5k Grid Stopper

[surfsup]

The current schematic omits a resistor on pin7 of stage 1. From Aikenamps:

The grid resistor on the preamp stages typically ranges from 0 to 68K, although very large values, such as 470K, are sometimes used in high-gain preamps to shape the frequency response and prevent "blocking" distortion in the preamp section under heavy overdrive conditions. The Miller capacitance of a typical 12AX7 is around 151pF, so the upper frequency response -3dB cutoff point of a stage using a 68K grid resistor is around 15.5kHz. The frequency response drops to around 2.2kHz if a 470k grid resistor is used. This "free" response rolloff can be used to tame the "buzziness" of high-gain preamp stages without having to add additional rolloff capacitors. Perhaps the most important grid resistor is the one that goes to the grid of the very first stage, right after the input jack. This resistor is the one that prevents oscillations and pickup of radio stations and other noise due to long or poorly-shielded cables. It is not usually a good idea to eliminate this resistor. Ideally, it should be soldered directly to the grid pins of the socket, with very short leads.

Seems like its a good idea to have one on a higher gain build. I added a 5k but am going to change that to either a 22k carbon comp or 100k metal film tonight. I am getting a squeal with G1/G2 maxxed and MV at 2pm or higher.

C for 12ax7 = 151pF = 0.000000000151F
f,22k = 1/(6.28*R*C) = 1/(6.28*22000*0.000000000151)
f,22k = 48.2kHz
f,100k = 10.6kHz
f,50k (two 100k in parallel) = 21.231kHz

f,5k = 212.3kHz (most recorded music will only range from 50Hz - 20kHz) so 212kHz based on the math almost seems pointless. I definitely need to change that.

[David Root]

That's right, 5K is not really doing anything. FWIW Dumble generally used 22K or 33K.

If you want to use 50K, why waste one 100K to do two in parallel when you could used one 51K?

[surfsup]

Yea I did the math afterwards. I don't have a 51k resistor. and the 22k is a carbon comp that someone posted a reply to me a while back that CCs are noisy and avoid them.

Maybe I'll put the 22k in there anyway.

[Cliff Schecht]

Actually CC's make excellent grid stoppers because of their very low inductance nature. I use them often without any noticeable addition in noise.

I have a question though.. I've noticed that some amps tend to darken up very badly when they have the standard 68k grid stopper, whereas no grid stopper makes for a very bright stage that works much better with the guitars volume (I'm thinking specifically of my 5C3 normal vs. bright channel). Has anybody found a value of grid stopper that doesn't badly attenuate the highs when the volume is rolled back (and no I will not use treble bleeds in my guitars ). It seems like some amps have this problem while others (*looks at Express in the corner of the room*) don't suffer from this problem at all. I know the bright cap is supposed to facilitate this but I was just wondering what other tricks people have used to manage excessive treble roll-off with grid stoppers.