Blocking 8Hz from Output Valves

Stu Blue

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3,167
To all the TGP experts, thanks for your contributions/expert views on the threads I've read. It must try your tempers that so many ignorant idiots feel obliged to be rude to the TGP experts.

My question is about blocking frequencies around 8Hz from the output valves. I got this from several sources including a specialist lecture (at the BBC) on a new (1965) cliff-face frequency blocker which my father dragged me along to. The point as I understood it was that output valves could not cope with that frequency at all and performance was dramatically compromised throughout the rest of the range if 8Hz was allowed through. I also remember this coming up in a guitar magazine in the '70s in an article by a respected amp designer (forgotten who) and that it was blocked in the guitar amp under discusssion in the article. I'm trying to find out some more about this ( I'm no electronics expert)... could be a forgotten piece of amp design lore....

Any comments/knowlege you have about this would be gratefully received. Any time will do... Thanks in advance ....
Stu
 

SatelliteAmps

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6,170
There are a variety of ways to block specific frequencies in a circuit. Making a notch filter is probably the easiest. I can get you a schematic if you want. Any basic filter can be adjusted to a specific frequency or range.

I do not know of anything specific to 8hZ at all in regards to tubes, but considering how extremely low that is, I don't think it makes much difference. 8hZ is way below human hearing, and way below what most musical instruments can produce. Blocking a frequency that isn't going through an amp won't make a difference one way or the other.
 

Stu Blue

Member
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3,167
I do not know of anything specific to 8hZ at all in regards to tubes, but considering how extremely low that is, I don't think it makes much difference. 8hZ is way below human hearing, and way below what most musical instruments can produce. Blocking a frequency that isn't going through an amp won't make a difference one way or the other.
I should point out that at that "60s lecture were people from Quad, Leak and all the guys from the BBC electronic workshop (who built the very first synth... with valves), and none of them questioned the need to prevent output valves from seeing frequencies below 10Hz. It was something about the physical nature of valves, not just how low 8Hz was. Unfortunately I was put in the Arts and Humanites stream at school, so I had/have precious little physics, and can't remember why valves can't cope with 8Hz. It was mentioned, but as something everybody there knew all about....

Anyone?
 

jay42

Member
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7,065
hmm...there have always been audiophiles who are in love with the idea of audio amps that go down to DC, with the assumption that the input will never go sub-audio. (bad assumption -- wind noise into a mic) There are no pesky series capacitors for SS designs and no phase shift from a roll-off circuit. Not real applicable to guitar and bass amps imo. Terrible idea for PA systems.
 

SatelliteAmps

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6,170
If there is no 8hZ going in or through a tube amp, then blocking it won't do anything. In some radio frequency situations, then this might be applicable. In which case it might just have something to do with a physical resonance of the tube, or glass.
 

pgissi

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2,481
8hz approx is the natural resonance of earths ionosphere and sometimes is referred to as Schumann Resonance and maybe was referenced in the lecture as a possible source of emi in tube designs. the ionosphere is energized by lightning and solar particles interacting with earths atmosphere and magnetosphere

reportedly, nikola tesla in some early experiments used earths ionosphere and this resonance to bounce rudimentary radio signals around earths circumference and was said to demonstrate the effect for some observers prediciting the timing of returning transmissions and if this is true, its the first use of the concept of radar. I think this formed the basis of teslas wireless power theory with the intention of using the ionosphere as some sort of region to use for wireless power distribution. Later experiments were said to take it further after the construction of a large tower funded by jpmorgan to conduct experiments.

I saw a pbs documentary some years ago on this topic and this elf (extremely low freq) stood out. Then there is the fringe theory that Tesla, using the ionosphere in a later atmospheric conduction experiment caused the Tunguska Russia event in 1908 where thousands of acres of trees were downed by an aerial blast :NUTS. I tend to doubt that since if that was the case, the wars that followed up to today would have seen this used over and over until there was nothing but rocks left.

