Why the single input should have weak gain is very very odd, when plugged into it the switches disconnect it completely from the parallel jack.
Solved. This was because the other input on the tube wasn't grounded. I didn't want to run two grounds and mis-wired to the switch from the parallel to the single jack. Giving each jack it's own ground was the answer.
As for the paralled jack, is getting 500K for the grid load unavoidable in this situation? I'm thinking this is also what you get when you use a Marshall style jumper cable plugged into two jacks. It's just how it is?
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Plugging into the High/Low jacks on a Marshall still gives a 1M resistance to ground. If you jumper from one channel to the other so you use both channels, then it is 500K. I wouldn't worry about it. With 500K you still have a resistance that is a couple orders of magnitude greater than the guitar. When your guitar volume is on 10, the resistance is about 6 to 8K depending on your pickups.
rp wrote:As for the paralled jack, is getting 500K for the grid load unavoidable in this situation? I'm thinking this is also what you get when you use a Marshall style jumper cable plugged into two jacks. It's just how it is?
If you've got "eagle ears" you may be able to detect the tiny bit of high treble that will go away with the 500K guitar loading. If you can hear the difference and it doesn't suit you why not swap 2M2's for the 1 meg resistors. FWIW Ampeg in the mid 60's typically had a 5M6 resistor across the input.
