500k vs. 1Meg Volume pots

[EETStudent]

Consider a typical Marshall master volume, with the master volume after the tone stack. What differences would a 500k instead of a 1Meg pot make? Less gain, signal?

[mooreamps]

You can figure this one out.
1. What is the output impedance of the tone stack.
2. What is the input impedance of the power tube.

-g

[Tubis]

Think about this. In an 2203 can you realy crank the master more than half way?
Thats only 10% on a log pot. Maybe you can swap the pot to a 100k log and put a 900k resistor before it? A 900k metaloxide and a 100k pot after it will sure sound better than a 1M pot only

[pdf64]

Also bear in mind that the tone stack is a bunch of RC low pass and high pass filters, with the R and C values setting the turnover frequency. In the case of the bass and mid controls these Rs are variable, within limits. But these filters aren't sitting there in isolation, they are being driven by a source which has an output impedance, and are loaded by the input impedance of the following circuit, in this case the MV. These source and load impedances will affect the turnover frequencies. Peter.

[booj]

I used a 100k for a master volume pot in a 2203 design (accidentally). After about two years of on-off troubleshooting time trying to figure out why it just had no "oomph", I totally diss-assembled the unit. Then I discovered the 100k pot, and it only took a short while for the hole in my head caused by scratching to heal.

[EETStudent]

What it seems to me is, a 1M pot means the signal is 'further' away from ground than a 500k pot, allowing more signal to go through hence more gain/volume. That makes sense to me, especially with respect to fixed resistors.

But, in the case of variable resistors its kind of hazy. With a 1M pot audio pot turned about halfway the resistance might be about 100k. With a 500k pot turned about 3/4 the resistance is about 100k. So, in these two instances is the output identical, even with different pots? Well, if you were one who never turned the gain control past halfway, then you could use a 1M or 500k and acheive the exact same results, just at different settings on the pot. right?

[booj]

You'd have a little less signal going to the inverter whatever position it's at given the use of the same type pot. The lower the value of the pot, the more signal goes to ground and the less goes to the inverter.

[Ptron]

You can figure this one out.
1. What is the output impedance of the tone stack.
2. What is the input impedance of the power tube.

-g

EET, if the answer to 2 is "pretty high" (it is) and the answer to 1 is "pretty low" (is it? I don't know) then the answer to your question is "not much". The change in pot value would not siginificantly load down or load up the output of the tone stack and the input of the next stage would not significantly load down the output of the MV. All that would matter is the ratio of restance in the pot itself. If the output impedance of the tonestack is relatively high, then the extra load the 500k puts on the tonestack might start to matter. As booj said, you would have a little less signal getting to the MV as more of it would be dropped in the tonestack.

[Wakculloch]

Mr EET:-
The resistance to ground may be the same but with potential dividers (like pots), one needs to think in terms of ratios.
100k above ground on a 1M pot gives 1/10th the input signal,
100k above ground on a 500k pot gives 1/5th etc.
Never works out exactly like this but i'm sure you get the drift.

[Tubis]

A lower resistance to ground after the tone stack will change the balance of the freq. in the tonestack. The tonestack will sound different.
You kan put a much lower resistanse than 1M without loading down a cathode follower thou. But a carbon pot has more affect on the sound than a metalfilm resistor. And if you dont need all the amplitude. Then just put a resistor in series with the pot to lower the amplitude and there will be much clearer sound. Just see to that the resistor and the pot together adds up to about 1M to not alter the sound of the tonestack.
But one kan change everything in the tonestack itself ofcourse.
If I put an effects loop driver after the preamp i lower the signal with 1M/56k to go to line level. Just a tip.

[EETStudent]

Ahhh, its all about the ratio. Makes perfect sense now. Thanks fellas