Shock mounting tubes?

[bob p]

i thought that was kinda funny -- mykey asking if Steve is ready to move ahead and tackle a high gain amp build. what a riot!

Enzo, i agree with your engineer friend's point about tube vibration -- there's a difference between bounces and jostles, and vibration, and what methods are useful in tackling them. that's the point that i was alluding to in my earlier post about how the military addressed these problems back in the 20th century.

about the antenna thing -- i always wondered what the purpose was of putting those little steel balls on the tops of antennae. i always thought that they made the antenna oscillate (acting like an upside-down pendulum) when you hit a bump. you know, kind of a simple harmonic motion kind of thing. but then maybe they designed antennae to have those steel balls on the end of them just so that you could anchor a sun ball onto them. i guess i'm showing my age.

there truly is not much new happening in the world of HiFi (discounting things like surround sound, if you even call that HiFi). even though there's really nothing new in HiFi, the pimps are out there trying to sell you everything that you really need. i'm surprised that some HiFi guru hasn't come along to recommend putting exotic O-rings around your preamp tubes.

[mykey]

[QUOTE=bob p;25510]i thought that was kinda funny -- mykey asking if Steve is ready to move ahead and tackle a high gain amp build. what a riot!

Actually, I didn't ask Steve any such question. What I wrote was:

"But don't be afraid to add another gain stage to get some grind, tunning the
amp for higher gain is trickier, and it seems like you are ready to tackle
the more advanced design phase. It's not impossible to balance higher gain
and reliability although some designers such as mesa have sacrificed reliability for gain."

But since you twisted and misquoted what I wrote in order to justify making an attack on me, I can gather you are a republican. what a riot!

[R.G.]

Mykey - are you ready to discuss a technical issue instead of just blathering it out and running?

Can you explain your assertion that electrical negative feedback in the first stage will lower microphonics? How does that work? Or do you just believe that it's voodoo?

I personally don't think you have a clue why this should be, or what the actual mechanism of a tube transducing mechanical vibration to sound would be.

[Enzo]

Bob, the point of the little balls on the end of antenna spikes is not as a weight, nor as an anchor. It is done to prevent atmospheric noises. A sharp point attracts static charges, or at least is attractive to them. This can result in charges accumulating, especially as the air flows past as you drive. And that in turn can cause pops, crackles and other unwanted noises in the reception. By putting the ball on the end of the spike, there is no sharp point, and atmospherics are substantially reduced.

The balls are not only on car antennas, look around and you will spot them on rabbit ears and other antennae.

[mykey]

Mykey - are you ready to discuss a technical issue instead of just blathering it out and running?

Can you explain your assertion that electrical negative feedback in the first stage will lower microphonics? How does that work? Or do you just believe that it's voodoo?

I personally don't think you have a clue why this should be, or what the actual mechanism of a tube transducing mechanical vibration to sound would be.

I thought you would never ask.
The frequency passed by the recommended 7 pf cap is rather selective and only interferes with the very highest response of the circuit. In this configuration most people really don't notice. by coupling the in phase signal at the control grid to the out of phase signal at the plate, some attenuation is realized from the small amount of negative feedback which effectively prevents the oscillation (ringing). as mentioned earlier, out of phase audio will cause cancellation, in phase will cause regenerative ringing.
this circuit has been used by Marshall and Fender in the higher gain guitar amps but it's origin can be more accurately traced to the design of phonograph pre-amplifiers dating back to the early 1940's; which is after all where high gain guitar amps originated.
Previously Mesa Boogie had used a 250pf cap from cathode to plate, but this sounds rather crappy (muddy) and in tests was not effective as the above method, the tube was still prone to oscillation as the in phase audio from the speaker acoustically coupled to the shell of the tube (causing positive regeneration). Also Mesa had attempted to select "special non- microphonic tubes" which were essentially low gain 12AX7A triodes. You could go through a whole box of those pups before you found one that wouldn't ring in the circuit.
It had been mentioned that coupling the plate to the control grid through a capacitor was an unsafe practice, but using a high reliability (silver mica) capacitor rated 1000 volts there have been no adverse problems reported by Marshall, Fender or other manufacturer that used this circuit. The DC current seems to have been effectively blocked.
I would also recommend soldering that 7 pf directly to the socket and keeping the leads short so as to prevent inductive AC ingress off the control grid (from the heater wires), which can be amplified in the audio path. I also like to move the heater wires away from the top of the socket pushing them towards the back of the chassis, keeping these far away from the control grids as possible.
So you see it really isn't voodoo after all, just a little negative feedback.

[R.G.]

I thought you would never ask.

