Grid stopper resistor question

[Mostro]

Hi people, I have a question that maybe some of you can help me answer:
I built a JCM800 preamp (actually there´s a post here about that), and I was getting hum (rectifier spike hum specifically). Eventually I solved the problem but when I was doing research about how to solve it, I read somewhere (can´t remember where on the net) that reducing the 1st valve grid stopper resistor value would lower the hum. The schem has a 68K, I tried a 33K and I recall that the hum was lowered, not a lot, a bit. Unfortunately I didn´t make recordings of before/after... this was not the only step to solve the hum issue, it was eventually solved by modifying the grounding. The 33K was the R value I settled on.
Is this idea of the grid stopper R value affecting hum correct? I know about the lopass filter formed by this R and the valve interelectrode capacitance and how the R value changes the cutoff point, but I can´t find any reference about this other concept... nor can I explain how this could work, if true (I´m really short on theory knowledge).
What do you think? Possible? Fiction? Is my memory betraying me? Plain ignorance? Should I take up gardening ?

Any help appreciated

Mostro

[morcey2]

Hi people, I have a question that maybe some of you can help me answer:
I built a JCM800 preamp (actually there´s a post here about that), and I was getting hum (rectifier spike hum specifically). Eventually I solved the problem but when I was doing research about how to solve it, I read somewhere (can´t remember where on the net) that reducing the 1st valve grid stopper resistor value would lower the hum. The schem has a 68K, I tried a 33K and I recall that the hum was lowered, not a lot, a bit. Unfortunately I didn´t make recordings of before/after... this was not the only step to solve the hum issue, it was eventually solved by modifying the grounding. The 33K was the R value I settled on.
Is this idea of the grid stopper R value affecting hum correct? I know about the lopass filter formed by this R and the valve interelectrode capacitance and how the R value changes the cutoff point, but I can´t find any reference about this other concept... nor can I explain how this could work, if true (I´m really short on theory knowledge).
What do you think? Possible? Fiction? Is my memory betraying me? Plain ignorance? Should I take up gardening ?

Any help appreciated

Mostro

Here's a good source:
http://www.aikenamps.com/InputRes.htm

Most of my recent builds have a 10k grid stopper on the first stage at the largest. My Hi-Octane went from a humming nightmare to a relatively quiet (for a high gain amp) amp when I dropped the grid stopper from 68k to 10k.

Grid stoppers are used for a couple of reasons. 1.) To kill any RF interference. 2.) To kill high-frequency parasitic oscillation. 3.) As part of the voicing of the amp.

For 1 & 2, a 10k is usually large enough on the first stage. Other ways to kill RF are wrapping the input grid lead through a ferrite bead a couple of times or running a very small (10-50pf) cap from the input jack tip lug to ground.

I won't worry about #3 right now.

The problem with grid stoppers is that they're prone to pick up noise, and the larger they are, the more noise they pick up as you've seen. I would say drop to 10k and see if the amp is stable. If it is, you should have significantly less hum than you do now.

Matt.

[Jana]

That first stage is very susceptible to hum. Before I would try to reduce hum by change that resistor, which is a bandaid type fix, I would try other means. First, where are the heater wires? do you have them twisted and as far away from other signal wires as possible? Second, the wires going to the grids, take a chop stick and play with them, move them around, see if you can find the quite spot where they want to be. Are the grid wires as short as possible? have you tried using shielded wires going to the grid(s)? Ground the shield at one end only, the input end. Also, how you ground things is very important too, do you have any ground loops? Where did you ground things? I ground the cathodes for the first stage, the filter cap for the first stages, the gain pot and the input jack all in the same place - a spot on the buss wire on the back of the pots close to the gain pot.

The bottom line is, do everything else you can think of to reduce hum before trying to fix it with a grid stopper resistor. It is possible, (I do it it) to have no hum (dead quiet) without having to resort to compromising the value of the grid resistor or using DC heaters or any of the other so called tricks.

Your answer, I believe, lies in the layout. Heater wires, grid wires and grounding - that is the key (in my humble opinion).

[dehughes]

Your answer, I believe, lies in the layout. Heater wires, grid wires and grounding - that is the key (in my humble opinion).

Agreed. I've found that to be the true 90% of the time.

[Andy Le Blanc]

grid stops need to be located properly or they will introduce noise which will
become more apparent as the number of gain stages goes up.... choice of
layout, grounding, and construction method has the most effect on noise....
if you choose a fender style approach it will be prone to the typical issues
caused by that layout choice and you may have to go lengths to de-bug it...
grid stops are supposed to be as close to the grid as physically possible....
right on the grid terminal of the tube socket.... shrink wrap them to the end
of a shielded cable and use the wire lead of the resistor to solder to the
socket pin.....ground the shield at the other end ....at the input of the unit.....
its not a panacea but it will shield the grid line and put the grid stop in the
right place so you can focus on the rest of the amps issues....

[tjauernig]

I agree, ,place the grid stopper right at the socket, and connect the shield at the other end. For me, this gets rid of the hum 99.99% of the time. Also, if possible, any grid wire should be shorter than the plate wire of that same stage, this also reduces noise. Also, in your preamp, you may consider shielded wire for all the grids. (but with a good layout, this may not be neccessary)

[Mostro]

Thanks people for the answers.

The problem with grid stoppers is that they're prone to pick up noise, and the larger they are, the more noise they pick up as you've seen. I would say drop to 10k and see if the amp is stable. If it is, you should have significantly less hum than you do now.

I guess there´s the answer to my question(s). Actually I solved the hum problems as I said by solving grounding issues, but I wanted to confirm/discard my theory of the grid stopper possibly contributing noise to a build, besides its role as RF and oscillation... ehmm... stopper...!

Thanks again