A while back I've build a Princeton. Sounds like it should, not getting any RF noise but today looking at my schematic I've noticed I've used 33k input resistor on my single input socket instead of the 68k on each of the two input sockets as the original has. Without opening it up I wonder what the difference between 33k vs. 68k might make if any at all? I wonder how critical this resistor actually is since some vintage amps never had one in the first place. Perhaps it's only important for high gain 1st tube distortion setup which Princeton really isn't?
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Tube itself has input capacitance, so grid resistor or grid stopper value forms a low pass filter.
which is why it works as RF rejection filter.
but also important to have grid stopper for stabilization.
So there always needs to be a grid stopper to help from parasitic
oscillation.
so anything from 22k to 100k is fine
depending on where you want the filter cutoff to be
basically the capacitance between grid and cathode and miller.
so you get a lowpass filter
12ax7 usually around 150 pf
so you can calculate with basic first order filter equation.
33k around 30 Khz cutoff
68K around 15 Khz cutoff
so some go up to 100k for around 10 Khz cutoff
which is good limit for musical instrument amp
for high gain distortion preamps they can go up to 470k
which gives a cutoff around 2K
but also needs to be high value, because the previous stage is
blasting it with higher gain/voltage.
so you can get blocking distortion with to high signal levels.
which is why it works as RF rejection filter.
but also important to have grid stopper for stabilization.
So there always needs to be a grid stopper to help from parasitic
oscillation.
so anything from 22k to 100k is fine
depending on where you want the filter cutoff to be
basically the capacitance between grid and cathode and miller.
so you get a lowpass filter
12ax7 usually around 150 pf
so you can calculate with basic first order filter equation.
33k around 30 Khz cutoff
68K around 15 Khz cutoff
so some go up to 100k for around 10 Khz cutoff
which is good limit for musical instrument amp
for high gain distortion preamps they can go up to 470k
which gives a cutoff around 2K
but also needs to be high value, because the previous stage is
blasting it with higher gain/voltage.
so you can get blocking distortion with to high signal levels.
Ok thanks. I got the 1M grid stopper there with the 33k so I guess it's all good than. I suppose I'd be hearing something if there was a problem but since I've discovered my error I just had to ask so I can sleep at night 🙂
The resistor on the input also helps keep grid current and the resulting DC upset that it causes when the dumm blond sets his pedal board on kill and drives 3 or 4 volts of signal into that input.