Microphonic pickup, guitar volume and amp interaction

[EFK]

Have looked around a bit trying to find an answer to this but can't seem to find much. When using an amp that makes a lot of gain, for example, and it's "dimed," one frequently can get a pretty annoying squeal if one is using a guitar with a microphonic or unpotted pickup and the guitar volume on 10. Usually, rolling back the guitar volume simply to around 9 or so banishes it immediately, unless you happen to be right on top of the amp. The actual perceived volume is not really affected much, but the squeal is gone. So what I'm wondering is, what kind of electrical interaction is occurring between the amp and guitar when that guitar volume is simply moved slightly which causes the squeal, or feedback, to go away? Might seem like a stupid question but I happen to be using a VERY microphonic SD Antiquity bridge pickup right now which is causing a whole bunch of amps to squeal which otherwise do not with all of the other guitars I've plugged into them (including vintage P90s, and fairly hot ones at that i.e. mid to upper 8K range, which I would think would get some squealing going... but do not...). Swapping input tubes does not seem to make any difference. Just a dull day wonderment going on here.

[pdf64]

There was a very recent thread which identified that rolling back the volume effectively isolated the pickup from cable capacitence, thereby affecting the pickups resonant characteristics / frequency. Moved it up and reduced the height of the resonant peak I think.
It electronic terms, it would be reducing the 'Q' of the guitar 'system', making it less suseptible to positive feedback etc.

On a similar theme, I've noticed that my P100 pickups are very microphonic unless the covers are removed, much more so than P90 or Kinman P90Hx.
From memory, the real P90s sound the most resonant / peaky. I've tried various covers, as the guitar came with chromed plastic covers, but changed to regular covers didn't help (the old P90 covers wouldn't fit though).
Pete

[soundguruman]

What kind of interaction? It's positive feedback, and the volume is causing a phase shift, which kills it.
The actual perceived volume is not changed much, but the sustain is GONE.

Putting a suppression cap on the first preamp tube will make all the difference. No more squeal, if it's correct.
The foam tube damper (from HR deluxe) will also help a lot.
But you see, if we take the feedback away, we have also killed the sustain of the guitar / amp.
So, learning how to control the guitar, + tweaking the amp, gets the situation manageable.
And that's why we take the covers off, and that's why we dip PUs in wax...etc...

[EFK]

I haven;t tried removing the pickup covers on this particular guitar because honestly, as an aesthetic thing, I hate the look of uncovered buckers.

Soundguruman, aka "current board lightning rod" ( I'm making a joke here!), specifically what kind of suppression cap do you mean? This particular circuit which I've lifted from a couple of late 1950s Supros, puts a 500pf cap either across the plate resistor or from plate to ground (I've seen it both ways). I did read in another thread that Chuck H was utilizing a 4.7 pf cap from plate to grid to mimic the use of a long cable, I've never run a plate to grid cap so I don't know if that would make any audible difference in consideration of the 500pf cap already hanging off the plate. ???

[EFK]

BTW S-man, your first two sentences are very interesting and I had not considered it in that way. However, it is not a pleasant kind of sustaining or harmonic feedback, it is an out and out microphone-type squeal, which nobody appreciates! Might try wrapping some different things around the preamp tube as well...

[soundguruman]

I haven;t tried removing the pickup covers on this particular guitar because honestly, as an aesthetic thing, I hate the look of uncovered buckers.

Soundguruman, aka "current board lightning rod" ( I'm making a joke here!), specifically what kind of suppression cap do you mean? This particular circuit which I've lifted from a couple of late 1950s Supros, puts a 500pf cap either across the plate resistor or from plate to ground (I've seen it both ways). I did read in another thread that Chuck H was utilizing a 4.7 pf cap from plate to grid to mimic the use of a long cable, I've never run a plate to grid cap so I don't know if that would make any audible difference in consideration of the 500pf cap already hanging off the plate. ???

well first you pick a good high gain preamp tube,
turn up the volume and tap on it
give it a whack with your fingernail, several times.
see if the tube sounds loose or microphonic...listen to the sound the tube makes when you whack it. go head give it a good tap tap.
ya gotta pick a good one, It's like where's Waldo.
Find the one with the least ringing, or mechanical vibration. It should sound like a dull thud when you tap on it.