I'm guessing that's because you thought you knew this one.

The frequency passed by the recommended 7 pf cap is rather selective and only interferes with the very highest response of the circuit.

Rather selective?

Capacitors have no frequency selection in and of themselves. Capacitors pass all frequencies equally well - until they're combined with a resistor or inductor. It is impossible to say what frequencies a capacitor passes unless you also say what the other parts of the circuit are.

Just to check - what's the rolloff frequency of a 7pF and a 3.3M resistor and how would you calculate it?

(It's ok, go look it up. We'll wait.)

In this configuration most people really don't notice. by coupling the in phase signal at the control grid to the out of phase signal at the plate, some attenuation is realized from the small amount of negative feedback which effectively prevents the oscillation (ringing).

Actually, putting a cap from plate to grid only increases the capacitance from plate to grid that's already part of the tube. What that does is add to the Miller capacitance that's already there and lower the high frequency response of the whole stage.

But this doesn't and can't help with microphonics.The problem is that microphonics is caused by the tube picking up MECHANICAL inputs in addition to the electrical one. The internal parts of the tube actually move around. Since there is a capacitance between all of the internal parts, the mechanical motion causes the spacing and hence capacitance of the internal parts to vary. The varying capcitance causes a varying voltage, that's how it becomes an electrical signal. Any plate-grid capacitance can only cut off response at some frequency, which doesn't help if the resonance is below the cutoff. Negative feedback only damps internal disturbances. The mechanical changes with vibration are effectively another input, so feedback can't change it. It's just like adding another voltage on the grid.

By the way, the frequency of the microphonic sensitivity is not determined by the tube, but by both tube and socket/chassis mechanical resonances. Both the tube and the acoustic path to the tube have to be passing the particular frequency for it to come through.

If you're thinking that there is no mechanical input, why does the microphonic stop oscillating when you touch (and therefore damp) the tube?

as mentioned earlier, out of phase audio will cause cancellation, in phase will cause regenerative ringing.

For an extra two points, who first said that? For an extra ten points, which of the years between 1930 and 1939 was it?

this circuit has been used by Marshall and Fender in the higher gain guitar amps but it's origin can be more accurately traced to the design of phonograph pre-amplifiers dating back to the early 1940's; which is after all where high gain guitar amps originated.Previously Mesa Boogie had used a 250pf cap from cathode to plate, but this sounds rather crappy (muddy) and in tests was not effective as the above method, the tube was still prone to oscillation as the in phase audio from the speaker acoustically coupled to the shell of the tube (causing positive regeneration). Also Mesa had attempted to select "special non- microphonic tubes" which were essentially low gain 12AX7A triodes. You could go through a whole box of those pups before you found one that wouldn't ring in the circuit.

I'm having a hard time translating that. It sounds a whole lot like flinging a lot of stuff onto the wall hoping something will stick.

Sure, Miller caps have been around a long time. Sure, guitar amps came out of hifi amps. I'm sure someone used a 250pF cap and it sounded muddy, but since it doesn't affect mechanical resonances, I'm also sure it didn't cure microphonics, either, and sure enough the tubes were still prone to microphonics. Tossing salt over your left shoulder at the crossroads at midnight is equally effective. As to what Mesa was doing with selected tubes - prove it. Show me the memos and reports? Were you the guy who did this for Mesa?

It had been mentioned

By whom? When? Where?

that coupling the plate to the control grid through a capacitor was an unsafe practice,

Define unsafe - unsafe HOW? Electrical shock? Earthquake? Natural gas explosion? OSHA reports? Riots and looting?

but using a high reliability (silver mica) capacitor rated 1000 volts there have been no adverse problems reported by Marshall, Fender or other manufacturer that used this circuit.

And there have been no elephants in my front yard for over two years, ever since I started using this here weed killer...
And there have been no reports of 50 foot pythons in town here ever since we switched to this new toothpaste, either.

(you know, I think I'm shooting too high again...)

The DC current seems to have been effectively blocked.

Well, duh. Caps do that. They block DC.

I would also recommend soldering that 7 pf directly to the socket and keeping the leads short so as to prevent inductive AC ingress off the control grid (from the heater wires), which can be amplified in the audio path. I also like to move the heater wires away from the top of the socket pushing them towards the back of the chassis, keeping these far away from the control grids as possible.

That's good advice! Keep the heater wiring away from the grid.

They had a saying in the Ozarks - even a blind hog finds an acorn every now and then.

So you see it really isn't voodoo after all, just a little negative feedback.

