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.