john55 wrote:
ILoveHiFi wrote:
You want 1M ohm to keep the grid ground, cathode or voltage refrenced, not in series with the gird.
In series with grid to help prevent high frequncy oscillation typically has a resistance value of k to 10ks of ohm, I don't know the orginal schematic but it probally should have its specifed grid resistor.
Not too much of a fuss to get perfect values but a value of 1.8-2.7k works well with most valves.
Thanks for the explanation, making more sense now. I'm removing the original 100k pot shown in (A). I assume (B) is correct now? Why would I not use a 100k resistor to ground? Why jump to 1M? Is it because the signal from the tone stack is different to say a signal from a CD player or iPod?
You want 1M ohm to reduce load on input capacitor and improve sound quality, smaller input caps normally mean less cap distortion. The second reason is to reduce load on your music source.
By having the other of capacitor connected to 100k but otherside at tube grid with 1M to ground will yeild about the same noise input to the amp.
Compared to that with larger cap and grid directly to ground with 100k.
A capacitor is like short in ac so having 100k to ground on either end yeilds about the same noise input to amp.
Lower input resistance means lower noise and less sensitive to noise pickup.
Not all grids like 1M to ground, bit too big for some tubes like KT88, it has specified maximum resistance of 220k denpending on bias method. Meaning you need to use 220k or smaller grid resistor to ground.