No, I was pretty sure it wasn't voodoo when I wrote that. I'm likewise still sure it's not negative feedback. You presented no info whatsoever that would explain how it could be, only a handwave about in phase reinforcing and out of phase cancelling.

Here's another extra points question: What are the minimum elements are necessary in a circuit for it to oscillate?

[Enzo]

The small cap from plate to grid is not there to suppress microphonics, it is there to suppress instability. it also suppresses RFI. ANyone who has ever picked up local AM radio stations on their guitar amp will appreciate that RFI is a real consideration.

Phono preamps want to roll off high freqs as part of the RIAA response curve - or at least the cheap phono approximation of it.

Guitar amps started off life as microphone amplifiers, not phono amplifiers, not that ther is a ton of difference after adding in the EQ..

Microphonic tubes are mechanically loose inside, so vibration will change the relative positions of the internal elements, which will affect the instantaneous gain of the stage. Hence the amplification of the mechanical stimulus. This is a completely separate phenomenon from electronic instability. Negative feedback will not prevent the innards of the tube from responding to the mechanical stimuli. And since often as not, the microphonic sounds of the tube are within the freq range of the guitar, we wouldn't find rolling those freqs off as a desirable method of contol even if it were effective. Ringing of an electronic signal is not related to a mechanically sensitive tube "ringing" in the soundfield or in response to chassis vibration. Electronic ringing in a guitar amp is usually refered to as parasitic oscillation - not microphonicity.

[Enzo]

Oh my, simulposts.

[Le Basseur]

What are the minimum elements are necessary in a circuit for it to oscillate?

I need badly some extra points so I'll take my shot:
TWO?

...wich reminds me a joke I heard the other day:
-What's the amp tech's definition for a great love making?
-Low-frequency resonance without phase-shifting!

[Steve Conner]

Mah thread is on fire! Damn Republicans!

To be pedantic, negative feedback will help to cancel ANY interfering input. Hum, noise, and I guess microphonics too. But it also reduces the stage gain, so I don't see how the proposed solution would work without making the amp sound muddy, like Enzo mentioned in his last post.

[Rob Mercure]

some materials rock n glue

Hey Steve,

I just wanted to mention a few materials that give you options for mechanical isolation of circuit elements - silicone "rubber" and hot melt glue. I still remember when GE "Silicone Seal" came onto the market in the 1970s and it was an instant hit with TV repairmen with it's high dielectric strength and the ability to stick to most anything - the solution to HV "cages" that had chronic arcing problems.

Anyhoo, both of these materials can be "cast" to custom form washers and isolation mounts and I've made several standoffs from silicone that has hardened in the applicator "tip" to form "plugs" of staggered diameter. Pretty easy to drill a hole in the middle of such a plug and make a "donut sandwich" or other isolation mount. Or you could take modeling clay and design custom molds and either fill them with silicone or hot melt - or perhaps a composite layered material. The silicone is tough but "springy" - some custom blends for automotive work set more firm and most mechanics could suggest which blend would meet your needs. The hot melt is less "rubbery" but a bit more dense - of course heat will remelt it so you've got to choose the application carefully but as it already comes as a cylindrical solid you've got a hand premade shape. I've used it to make/replace "feet" for chassis and stompboxes.

Lastly there is steatite - "soapstone." The largest commercial quarries in the USA were in Nelson County, VA (about 6 hours east of me) and years ago I picked up slabs of "rejects" with calcite streaks (laboratory sinks were made of the material so you can imagine the size "blanks" the quarry was seeking) and brought a few hundred pound home with me. Soap stone is easily worked and can be used to dampen vibration by providing custom shaped/sized "masses" - you can probably purchase small blanks at a hobby store and use them to "stop" vibration from being transmitted from one place to another. Fer instance, if you were really fanatic you could drill out a soapstone "tube shield" that was so massive that no manner of vibration could set the tube's elements moving - say a couple kilos of rock per tube <grin>.

Rob

[Steve Conner]

Hi Rob,

Tube shields made out of rock sound a little extreme. But I have a piece of heavy gauge brass somewhere that I guess would make a nice sub-chassis. I also have a set of those little flexible mounts, so no need to cast them out of silicone, I hope.

[mykey]

[QUOTE=R.G.;25577]

But this doesn't and can't help with microphonics.

Actually it does. Fender and Marshall have both used it. the attenuation realized by the negative feedback is successful in stopping the ringing oscillations from occurring. anyone who has removed this from the circuit of a Fender or Marshall amp would realize that the capacitor really does work, and it's much easier than selecting a "non microphonic" preamp tube.
although grabbing the tube with the fingers will stop the ringing, attenuating the hot frequency will also stop it.
the same method is used in professional sound systems. equalizers utilizing negative feedback attenuate hot frequencies to prevent the sound system from ringing (oscillating feedback). this is used quite successfully in all professional PA systems. applying the same idea to a guitar amp works just as well, and is highly recommended. (an equalizer will also work).
I would encourage anyone who has found problems with ringing preamp tubes to try this method, it really works very well.