One thing that can be said is true is that the ionosphere is resonant at just below 8hz, something like 7.8 or there abouts and since EMI (electromagnetic interference) and its negative effects are to be considered in all electronics design, the only logical conclusion here is that the lecture was entertaining the potential for this sort of interference in signal processing, relevant or not as it may be

this is the only page I could find that presents some information while avoiding the new age bent-
http://www.teslaradio.com/pages/tesla.htm

limitless wireless power, yeah thats what the enemies of civilization need
 

pgissi

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2,481
Can't imagine a guitar amp would pass a 8hz signal, or would it?
possibly via parasitic oscillation, I think its assumed its rarely occurs in the elf range but maybe under the right conditions and certain designs its something to be considered
 

GearHeadFred

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1,643
Wow - Never thought I'd see a reference to the "the Tunguska Russia event" relating to guitar amps. That's pretty cool info... I wonder what Dr. Dumble would have to say about this? (sorry, don't mean to be a jerk - just 'cheeky' after a long day at work!)
 

schmidlin

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5,257
The OP is suffering from idealogical fidelity crapola. Hi-fi nuts use this to sell their wares. It's part of a sickness they feed on. Not my scene. Only Jim Jones could trump this with his cool-aid.

Use your EARS and stop the hype.
 

ohmslaw

Member
Messages
208
The OP is suffering from idealogical fidelity crapola. Hi-fi nuts use this to sell their wares. It's part of a sickness they feed on. Not my scene. Only Jim Jones could trump this with his cool-aid.

Use your EARS and stop the hype.
What he said. I've been playing for 38 years and I've never heard of such an oddball thing. I have been hearing a lot of wierd stuff on this forum since I jointed, to be honest.
 
Messages
165
Don't know about tubes, but to me 8Hz is a DC offset as much as it is a "signal". If you waste all your headroom amplifying DC, you clip the hell out of any audio that's riding on top of that. And at 8Hz, the "kerplunk" that goes with plugging in a gutar or something like that can be massive. So it's just a bad idea in general to have that there unless you really need those frequencies. In the context of a radio or TV station with long complex signal paths and a need to maximize their S/N ratio, even more so. I wouldn't be surprised at all to hear the BBC uses a real steep filter in front of their system at some point. Is it possible that they weren't actually referring to tubes specifically but rather audio stages in general? In '65 "output valves" could be a general reference to a power stage.

Regardless, the presentation sounds like a neat bit of audio history. Wouldn't surprise me if there was a tape recorder running in that room somewhere - I'd love to hear it! What are the other sources?
 

Stu Blue

Member
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3,167
Gentlemen. Woke up this morning with more remembered info on this 7-8Hz thing. It's to do with the ease with which these frequencies saturate the valves...er, and that's it. Sorry for my lack of physics and the absence of a detailed explaination. Please don't dismiss this as audio myth. The expert people in the lecture room designed some of the most famous valve amps in the world... they certainly knew more than the average tech or amp cloner today. I just wondered if that might be the explanation why some amps seem special... since I never see it mentioned today.
 

pgissi

Member
Messages
2,481
Wow - Never thought I'd see a reference to the "the Tunguska Russia event" relating to guitar amps.
Well now you have, ha aha aha aha h

We live within the realm of the electromagnetic spectrum and all electronics is susceptible to emi from natural earth electrical phenomena
and the tesla tunguska link was simply a side reference to the fact that the ionosphere has an approx 8hz resonance (as mentioned in the OP) and since emi is originates from earths atmospheric interaction with solar particles and earths own magnetic field, I suggested that maybe the lecture was a discussion on extreme emi filtering, thats it


The OP is suffering from idealogical fidelity crapola. Hi-fi nuts use this to sell their wares. It's part of a sickness they feed on. Not my scene. Only Jim Jones could trump this with his cool-aid.

Use your EARS and stop the hype.
This is a harsh statement, I dont see any hype here, maybe speculation but not hype. So now the OP has recalled possibly some key words of the discussion "It's to do with the ease with which these frequencies saturate the valves", maybe this is the key and maybe we could learn something and save the snide comments and open our minds.

He mentioned the BBC in the op and since they are a broadcasting company and utilized tube based radio an tv broadcast equipment meaning, tube gear operating at higher bandwidths, I dont think its hype, since at these upper ranges, susceptibility to emi and rfi is exagerrated and design is most critical.

In comparison, a tube guitar amp is a primitive design. Next time you tune in a game from an am broadcast on your amp, think about that, this is unacceptable in broadcast gear operating at higher bandwidths and ultimately broadcast power, it could fry some expensive equipment in a successive chain of failure.


So it's just a bad idea in general to have that there unless you really need those frequencies.
It seems that the assumption here is that the lecture was a discussion on the inclusion of 8hz, I dont understand how anyone here would derive that from the op even if it did not come out and state the obvious but the keys words are-

blocking frequencies around 8Hz

a specialist lecture (at the BBC) on a new (1965) cliff-face frequency blocker

a respected amp designer (forgotten who) and that it was blocked in the guitar amp


Its obvious this lecture was about improving performance through careful design and maybe it is overreaching in regard to guiitar amps but once again, it was a BBC related lecture.