Then after finding a nice solid tube...
(remove waldo from the socket)
install a 7 pf 1000 V silver mica cap between the grid and the plate (on the socket) of the first preamp stage.
or as I did it, two 12 pf 500V caps in series.
you can make the value smaller and loose less highs, the bigger the value is, the more highs are lost.
7 pf works out pretty well on Marshall and Fender high gain amps.

The "stabilizer" which is really a damper, adds effective mass to the envelope of the first preamp tube.
It's a high temperature, high density foam collar that will discourage the glass from vibrating.
1X Hot Rod Deluxe Foam Tube Stabilizer part number 0059718000

Then if you have done all the stuff, the cover, the preamp tube, the cap, the foam, etc...
you should be getting somewhere.

OK the pickup could really use replacing, with a humbucker that is made to go into the exact spot.
without modding the guitar.
This will produce a pretty noticeable improvement.
and seriously, that's what you should do.

[Chuck H]

I'd like to straighten out a couple of misconceptions.

In the case of squealing pickups the feedback is between the speaker and the guitar pickup. Not the amp circuit and the guitar pickup. Tube microphonics are a different and unrelated issue. A microphonic guitar pickup squeals just as badly under the same gain conditions with any amp. Tube or solid state.

The volume control doesn't squelch feedback by shifting the phase. It does it by reducing sensitivity and top end. If an amp is clipping, any reduction in the guitars volume control setting that doesn't un-clip the output signal does not change the volume of the amp. But it does reduce the sensitivity. It's simply a gain reduction. That's what the volume control is for. With a clean output the gain reduction is analogous with a volume reduction. That's why it's called a volume control. And that, incidentally, is why some modern amps differentiate between volume and gain for certain control designations. But guitar "gain" controls are still called "volume" controls. The small added series resistance creates a voltage divider. So the series resistance acts partly as a low pass filter. And yes, changing the voltage division also changes the resonant peak of the pickups normal function. Which also has an affect.

Any loss in sustain is strictly a matter of reduced sensitivity. This could include reducing gain at specific resonant points that would normally produce acoustic feedback. It should also be noted that in some instances a small reduction in the guitars volume control has been known to align the resonant relationship between the guitar and cabinet in such a way as to ENHANCE acoustic feedback. For any given guitar, amp, room, speaker, pickup, etc. it's a crapshoot. But certainly shouldn't be considered a sustain killer. It's a change of gain and resonant relationships for better or worse.

7pf is a huge value for a plate to grid capacitance. Especially on high gain amps. The capacitance is part of a multipying effect relative to plate swing. The more gain and dynamic peak, the more capacitance. 7pf squelches A LOT of upper harmonics. This is rather contrary to the desired effects of high gain. I used 6pf for awhile, But that was too high. I've been using 4.7pf as of late and it's just OK. AND...

This isn't certain to stop the squeal anyhow. It depends entirely on what frequency the pickup is suealing at. I've known plenty of pickups that were quite low in their microphonic resonance. Most, IMHE, are microphonically resonant at a frequency below what would be squelched by any reasonable value plate to grid cap.

We do need to pick good tubes for high gainers. The outline above is the method used by most. But this IS an issue unrelated to microphonic pickups.

The only ways to stop a microphonic pickup from making noise are to:

Pot the pickup to stop the moving parts from moving. (this gets my vote only if you love the tone of your current pickup)

Reduce sensitivity by reducing gain. This can be done at the guitar OR amp. Reducing gain includes reducing gain @ frequency targeted below the pickups microphonic resonance. Which could include turning down the treble control (or in one strange case I dealt with, turning down the bass control). This is obviously counter intuitive to the high gain objective.

Replace the offensive pickup that has loose parts with a good pickup that doesn't have loose parts. This is the most popular. Most modern pickups are well potted and virtually non microphonic.

Peace.

[soundguruman]

I'd like to straighten out a couple of misconceptions.

In the case of squealing pickups the feedback is between the speaker and the guitar pickup. Not the amp circuit and the guitar pickup. Tube microphonics are a different and unrelated issue. A microphonic guitar pickup squeals just as badly under the same gain conditions with any amp. Tube or solid state.

