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.comCc: 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