grid stops in modded Marshalls circuits

[pjd3]

Just curious on your quick thought (or lengthy thoughts if your in the mood).

I've done a bit of looking at the function of grid stop resistors in guitar amps but would like your opinion on something.

Should you be building a modded Marshall circuit such as a 2204 with JEL mods and other gain increasing mods, and, your guitar is a single coil Strat, does it make any sense to set your grid resistor values to aim for a freq cutoff of say, 7-8K? Without having actually played with this as of yet, it would seem like a reasonable thing to do seeing that most speakers don't seem to be letting out much more than 5-6kHz along with the fact that you are using single coils with a moderate amount of gain that is generating even an excess of high harmonics that you wouldn't want to be hearing (if you don't want a screechy strident harsh tone - I don't).

It also seems like it wouldn't hurt the greater RF suppression and maybe even aid the stability of the tube stage.

Has anyone even gradually worked up the value of their grid resistor(s) in a high gain Marshall until they reached their desired Low pass Freq cut off point?

Thank for your comments.
Phil D3

[thetragichero]

i'm never really that scientific about it. generally use 10-39k even on lower gain (like fender) builds for grid stoppers (every preamp stage has one, not trying to save a few pennies like fender usually did). if it's something high gain 100k or 220k like soldano did

that being said, it's an interesting idea for tone shaping. could likely result in not having to use anode bypass capacitors which is a plus

[Reeltarded]

Early stages I start with 33k, generally. 3rd stage gets a 33-47k possibly. Added stages might get around 68k. Always small but larger as they get later in the circuit.

I have used grid stops to lowpass and it ripped the life out of the monster. Sticking with smaller grid stops let's harmonics develop, so think about a stepped solution to shaping highend.

Pick a high mid cutoff cap and a tiny high cut cap. Using small trimmers, set the amp to neutral ball park what you like, start trimming the high mid until there is a smoothness in that range then back off the trim very slightly. Now trim the highs doing the same until deciding if you need to revisit the mid trim. Tinker with this back and forth and live with your results for a while. When you are sure you found some magic, mearure the trimmers and replace with resistors.

Here is the thing: Keep these simple circuits as late as you can with the high cut being great just before the PI input, and the mid cut just before the CF driving stage, maybe on the mixer junction over the ground leg on an 800, or junction to ground on 4 holer.

It is a balancing act that you will tend to overshoot until you have played and adjusted your tone controls several or many times. If you are supressing highs with plate bypass you can lower the cap value there (as in from 470p to 100-150p) until the idle hash is the only thing cut instead of shaping with it, because this is also not a very delicate solution.

This can make the amp incredibly smooth and yet still retain loveable harmonics.

[pjd3]

Thanks guys,

This is some great insight into setting your own grid stops. It somehow makes alot of sense to let the harmonics develop through the first few stages - allow the harmonic energy to establish first, then roll it off. Plus I believe it would be an accumulative thing to roll off the roll off the roll off over many stages. I can see how that might deaden the upper harmonics.

Great stuff. I'm printing this one out for my work bench folders!

Thank you,
Best,
Phil D.

[pdf64]

The 68k grid stopper on the higher gain input of a Marshall 2204 seems a somewhat wrong headed, as it probably adds more noise than necessary, eg compared to a 33k grid stopper.

[thetragichero]

Keep these simple circuits as late as you can with the high cut being great just before the PI input

again not high gain, but fender used a lot of 500-1000pf coupling caps right before the phase inverter which follows this suggestion

[pdf64]

Keep these simple circuits as late as you can with the high cut being great just before the PI input

again not high gain, but fender used a lot of 500-1000pf coupling caps right before the phase inverter which follows this suggestion

Grid stoppers act as part of a low pas filter, whereas coupling caps act as part of a high pass filter.

[Reeltarded]

The 68k grid stopper on the higher gain input of a Marshall 2204 seems a somewhat wrong headed, as it probably adds more noise than necessary, eg compared to a 33k grid stopper.

