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Josh @
9/9/2001 5:24 AM
Minimum value for grid stopper?
Hi
I have a preamp in my amp comprised of one 12AX7. After the volume pot, in between the stages, I have a 470k resistor with another 470k tied to the ground. this sounds
good, but I am fond of the way the 12AX7 sounds with no grid stoppers after the Volume. Just before it starts to oscillate, it develops a very squished, Neil Young like tone. My
question is, What is the minimum value of grid stopper I would need to prevent oscillation? I want to keep the massively compressed squished sound which occurs just before
the squealing does.
I have a 68k resistor. would this work?
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Ray Ivers @
9/9/2001 12:28 PM
Re: Minimum value for grid stopper?
Josh,
I personally wouldn't go below 10K, although I've never had an oscillation problem attributable to a lack of grid stopper in a preamp gain stage. Is this a high-gain preamp
design?
The 'squished' tone you refer to is probably the effect of the driven tube's drawing (a small amount of) grid current, which results in a big drop in input impedance that loads
down the driving tube.
Ray Ivers
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stephen conner @
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9/9/2001 12:46 PM
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Re: Minimum value for grid stopper?
An audible squeal isn't likely to be caused by lack of a grid stopper, these are meant to cure RF oscillations. Squealing is more usually a layout/shielding issue, and by giving
attention to this you should get it running fine with no grid stoppers and no squeal. Unless the dirty sound you like is actually _caused_ by the onset of oscillation. Sounds silly
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but I had a homebrew like that once.
Steve C.
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Josh @
9/9/2001 5:00 PM
Re: Minimum value for grid stopper?
No, this isn't any sort of hi-gain preamp. Just two stages of a 12AX7 with a pot between them.
This "squeal" only happens when the volume is MAXED with no grid stopper whatsoever. Its not really that loud, i'd just prefer my amp not to do that. What does an RF
oscillation sound like? I'm pretty sure this is directly a result of no grid stopper.
I guess this is just a matter of "how low can you go" whithout having any problems.
Josh
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Ray Ivers @
9/9/2001 7:39 PM
Re: Minimum value for grid stopper?
Josh,
Maybe if I read your original post a little more closely, I would have seen your mention of a single 12AX7 right up front.
Sorry about that.
Your present circuit is actually more of a voltage-divider than a pure grid stopper, although the series 470K certainly performs that function (as Steve said, the grid stopper
function is more of an RF/stability/isolation thing). Right now, your voltage gain is cut in half right before the input to the second stage, and I think it's the removal of this gain
reduction that's responsible for the feedback. If so, you may need quite a large value of series resistance to achieve stability, your problem may come back when you change
tubes, and you may have bursts of ultrasonic oscillation from time to time - all very undesirable IMO.
As you can imagine, you can't really hear ultrasonic oscillation directly - but you can observe its side effects; usually a sudden, total lack of hiss and/or hum, and possibly your
output tube plates glowing red all of a sudden. Sometimes as the volume is increased, the amp will give a small 'chirp' as the amp goes into oscillation. It's kind of a hard thing
to describe, but it's easy to hear if you listen closely.
I would recommend going over your lead dress carefully, using shielded wire if you're not already, and checking the ground point of your first gain stage's HT filter/bypass cap, if
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any. There are so many things that can cause squealing, but it's usually one of just a few things. This isn't microphonic feedback, right?
Ray
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Josh @
9/9/2001 9:46 PM
Re: Minimum value for grid stopper?
Wow, Ray, thaks for the input. The wiring on this thing is impecable, I don't think this whistling is being caused by it. it also has a very good star ground scheme on it. Its also
not microphonic feedback, I've used different tubes with the same results.
I am not sure if the two sections of the 12AX7 can have no resistance between them and still be perfectly fine. Have you ever been able to?
What effect would having no series resistance between the stages, but a resistor going to ground after the volume pot? would this help to stabilize the circuit or have no effect?
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Ray Ivers @
9/10/2001 4:52 AM
Re: Minimum value for grid stopper?
Josh,
Yes, I've even had some super-high-gain preamps (four cascaded gain stages, no voltage dividers) that could run flat-out without squealing. That's not to say that they sounded
especially wonderful; aside from the obvious noise problem, I always added at least a 15K series resistance to the second gain stage's grid to prevent 'squash-out' with the gain
dimed. An even better solution IMO is a double pot for gain, with an additional pot section between the 2nd and 3rd gain stages for better control, and sometimes additional
series resistances were required to dial in the tone, as well.
The resistor to ground would basically lower the impedance of the grid circuit, which is usually a good thing if the driving stage can handle it when the gain control's maxed out. I
used to use 220K resistors across 500K gain pots from time to time, which would clean up muddiness, add stability, and drop the noise a bit as well - but this was only with higain distortion circuits which were not expected to clean up real good with the gain control backed off.
You mentioned a star grounding scheme. Bear in mind that each gain stage's cathode resistor ground, grid resistor ground, and filter/bypass capacitor ground should be
connected together and then grounded. Since you are having a problem, I wouldn't assume the grounding is perfect because it's a star ground scheme; it's extremely difficult to
achieve zero ohms between all points of a star ground, which is the only situation that will guarantee satisfactory performance under all conditions.
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Ray
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Chris Harden @
9/9/2001 10:24 PM
Re: Minimum value for grid stopper?
Just to wander slightly off sideways. I've had a few oscilation problems probably due to lead dress which I think I've solved. However, when I turn the amp off, it produces some
wonderful tones and whistles as the filter caps discharge. Where these oscilations are being produced I haven't a clue. It's not a problem, just strange.
If anyone has ever experienced this and found out what causes it I'd be interested to know.
Chris
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R.G.
9/9/2001 9:38 PM
Re: Minimum value for grid stopper?
There's not a good single answer to that. The squealing is a consequence of the individual tube, the layout, the power supply, the interlead capacitance, stuff like that. And a grid
stopper is not the only way to kill the oscillation. You could sub in resistors until you find the value that you like.
There is a quite audible difference in the sound out of a triode when it's driven from a low impedance (hence, low or no grid stopper) signal source, and from a high impedance
(or high grid stopper) source. However, your volume pot is probably preventing this effect.
R.G.
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Bruce /Mission Amps @
Date:
9/10/2001 12:33 AM
Subject:
Re: Minimum value for grid stopper?
I've thought about this too and sometimes, depending on the power tube choice, NFB or no NFB, overall tone stack tuning... etc., I like more or less series resistance on the grid
of the first stage, not considering the actual total grid load to ground.
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I find that the low power, easily overdriven brightly tuned EL84 amps seem to repond nicely to quite a bit of grid stopper resistance, for example.
Anyone else messed with this?
Bruce
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Mark Lavelle @
9/10/2001 1:19 AM
Re: Minimum value for grid stopper?
Josh,
I recently got a lot of good advice on squeal, etc. from Mike (MBSetzer) over in the "Noise" area - you might want to check it out: http://firebottle.com/fireforum/fireBB.cgi?
forum=ganoise&thread=136638-000000.msg
Good luck,
Mark
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