Daver 7/16/2006 10:29 AM
Input Impedance for 2 Triodes
If I have a single input feeding two triodes for two separate channels do I need to adjust the value of my 1M resistor that goes to ground? Each triode has its own 34K grid resistor. Should I raise the value of the 1M to 2M or doesn't it matter much? Thanks. Dave
 
Bob-I 7/16/2006 5:30 PM
[QUOTE=Daver]If I have a single input feeding two triodes for two separate channels do I need to adjust the value of my 1M resistor that goes to ground? Each triode has its own 34K grid resistor. Should I raise the value of the 1M to 2M or doesn't it matter much? Thanks. Dave[/QUOTE] Actually, you don't need to 2 grid stoppers, one before both grids will be fine. The impedance will be reduced because the tubes are in parallel, but it's not enough to matter. Stay with the 1M.
 
Daver 7/16/2006 5:47 PM
Bob, the tube halves aren't in parallel. They have separate plate and cathode resistors and are the first stages in separate channels. Maybe I didn't make it clear. :confused: There is only one input, a 1M to ground and then the signal splits to two different tubes each having its own 34K grid stopper. Does that make sense? :) I know a schematic would be better.......
 
Bob-I 7/16/2006 5:56 PM
[QUOTE=Daver]Bob, the tube halves aren't in parallel. They have separate plate and cathode resistors and are the first stages in separate channels. Maybe I didn't make it clear. :confused: There is only one input, a 1M to ground and then the signal splits to two different tubes each having its own 34K grid stopper. Does that make sense? :) I know a schematic would be better.......[/QUOTE] No that makes since, but now I'm confused as to the input impedance as well, sorry.
 
Andy 7/16/2006 8:04 PM
A common-cathode gain stage has a very high input impedance. unless you drive the grid positive, it is close to infinite (as close as the high resistance of glass and vacuum will allow). one 1M resistor should be fine.
 
Enzo 7/16/2006 8:34 PM
If that 1 meg resistor is on the input jack end of the grid resistors, it is in parallel with your pickups, and will have little effect.
 
Ray Ivers 7/16/2006 8:47 PM
Andy, A grounded-cathode triode gain stage (I assume you meant this - the typical guitar-amp GS - rather than the common-cathode circuit shown below) has an extremely high input resistance at DC, but the effective AC input impedance is much lower due to the tube's interelectrode capacitances combined with the Miller effect. FWIW, here are some ballpark Zin's @ 20kHz, using a 100K Rp and unbypassed 1.5K Rk: 12AX7 - @ 90K ohms 12AT7 - @ 110K ohms 12AU7 - @ 260K ohms The input impedances will be correspondingly higher at the lower bandwidths typical of most guitar amps. To Daver: the 1M resistor will be fine in your circuit IMO. Ray [IMG]http://music-electronics-forum.com/attachment.php?attachmentid=98&stc=1&d=1153103998[/IMG]
 
Daver 7/17/2006 8:50 AM
Thanks, gentlemen. Excellent explainations! I didn't think it would be a problem, and it does work fine ;) , but I wanted to make sure I wasn't missing something. I appreciate it. Dave