grid stopper question

[iknowjohnny]

In a typical 3 gain stage JCM sort of preamp, would you consider a 220k grid stopper on the first 2 gain stages a lot? Do you think it would cut too much hi end ?

[tubeswell]

anything between 100k and 470k is possible. Some people like the hi cut that bigger stoppers provide in hi-gain amps. If you don't like it you can easily partially bypass it with a smallish cap.

[Torquil]

Go here:

http://www.muzique.com/schem/filter.htm

Input resistance 220000 ohm
Capacitance: 0.00015 uF

Gives a low pass corner frequency of around 5000Hz. It simply calculates the corner frequency using the formula

f = 1/(2*pi*R*C)

The capacitance 150pF is an estimate I've seen for the total capacitance for a 12AX7 triode. You can look it up e.g. on Merlin's or Aiken's web pages.

You can check different resistances to see how the corner frequency varies.

[billyz]

Nice Calculator.

[iknowjohnny]

Thanks. Sure seems like theres a lot more hi attenuation that that with 220k, but i guess calculators don't lie.

[vibratoking]

Sure seems like theres a lot more hi attenuation that that with 220k, but i guess calculators don't lie.

Keep in mind that the formula calculates the -3dB point. There is attenuation below this value. Substitute some different values and calibrate your ear to what the calculated -3dB point sounds like.
You can also measure the -3dB point if you have a scope and signal generator.

The calculator isn't lying if it is implemented correctly. I assume it is, but I haven't checked it. Be sure that you are putting the correct values into it.

[Torquil]

Thanks. Sure seems like theres a lot more hi attenuation that that with 220k, but i guess calculators don't lie.

The calculator doesn't lie, but thereare a few caveats. First of all, the proper resistance value to use in the calculator should include the output resistance of the preceeding part of he circuit. If this is e.g. 50k or even more, then you should use 270k instead of 220k.

Also, there is the question of how sensitiveyour ears are and what kind of tone you are using.

So your best bet is to just use the calculator to estimate an initial value to try, then adjust from there using your ears. Actually, it might be best to use a pot and adjust to your liking. When you are done, you can use a fixed resistor in its place.

That JCM preamp is so well-known that I bet there are numerous ways of reducing treble that are documented/reviewed on the net. E.g. removing/changing bleed caps or e.g. adding small bypass caps on the plate resistors. I would try to remove the bleed cap in the 470k/470k voltage divider in front of the third triode.

If a goal can be achieved by reducing the number of components, then I'm all for it!

[iknowjohnny]

for the record i'm not trying to remove highs. In fact i was wondering about lowering the grid stoppers to get MORE hi end. But it's such a pain to lower one, play it for a week, lower it more, play again, try the other, etc etc. It's a subtle thing really to get it just right so i thought i'd ask. thing is, the amp sounds so perfect right now, but in a mix *at times* i feel it could be a tad brighter. I will probably think about other ways to accomplish this tho, possibly in the PA because i have the preamp so right at the moment and i don't want to mess with it. By the way, it's not a JCM 800. I just said "jcm sorta preamp" as a general description so it would at least be in the ballpark. But it's actually quite different, tho the JCM is close enough to give a general idea of what i'm dealig with. In fact, i don't even have a cathode follower, tho it did till a few months ago.

[Jana]

If it's perfect now, then why change it? It sounds like you are chasing your tail with this one and will never catch it! Why? Every room sounds different, playing with a band is different than in the carpeted living room, standing in front of the cabinet instead of off to the side because the drummers cymbals are melting your ears changes it. Forty people in the bar is going to sound different than when there are 200. Even if nothing else changes, it's going to sound different at 11 pm than it did at 9 pm (your ears becoming fatigued).

I have an amp set up in my practice space and nothing changes in that room. Some days it sounds different than the previous day. What happened? I don't know. Humidity, my finger calluses changed, my ears are hearing things differently, who knows. I don't worry about it, I just go with the flow. It's the "tone for the day." Life is too short to be chasing mojo fairies.

Use the tone controls and the presence control to "tune" it to the situation. They are there for a reason.

[iknowjohnny]

I know. it's just that overall when taken as an average, it seems darker than i believe is where i normally like to be. It's in a bright room too, and i usually have more issues with brightness here then the rare occasion i play out somewhere. I suppose i am pretty d@mn anal tho ! But i think a 100k feedback resistor may be in order. the 47k sounds great, but i think it's not so much brightness it needs as that little extra cut and "in-your-face-ness" the 100k NFB resistor will give that may be just what it needs. Somehow that doesn't so much add treble, it just sorta removes a corner of the proverbial blanket over the speaker.

Everyone who knows me knows i'm a perfectionist and they often tease me about it. I'm not particularly fond of being that way, but it's not something i've had much luck trying to change.

[dukeamps]

Hi iknowjohnny, have you got a bright switch on the amp. I mean the same that is on the trainwreck amps. I have put one on my amps, and helps in those situations where you need that little bit extra. I went with 100pf and 250pf, but I usually only use 100pf.

Dukeamps

[FUCHSAUDIO]

It alters how the stage will clip and you also may gain noise too. Keep that in mind as well.

[Colossal]

Don't forget that while 220k may be the chosen grid stopper, the guitar volume pot adds series resistance with the grid stopper when turned down so you may lose even more high end under those conditions.

[iknowjohnny]

I didn't mean to say i have a 200k on the input grid. Thats 10k

[Colossal]

I didn't mean to say i have a 200k on the input grid. Thats 10k

Ah, ok! 10k is an effective RF deterrent without much loss of highs. In determining the low pass filter, the input tube (Miller) capacitance is taken into account as the calculator shows. You can add more capacitance from grid to cathode to take advantage of the Miller effect. This will keep the grid stopper value small while attenuating more highs (if desired) at the input.

I've enclosed a PDF showing the effects of different sized grid resistors on the paralleled V1 input of an 18W amp. The electrode to electrode capacitance for that is 260pF because the triodes are run in parallel so for a single 12AX7 it would be about 1/2 that. I wanted to see the effects of running two grid stoppers (in parallel) or just using one and jumpering the inputs and the attenuation due to size. The gain of the stage figures into the calculation too so it's not just about R and C.