Bias Trimmer Value Question

[sine_wave]

Recently, I've been offered a NOS RadioSpares slider/trimmer (the slider looking kind you see in old Marshalls).
It's in great shape but it's value is 100k.

I know that for the amp I'm building (12,000) the specified value for the trimmer is something like 27k or thereabouts.
Two questions… If 100k is waaay out of spec with what should be there, are there any alterations/substitutions with resistors, etc that I can make elsewhere in the circuit to accommodate it (without altering the fundamental tone)?

Secondly, on the 1230X schematic George posted a while back I think I'm seeing a bias trimmer with a value of 100k. Am I mistaking VR6 for another pot, or am I correct? I'll post the pertinent part of the schematic below. Any answers or advice are more than welcome. Thanks all!

Cheers,
John

[Tazin]

Marshall used whatever Radiospares sent them; whether it was a 27K, 50K, 75K, or 100K slider pot. This particular example ended up with a 100K slider installed. Personally, I would stick with the normal Marshall fixed resistor values for the bias circuit and go with a 25K trim pot.

[Strat78]

Ha ha, back when I was scouring the net for these I ended up with a dozen 2meg and 500ohm sliders, both worthless for this purpose. I did find a 27k and it worked great with a 5.8k resistor by the diode. Tazin's advise is good, but if you don't want that 100k slider, I'll be glad to give it a new happy home.

[sine_wave]

Thanks guys.

So from what I gather from both your posts, a 100k trimmer could actually work, (unless I got the wrong end of the stick).

Seems it would be much more problematic if the slider's value was below 27k, right?
More importantly, anything dangerous about using a value like 100k? Are there any perceivable differences between using a 100k trimmer rather than 27k? Besides any possible affects it might have on the circuit, could it cause any audible difference? I'm just trying to exhaust any possible disaster situations before I commit to anything.

- John

[Haze13]

I looks like a simple Voltage divider, 15k and 100k pot (that which can be any thing from 0 to 100k) and 10uf to remove the voltage ripple. The voltage you get is the voltage across the 100k pot (or some thing from 0 to 100k). The min voltage what you can get with 27k pot is 27k*Vin/(15k + 27k). With a 100k pot is less. So if you need for example a 15k value on the pot, to get this value more accurate with 27k is easier since it's some where in the middle of it's "track" (if it's linear Pot), but with a 100k pot it's just little more harder, because this value is in the beginning of the "track". That's it I think...

[Tazin]

You have to remember that this era circuit is slightly different compared to an amp from say mid 1969 in that it does not have a fixed value resistor (47K or 56K) in series with the trimmer pot.