Input impedance with internally jumpered channels

[Silvio55]

Hi! I made a 1987 50W plexi clone, with both channels internally jumpered at the tube socket, I used a single 1M resistor at the input with one shielded lead going to pin 7 and jumpered with pin 2 at the socket. My question is: is it the same as using two leads going to v1 grids with each lead having its own 1M impedance resistor? or does the impedance change this way? thanks!!

[Chuck H]

The impedance is different. There are also dividers created when the input grid resistors are incidentally wired in series. The circuit becomes quite specific and any difference from yours, for better or worse, will be small. You might just analyze the stock circuit and tack solder it in to see what you think.

[Silvio55]

Yeah, the voltage dividers created by the gridstoppers, but I didnīt use gridstoppers, I go straight to the grid, what I was thinking is if using just one 1M impedance resistor would give me an impedance of 1M for both grids or, being shared, each grid sees twice the impendance (2M).

[Tom Phillips]

This is a little hard to visualize without a schematic of your build. My take is if you have one 1 Meg Ohm "grid leak" resistor to ground connected to two triode grids in parallel, then the input impedance is essentially 1 Meg Ohm.

[Silvio55]

Yes, that what I have. so itīs ok then, I was afaid that I had to halve the value of the grid leak resistor, thanks!!

[Tom Phillips]

Yes, that what I have. so itīs ok then, I was afaid that I had to halve the value of the grid leak resistor, thanks!!

Right. The input impedance of the valve grids is so high that the value of the grid leak resistor is essentially the input impedance of the overall circuit.

[J M Fahey]

Use a single 1M ground reference for both input triodes, but personally Iīd use an individual 68K (or 47K) resistor in series with each grid, to minimize RF pickup ("radios truchas" )
Place resistor close to input grid and heat shrink it for mechanical strength.

[Silvio55]

I avoid the RF interference problem by locating my ground point right at the input jack, as recommended by Paul Ruby, it worked perfectly in my two amp builds, this way you can avoid using a gridstopper and reduce noise. In my first build, I forgot to add the gridstopper, and was using an insulated swtichcraft style jack, my amp worked better as an FM receiver than as a guitar amp! then I just removed the insulation and voila! no more radios truchas through the speaker!