[bob p]

But this doesn't and can't help with microphonics.

Actually it does. ...

Give up -- there's no point in casting more pearls... <sigh>

i thought that was kinda funny -- mykey asking if Steve is ready to move ahead and tackle a high gain amp build. what a riot!

Actually, I didn't ask Steve any such question. What I wrote was:

"But don't be afraid to add another gain stage to get some grind, tunning the
amp for higher gain is trickier, and it seems like you are ready to tackle
the more advanced design phase. It's not impossible to balance higher gain
and reliability although some designers such as mesa have sacrificed reliability for gain."

Mince words if you want to, the meaning is the same. The bottom line is that you seriously underestimate one person's expertise while seriously overestimating another's. I found what you said to be amusing, so I laughed.

But since you twisted and misquoted what I wrote in order to justify making an attack on me, I can gather you are a republican. what a riot!

There's a difference that is not at all insignifiant -- I did not attack you. I just laughed at something that you actually said. Instead of responding to something that I have actually said, you engaged in speculation about my political affialiation and laughed at your own conclusion. You can laugh at your own thoughts if you want to, but whether or not I have a political affiliation is not at all relevant.

I find it amusing when people who are losing ground in a discussion try to change the subject to something totally unrelated, in a desperate attempt to redirect the focus of the conversation away from their inadequacies. As someone has pointed out already in the Safety thread, you are a master of this technique. Whether I am registered as a Democrat, Republican, or Independent isn't germane to the discussion. Please stick to the discussion at hand. If you want to engage others in political discussions, the Guitar Amps > Theory & Design forum is is not an appropriate place to do it.

And if its not too much trouble, please familiarize yourself with the quote feature of the board. It isn't fair to expect others to repeatedly clean up your botched BBCode.

[R.G.]

But this doesn't and can't help with microphonics.

Actually it does. Fender and Marshall have both used it. the attenuation realized by the negative feedback is successful in stopping the ringing oscillations from occurring.

mykey, you're either not comprehending or willfully ignoring what I wrote.

1. A feedback cap does NOTHING except roll off treble response.
2. Microphonic tubes are tubes which produce an electrical signal in response to mechanical vibration.
3. A treble rolloff CAN'T stop microphonics except abusively by deleting all of the audio content at whatever frequency the mechanical resonance happens.

You could for instance use this same "fix" on a microphone which squeals - put a cap across it. That would fix the squeal all right, by making the microphone insensitive to audio.

A treble rolloff might fix internal electrical oscillation, but not microphonics.

THEY'RE DIFFERENT PHENOMENA!

Beyond that, negative feedback does not and cannot stop output from a valid (if obscure) input.

Fender and Marshall may well have used rolloff caps. But they've used lots and lots of other things too. You need to understand that because a big company did something, (a) does not mean it fixes what you're trying to fix and (b) does not mean that you understand why they did that.

I need to go look up the formal logic terms again. There is a specific name for the logical fallacy you're promoting.

although grabbing the tube with the fingers will stop the ringing, attenuating the hot frequency will also stop it.

So will turning off the electricity.

You elected not to participate in my questions to you. If you had, you might have realized that the questions were a short course in the history and theory of gain-phase oscillation.

Sustaining any oscillation needs two things (yep, that's the answer to one of the questions!) - unity or greater gain from input through the amplifier and feedback path back to the input and a phase of integer multiples of 360 degrees so the input is in phase to reinforce. Take away either one and the oscillation stops.

Your suggestion of treble rolloff will only work by cutting audio as well. But if you then up the gain again (this was for a high gain guitar amp, remember) the next frequency which has the right phase will start oscillating. Then you get to cut more treble.

If you instead stop the mechanical feedback by damping the mechanical input, you stop all of the gain-phase feedback paths at the same time, and you are left with only electrical issues to deal with.

Negative feedback reduces gain. You would get the same effect by turning down the gain, but that's not what was being discussed.

I would encourage anyone who has found problems with ringing preamp tubes to try this method, it really works very well.

I'm sure you would. But there are much better ways to deal with it when you understand what you're doing. I would encourage anyone with ringing preamp tubes to first understand what's going on and then fix the problem, not to try something because Fender did.

But meanwhile back at the ranch...

Exactly what is it you do? What experience do you have?