The volume control doesn't squelch feedback by shifting the phase. It does it by reducing sensitivity and top end. If an amp is clipping, any reduction in the guitars volume control setting that doesn't un-clip the output signal does not change the volume of the amp. But it does reduce the sensitivity. It's simply a gain reduction. That's what the volume control is for. With a clean output the gain reduction is analogous with a volume reduction. That's why it's called a volume control. And that, incidentally, is why some modern amps differentiate between volume and gain for certain control designations. But guitar "gain" controls are still called "volume" controls. The small added series resistance creates a voltage divider. So the series resistance acts partly as a low pass filter. And yes, changing the voltage division also changes the resonant peak of the pickups normal function. Which also has an affect.

Any loss in sustain is strictly a matter of reduced sensitivity. This could include reducing gain at specific resonant points that would normally produce acoustic feedback. It should also be noted that in some instances a small reduction in the guitars volume control has been known to align the resonant relationship between the guitar and cabinet in such a way as to ENHANCE acoustic feedback. For any given guitar, amp, room, speaker, pickup, etc. it's a crapshoot. But certainly shouldn't be considered a sustain killer. It's a change of gain and resonant relationships for better or worse.

7pf is a huge value for a plate to grid capacitance. Especially on high gain amps. The capacitance is part of a multipying effect relative to plate swing. The more gain and dynamic peak, the more capacitance. 7pf squelches A LOT of upper harmonics. This is rather contrary to the desired effects of high gain. I used 6pf for awhile, But that was too high. I've been using 4.7pf as of late and it's just OK. AND...

This isn't certain to stop the squeal anyhow. It depends entirely on what frequency the pickup is suealing at. I've known plenty of pickups that were quite low in their microphonic resonance. Most, IMHE, are microphonically resonant at a frequency below what would be squelched by any reasonable value plate to grid cap.

We do need to pick good tubes for high gainers. The outline above is the method used by most. But this IS an issue unrelated to microphonic pickups.

The only ways to stop a microphonic pickup from making noise are to:

Pot the pickup to stop the moving parts from moving. (this gets my vote only if you love the tone of your current pickup)

Reduce sensitivity by reducing gain. This can be done at the guitar OR amp. Reducing gain includes reducing gain @ frequency targeted below the pickups microphonic resonance. Which could include turning down the treble control (or in one strange case I dealt with, turning down the bass control). This is obviously counter intuitive to the high gain objective.

Replace the offensive pickup that has loose parts with a good pickup that doesn't have loose parts. This is the most popular. Most modern pickups are well potted and virtually non microphonic.

Peace.

Although I have maximum respect for our associate Chucky,
I have to disagree.
The pickup, the tube, the circuit, are all interrelated; are all principle elements in producing squealing feedback, which is often blamed on the guitar or the amp alone...
and so as a result of decades in experimenting, testing, measuring and tweaking, if you want to control the problem, it's a multifaceted approach.
Other than, installing an audio frequency notch filter, to eliminate the resonant frequency all together. (which would, after all solve the whole thing)

[big_teee]

Usually it's a guitar pickup issue.
Pickups with covers, pickups with Tele base plates, are worse.
Also in the mix can be a microphonic tube, and the amount of gain used.
Pickup Potting is always the place I start.
When I make pickups, the last thing I do is temp the pickup to the guitar cord.
I dangle it in front of the amp and compare it to known quiet pickups.
You can make the quietest pickup squeal in this test, up close to the amp with lots of volume and gain.
That puts only the Pickup and the front end of the amp in the equation.
I did weed out a microphonic tube, by this method.
Keep changing V1 until you get a quiet one.
The Hardest Pickup assembly to get dead quiet is the Tele bridge Pickup with steel base plate, mounted in a steel bridge plate.
Everything has to be well potted and mounted solidly to make it quiet.
T

[Chuck H]

I agree to disagree. IME a microphonic tube that's problematic will ring on it's own with high gain. The possibility of the microphonic resonance of a given pickup being sympathetic to the microphonic resonance of a given preamp tube seems remote to me. But I concede that it certainly could have happened to you. And this would merrit advising caution on the matter.

[Steve Conner]

I agree, I think if you find a tube that stops your pickups squealing, it's simply a low gain tube.

Another "gotcha": If a pickup is placed too near the speaker of a high-gain amp, it can pick up the magnetic field from the voice coil and shriek horribly, no matter how well potted it is. This is electromagnetic feedback, not acoustic. I guess single coils would suffer worse than humbuckers from it.

