Let's talk Plexi's (couple questions)

[Ripthorn]

I have recently renewed my interest in amp building after a string of guitar builds and wanted to do a submini version of the plexi. I have it set up on my breadboard right now and it sounds great. However, there are two questions that I still have about this amp in general.

1) If I turn either inputs gain to 0, I get no output. This make sense looking at the schematic, since each pot's output has a path back to the other gain pot, but I wasn't sure if this is "normal" operation, as I have never played an actual plexi.

2) If I play into the Input II input, I get very low output and lots of noise (even a little squealing sometimes). In experimenting, I found that the problem goes away and everything sounds great if I don't have that channel's output connected to the other channels output as shown in the schematic. The strange thing here is that if I play into Input I with it's output connected to the other channel's output as shown in the schematic, there is no problem, it sounds great either way. This doesn't seem like it should be normal. Any idea what is happening here?

Another interesting bit regarding question 2 above is that if I jump the inputs together, I don't get the problem either, only when I plug straight into Input II.

If possible, I would like to make it so that the gain controls are independent, but I also want to be able to jump the inputs. I would appreciate any input (pun intended) you guys have about these issues or plexi's in general. Mine is sounding great, I would just like to get these two issues at least understood (if not resolved) before putting the amp on a board. Any help is appreciated.

[Jana]

Something isn't right. You should get input from either channel even if the other channel is turned down to 0. Do you have the mixer resistors hooked up right?

[paulster]

The gain on one pot should affect the other since it is altering the impedance at the mixer resistors but so long as you have the mixer resistors in there you can't get it to zero.

What you should find is that if you use channel 1 and have channel 2 on 0 or 10 that you'll have a level of gain, and then if you change the channel 2 volume to about 7.5 with nothing plugged into it the gain on channel 1 will increase slightly because you're now increasing the resistance to ground at the mixer to its highest point.

[Ripthorn]

Thanks for the responses, guys. I checked and it turns out I had 470R mixer resistors as opposed to 470k (poor schematic resolution). That fixed the gain pots. The problem with the noise and such was that I didn't have any connection to ground for the unused channel (looks like the original used switched jacks), so a 1M pulldown resistor fixed that issue. It is now sounding just as I would expect. Again, thanks for the help.