I'm putting a Lar Mar Type 2 PPIMV on my Tweed Twin, like this:
I'm targeting a nominal 220K/220K pot value. The safety resistor value with an actual 250K pot is 1.8M as in the above drawing. Out of 20 Alpha dual 250K pots, my closest-matched pot tests as a 238K/235K pot, which if I average that at 236.5K makes the safety resistor value about 3M3. Is that too large a value to be "safe"? If the pot fails and my PT grid leak resistor is instantly 3M3, is anyone going deaf or jumping for the fire extinguisher? Just curious before I wire this up. Thanks.
You don't have to hit 220k exactly. 214k [1 / (1/237k + 1/2.2m)] is only 3% off from 220. If it were me, I'd just use the 2.2m and not worry about it.
Mike
EDIT: Just noticed your schematic mentions 1.8m and not the 2.2m I've always seen with this type of MV. Using 1.8m safety resistors will put you at 209k which is still only 5% off of 220. I might still bump them up to 2.2m if you have those. Otherwise 1.8m should also be ok.
If the pot center wiper fails even 1.8m is probably way over the maximum grid leak for these tubes (don't have the data sheet memorized). But having bias voltage on the grids is still probably way better than not having it. 2.2m is pretty close to 1.8m while 3.3m is considerably larger than 2. So, if it were me, I don't think I'd tempt fate with the 3.3m. I know folks have used this circuit with values around 2m. However I don't know if I've ever seen anyone test this setup in the failure case (center wiper disconnected)...
Just curious before I wire this up. Thanks.
