I'd say "typical." The 1M can be disconnected or not, the difference being the input impedance seen by the guitar pickup, which will make a small difference in its resonant peak. The tube's grid is going to be looking at ~10k either way.
I'd say it's required to insure that the input grid always has a dc ground reference. Who can say what the other end of the instrument cable is plugged into. For example, it may be plugged into a 6G15 reverb unit, in which case there is no dc ground reference for the grid and bias is lost.
sluckey wrote: ↑Thu Oct 27, 2022 8:19 pm
I wonder if the original schematic in Dave Hunter's book shows the input jack drawn like that.
There is no schematic, just this layout, but it clearly shows the 1M going from the grid stop to ground. The grounding switch is not connected. The jack is specified in the parts list as a "switching jack," with no part number given.
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martin manning wrote: ↑Thu Oct 27, 2022 5:16 pm
I stand corrected. I wonder how many of those have been built with the 1Meg on the shunt lug?
So you and @sluckey got me wondering, so I pulled out my two stroke and sure enough, the R3 resistor is wired to the switched leg of the jack and only connects to ground when the plug is inserted.
Should I move this to sluckey's suggested point? From a layman's perspective, what will this change do?
A couple of points were made above re losing the ground reference if the guitar end of the input cable is unplugged, or plugged into a device that doesn't have a ground-referenced output connection. These are both good reasons to move the 1Meg to the tip lug of the input jack, which is the standard location. I don't think you will hear any difference when playing a guitar into the amp, and it's not likely that any damage has been done.
Magnatron wrote: ↑Fri Oct 28, 2022 12:02 am
sure enough, the R3 resistor is wired to the switched leg of the jack and only connects to ground when the plug is inserted.
You have that backwards. The tip is only connected through R3 to ground when the plug is not connected.
Should I move this to sluckey's suggested point? From a layman's perspective, what will this change do?
Yes. Doing so will insure the preamp tube grid always has a resistive path to ground. This means the tube in that cathode biased preamp will always be biased correctly.