[soundguruman]

I agree, I think if you find a tube that stops your pickups squealing, it's simply a low gain tube.

Another "gotcha": If a pickup is placed too near the speaker of a high-gain amp, it can pick up the magnetic field from the voice coil and shriek horribly, no matter how well potted it is. This is electromagnetic feedback, not acoustic. I guess single coils would suffer worse than humbuckers from it.

Actually, I think there are high gain tubes with low microphonics, and tighter internal construction.
But basically (and statistically) you are right, a low microphonic tube generally does have lower gain, and I wrote that myself (previously).

There actually are exceptions. High gain preamp tubes, with low noise and microphonics.
And these are difficult to select, out of a box containing 100 tubes, you typically find 2-3 good ones. You "might" find more.
This is why brands like Bugle Boy, Groove Tubes, NOS Mullard, became so popular. And there are bad ones mixed in w/ the good ones too.
Although those 3 mentioned are much better, they are not all perfect either.

(Let's be clear, there is no such thing as a preamp tube that is absolutely free of microphonics, even though some people want you to believe it)
It's hard to buy one.

And that is why the foam tube damper, and that is why the suppression capacitor (works quite well actually), because it helps to make a less than perfect tube
function in the circuit.
And those methods (cap and damper) are not my ideas, those are the ideas that the factory installs.
A. The circuit tracks are placed closely together (to increase interelectrode capacitance)
B. the foam damper (Fender HRD)
C. The shielded wire (plate connects to outer shield, grid connects to inner conductor) in Marshall designs.
D. the 7.5 pf cap installed between plate and grid on Fender and Marshall production Amps.

The humbucking pickup, with the two out of phase coils, has a much better signal to noise ratio.
You will amplify much less noise with a high gain amplifier. You will have greater overdrive because the PU has more useable signal to start with.
The humbucking pickup produces less high frequency, but not too little. You can raise the volume higher w/ less feedback.
Yes, less ultrahigh frequency (compared to single coil), but still a very acceptable tone and performance, that accomplishes the ultimate goal.

There has always been the need for a compromise to accomplish the end result.
One more compromise is to notch the resonant frequency out of the audio path entirely. (there is such thing as a parametric EQ)
I mean, that's going to stop the squealing feedback too. It just depends on where you are willing to install that compromise.
In a mechanical fashion, or a electrical / electronic fashion, or a combo of all available.
But, you can't really stand there and whine:
"I don't want to make any changes to the guitar or tubes, or circuit, but I want the problem to go away."

Because you will go nowhere.

[madkatb]

Just to toss another cat among the pigeons...
I've heard that short-plate 12ax7's are better in V1 as their structure makes them less likely to be as microphonic as long-plate versions. Any thoughts?

[big_teee]

I agree, I think if you find a tube that stops your pickups squealing, it's simply a low gain tube.

Another "gotcha": If a pickup is placed too near the speaker of a high-gain amp, it can pick up the magnetic field from the voice coil and shriek horribly, no matter how well potted it is. This is electromagnetic feedback, not acoustic. I guess single coils would suffer worse than humbuckers from it.

If you dangle the pickup, I usually get the most magnetic noise near the PT Iron.
Sometimes you can get squeal with the pickup dangled near V1.
It is just a basic test, and after you do it a while with different pickups, you get to know what is normal , and what is excessive.
You can tell real quick if the base plate is properly potted on the bottom of a tele bridge P/U though.

[Alan0354]

If your amp is high gain and you stand close to the amp with your guitar, the guitar directly talk to the tubes or OT. I tried using the THD Hotplate to lower the speaker level to room level and still feedback. It has nothing to do with the speaker. I triple and quadruple verified already. The fix is very easy, a few hundred pF cap from the plate of one of the preamp tube to ground will fix that. You just need to tweak the value so it stop the squeal but not affecting the sound. The higher the gain of the amp, the more susceptible it is for this. Don't believe me, if you take a Mesa and crank the high gain up, then turn down the volume on your guitar to get clean sound. You'll notice it absolutely sounds like $hit. It's like you turn down the tone pot all the way, all muffled. Why? because they use so many caps in the signal chain to drown out the feedback.

For the record, I think Mesa make the worst amp amount all the big names. They put so much crap in the signal chain there is no more tone left.