It probably does. The first stage being the most crucial because whatever you gained you have to live with and whatever you snubbed you will never get back. Miller was a witch!

I see a lot of bat eared guys using 12-19k and there are a lot of TW amps with 22k on the first grid. I live about three miles from a military flight path. I have to cut <40kHz. I do have a pedal that modulates Japanese shipping channels. Sounds like a Godzilla movie intro twice a year for about a week.

[Roe]

The 68k grid stopper on the higher gain input of a Marshall 2204 seems a somewhat wrong headed, as it probably adds more noise than necessary, eg compared to a 33k grid stopper.

It probably does. The first stage being the most crucial because whatever you gained you have to live with and whatever you snubbed you will never get back. Miller was a witch!

I see a lot of bat eared guys using 12-19k and there are a lot of TW amps with 22k on the first grid. I live about three miles from a military flight path. I have to cut <40kHz. I do have a pedal that modulates Japanese shipping channels. Sounds like a Godzilla movie intro twice a year for about a week.

10k is sufficient with a 100pf snubber like the 2203/4s have. but grid stoppers later in the circuit is a good idea, esp if you slam the phase inverter and get blocking distortion

[pdf64]

Merlin advises how to use a lower value grid stopper whilst retaining the filtering action of a higher value grid stopper http://www.valvewizard.co.uk/gridstopper.html

It’s important to filter RFI at the input, as otherwise radio transmissions may becoming demodulated by the input stage’s g-k diode. As once it’s been demodulated, you’re stuck with it in the signal path.

Fender commonly used two jack sockets each connected to 68k grid stoppers which appeard in parallel when using only the 'hi' input, making 34k. Close enough.

However, the input grid stopper adds the most amount of Johnson noise (hiss) of any resistor in the amp. (A 68k stopper generates at least four times more hiss than a typical 12AX7!) Can we use a smaller resistor to reduce noise, but still keep the bandwidth the same? Yes, it's easy. We simply add a little extra capacitance to make up for the lower resistance. I recommend a 10k resistor, which will make the amp much less hissy, together with an extra 100pF to 470pF capacitor from grid to ground.

[Reeltarded]

10k is sufficient with a 100pf snubber like the 2203/4s have. but grid stoppers later in the circuit is a good idea, esp if you slam the phase inverter and get blocking distortion

I stop every stage but I let the PI be free. I have never needed even 10k at the PI.

[Reeltarded]

This is useful! I forgot that I run this. Found accidently. 100p

Merlin advises how to use a lower value grid stopper whilst retaining the filtering action of a higher value grid stopper http://www.valvewizard.co.uk/gridstopper.html

It’s important to filter RFI at the input, as otherwise radio transmissions may becoming demodulated by the input stage’s g-k diode. As once it’s been demodulated, you’re stuck with it in the signal path.

Fender commonly used two jack sockets each connected to 68k grid stoppers which appeard in parallel when using only the 'hi' input, making 34k. Close enough.

However, the input grid stopper adds the most amount of Johnson noise (hiss) of any resistor in the amp. (A 68k stopper generates at least four times more hiss than a typical 12AX7!) Can we use a smaller resistor to reduce noise, but still keep the bandwidth the same? Yes, it's easy. We simply add a little extra capacitance to make up for the lower resistance. I recommend a 10k resistor, which will make the amp much less hissy, together with an extra 100pF to 470pF capacitor from grid to ground.

[thetragichero]

Grid stoppers act as part of a low pas filter, whereas coupling caps act as part of a high pass filter.

yeah i confused some things here and muddied the waters. this is what i get for trying to multitask

[thetragichero]

I see a lot of bat eared guys using 12-19k and there are a lot of TW amps with 22k on the first grid. I live about three miles from a military flight path. I have to cut <40kHz.

yeah i have a local npr affiliate in my small town. had to add a grid stopper to the low gain input of a master volume super lead because i was getting npr super clear to the speaker

[Reeltarded]

Still, very smooth!

"You're listening to NPR..."....