this modification." I don't know how much of a difference that would
make, or how that could help. The suppression grid deflects "loose"
electrons back towards the plate. Generally it's connected to the
cathode, but this is a rather unique circuit. It should be more
negative than the the plate (which is a given) and screen grid (which
carries the control voltage of the VCA) on either side of it, so that
it does not draw current.

Mark wrote:

> I'm also interested in envelope followers and
> twin-T filters, which I will probably end up building on perfboard
> and putting in project boxes, since I can't seem to find satisfactory
> PCB's/panels.
>

Paia still offers the drum tone board which is basically a collection of
6 twin-T filter circuits.

Dave

--- In ModularSynthPanels@yahoogroups.com, James Elliott
<johans121@...> wrote:

>
> Now, it very well may be possible to use a SPDT switch to do this,
> however upon a novice's first (or second, third, fourth) glance it
> seems that the jumper switches between to pins on the tube - so I
> don't know that a SPDT will work without some other circuit voodoo
> magic. I'm writing this at work so I can't necessarily verify what
> I've just said at this moment....

Since I'm building one this evening, thought I'd follow up.
There is a bit of voodoo. It looks like Ken intends for
pin 4 of the tube to be at the same voltage as the 330K
bias resistor so the 'outside' of both options are actually
connected on the board. (You should be able to follow this
on the PCB with little effort.) The voodoo then is to send
the three pads involved in the jumper option (i.e. not the
330k resistor option) to the SPDT switch. Then solder the
330K resistor from either of the two left pads (they're
connected) to the center pad of the jumper set. (It will
actually work better if the resistor is put in first in
vertical position giving you a solder point for the middle
wire of the SPDT run.) The voodoo makes it a bit harder
to try tweaks but that's the tradeoff...

I just started my coffee, and haven't built my cgs65 at all, but I'm
still thinking that you would only need one 330K resistor between
the signal grid and the actuator of the switch.


On 3/29/08, basdbg963 put forth:

>--- In ModularSynthPanels@yahoogroups.com, James Elliott
><johans121@...> wrote:
> >
> > Now, it very well may be possible to use a SPDT switch to do this,
> > however upon a novice's first (or second, third, fourth) glance it
> > seems that the jumper switches between to pins on the tube - so I
> > don't know that a SPDT will work without some other circuit voodoo
> > magic. I'm writing this at work so I can't necessarily verify what
> > I've just said at this moment....
>
>Since I'm building one this evening, thought I'd follow up.
>There is a bit of voodoo. It looks like Ken intends for
>pin 4 of the tube to be at the same voltage as the 330K
>bias resistor so the 'outside' of both options are actually
>connected on the board. (You should be able to follow this
>on the PCB with little effort.) The voodoo then is to send
>the three pads involved in the jumper option (i.e. not the
>330k resistor option) to the SPDT switch. Then solder the
>330K resistor from either of the two left pads (they're
>connected) to the center pad of the jumper set. (It will
>actually work better if the resistor is put in first in
>vertical position giving you a solder point for the middle
>wire of the SPDT run.) The voodoo makes it a bit harder
>to try tweaks but that's the tradeoff...

On Sat, 29 Mar 2008, Mark wrote:

> I just started my coffee, and haven't built my cgs65 at all, but I'm
> still thinking that you would only need one 330K resistor between
> the signal grid and the actuator of the switch.

Correct. I've a picture I'll try to upload somewhere...

--
Monty Brandenberg

I've uploaded a small image showing how I wired the SPDT switch
to control bias to two grids. In case it isn't clear, the
'top' end of the resistor is wired to the pole of the switch
and to the common point of the two on-board jumpers.

--
Monty Brandenberg

On 3/29/08, mcb, inc. put forth:

>I've uploaded a small image showing how I wired the SPDT switch
>to control bias to two grids. In case it isn't clear, the
>'top' end of the resistor is wired to the pole of the switch
>and to the common point of the two on-board jumpers.

Two grids?? Do you mean you are switching the suppression grid as
well?? My plan was to switch the bias on the signal grid between two
values. Did you take any voltage measurements??

Where would I find this picture??

Thanks :)

On Sun, 30 Mar 2008, Mark wrote:

> Two grids?? Do you mean you are switching the suppression grid as
> well?? My plan was to switch the bias on the signal grid between two
> values. Did you take any voltage measurements??

The jumper alternatives on the board (one solid line, one dashed
line) switch the suppresion grid as per Ken's modding suggestions
on his website. I'm *guessing* that since he changed the board
layout, he went to the trouble of testing this first but I wanted
a challenge anyway.

Photo is in the root directory of the files area with a 'CGS65'
prefix.

Comments welcome (mostly)...

--
Monty Brandenberg

On 3/30/08, mcb, inc. put forth:

>On Sun, 30 Mar 2008, Mark wrote:
>
> > Two grids?? Do you mean you are switching the suppression grid as
> > well?? My plan was to switch the bias on the signal grid between two
> > values. Did you take any voltage measurements??
>
>The jumper alternatives on the board (one solid line, one dashed
>line) switch the suppresion grid as per Ken's modding suggestions
>on his website. I'm *guessing* that since he changed the board
>layout, he went to the trouble of testing this first but I wanted
>a challenge anyway.

I still don't know how you switched two grids with one SPDT switch.

While Ken suggests connecting the suppression grid to a different
voltage when connecting the signal grid to a different bias voltage,
I don't think that would be necessary. The function of the
suppressor grid is to reduce noise. It works by keeping electrons
emitted by the plate off the signal grid, and in order to repel
electrons it needs to be connected to a relatively negative voltage.
So it's generally connected to the cathode. Things are more
complicated with this circuit because it uses a directly-heated
cathode, a bipolar supply, and the unmodified circuit has diodes in
series with the cathode. Yet I don't think having it 1.2V more or
less negative is going to make a difference. What you don't want is
a path between the signal grid and the suppression grid because that
would defeat the purpose of the suppressor grid.

>Photo is in the root directory of the files area with a 'CGS65'
>prefix.

Thanks :) I found the picture, but couldn't figure out anything by
looking at it.