Bias supply "idiot resistor"

[PharmRock]

I am in the process of laying out a JTM45 board, and have a question about the bias supply circuit. 
Valve Wizard has a very nice article on the topic, from which I obtained the screenshot below.  In this article he states:
"It is good design practice to limit the range of voltage adjustment so it is impossible to reduce the bias to nothing (0V). This at least gives a modicum of protection against accidental or unskilled adjustment. This limit is provided by R2 which forms an end-stop for P1, also called an ‘idiot resistor’!"

Most bias supply schematics I have come across use this exact approach, where R2 (idiot resistor) is between the trim pot and ground.  However, when looking at the Metro build guide for the JTM45, R2 (68K) is prior to the trim pot, coming off the junction of C2 and R3.

Since the trim pot and R2 are in series, does it matter which order these are in?  (I have a feeling PRR will ask me, if my car needs a wash and gas, does it matter which order? https://el34world.com/Forum/index.php?topic=27216.msg299761#msg299761  )

When looking at Plexi schematics (e.g 1959, 1987, etc.), as well as Doug's 5F6A schematic https://el34world.com/Hoffman/files/Hoffman_5F6A.pdf, the "R2" resistor comes after the trim pot and connects directly to ground.

Thanks for your help

[acheld]

Simple answer is that no, it does not matter in terms of the electrical circuit.   Resistors in series are what they are . . .

It might matter in terms of the potential failure modes of the trim-pot you are using (though I can't think of anything like that), or layout considerations.

[PRR]

> if my car needs a wash and gas

It matters, if you pick up your lover and go out of town and "oops, ran out of gas!" so you can neck a while.....

The pot in Merlin's plan is no longer in play. Instead there is a variable resistor (yes, a pot, but wired different) and a 68k "stopper" so you can't turn bias down to zero.

You got the parts there? Tack it up on a scrap board and try it. It will even work with a safe/handy 6VAC feed instead of the 300VAC feed, if your meter reads mV of DC (most do now).

[PharmRock]

Simple answer is that no, it does not matter in terms of the electrical circuit.   Resistors in series are what they are . . .

It might matter in terms of the potential failure modes of the trim-pot you are using (though I can't think of anything like that), or layout considerations.

Thanks for the feedback....all I can think of is that it has to do with the layout. I'll keep looking at it and see which version best fits my board.

> if my car needs a wash and gas

It matters, if you pick up your lover and go out of town and "oops, ran out of gas!" so you can neck a while.....

The pot in Merlin's plan is no longer in play. Instead there is a variable resistor (yes, a pot, but wired different) and a 68k "stopper" so you can't turn bias down to zero.

You got the parts there? Tack it up on a scrap board and try it. It will even work with a safe/handy 6VAC feed instead of the 300VAC feed, if your meter reads mV of DC (most do now).

PRR...haha well that's one way to look at it (don't tell my wife though).

I have a scrap board that may work, so I will give this a shot this weekend. 

Attached is Doug's 5F6A bias supply, with the trim pot wired as a variable resistor.  The 47K resistor is after the VR.
Also attached is Mark Huss's Plexi 6V6 Bias supply, where he has the resistor prior to the VR, like the Metro JTM45 board layout.

Thanks for all the help

[sluckey]

Another alternative is to not use any idiot resistor. Makes bias adjustments much more exciting! Kinda like this Weber amp...