Now I am off to consilt with Edgar Cayce on the future of the human race :rotflmao
 

donnyjaguar

Member
Messages
4,194
In proper tube power amplifier design you set a minimum frequency at which you want to amplify. Is this what you're getting at? If so, there is a definite reason to keep these ELF frequencies out and that is for stability reasons in a feedback amplifier design. This is normally accomplished by keeping any excitation signals out of the input to the power amp to begin with and accomplished with bandwidth limiting upstream. In the power amplifier section itself you generally address this with cathode bypass capacitors that will reduce the gain below a certain frequency. Further to that you select coupling capacitors and bias-feed resistors values, considering the impedance of the bias circuit itself, to act as a high-pass filter going into the output tubes. Usually this is enough. Where it isn't enough is where you have too much gain in the power amplifier itself as the phase tends to reverse, or distort badly enough, going through the output transformer at subaudible frequencies. This creates a more positive feedback scenario aka oscillator. This generally as a result of lack of inductance and magnetic permeability of the core and is very common, especially in musical instrument amplifiers.

Hope this helps!
DJ
 

jawjatek

Member
Messages
714
Hmmm, well, thinking out loud here, its difficult to build a "cliff-face" or brick wall filter (especially in 1965) without a lot of gain loss and phase shift. IMO you don't need to worry about it too much in typical tube guitar amp circuits, because the typical common cathode gain stages have a 1st order low-freq roll-off (HPF) characteristic that is a function of the cathode resistor and the cathode bypass cap. The roll-off freq is equal to 1/2piRC, which for a typical Fender gain stage, R is 1500 Ohms and C = 25uF, which gives a corner frequency of 4.244 Hz (simplifying here). Many amp tweakers find that dropping the cap values here helps tighten up the bass some (it moves the corner freq up). The Fender fat switch modifies this roll-off freq, for instance. If you want it to start rolling off at say, 20 Hz, make it 5.3uF (use a 4.7uF). The other big inherent high-pass filters in the typical tube guitar amp are the PI output coupling caps, and the tranny. IMO there is very little energy in the 8Hz range making it thru a typical healthy amp. No common guitar speaker is going to put out much 8Hz anyway, they physically cannot produce the wavelength, i.e. they will simply produce higher harmonics of 8 Hz.
 

SatelliteAmps

Member
Messages
6,170
Gentlemen. Woke up this morning with more remembered info on this 7-8Hz thing. It's to do with the ease with which these frequencies saturate the valves...er, and that's it. Sorry for my lack of physics and the absence of a detailed explaination. Please don't dismiss this as audio myth. The expert people in the lecture room designed some of the most famous valve amps in the world... they certainly knew more than the average tech or amp cloner today. I just wondered if that might be the explanation why some amps seem special... since I never see it mentioned today.
Not trying to dismiss it as a myth, just trying to show why it might be important for something else, but probably not guitar amplifiers. Doesn't mean there is no reason for it happening or affecting certain types of circuits.

It is not a reason why some amps seem special. There isn't any common guitar amplifier out there that has anything in it to block 8hZ. (Haven't seen a tube hifi with one either, but there might be something out there that has one. That is where a lot of the tube science used to be directed, not at guitar amps.)
 
Messages
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It seems that the assumption here is that the lecture was a discussion on the inclusion of 8hz, I dont understand how anyone here would derive that from the op even if it did not come out and state the obvious but the keys words are-
The "that" in my post was referring to 8Hz... meaning, it's bad to have it there unless you have a reason to want it there. The rest of my post was pretty "anti 8Hz".

Please don't dismiss this as audio myth. The expert people in the lecture room designed some of the most famous valve amps in the world... they certainly knew more than the average tech or amp cloner today. I just wondered if that might be the explanation why some amps seem special... since I never see it mentioned today
Awe, now you're just trying to soften us up. :) Maybe it's just a bunch of horse crap and *that's* why we've never heard of it and a google search brings up nothing. Mythologizing the great designers of the past doesn't do any good especially when you can't explain their concepts properly (or at all as the case may be). It is possible they were completely wrong - it's happened before. They didn't get to be great designers by having to be right all the time either (which seems to be a problem with many audio people I've noticed). Sorry but yeah, apparently nobody here has heard of this and since you can't really tell us any details at all, what else are we gonna do but try and hypothesize about what they were talking about. If there's something special about 8Hz that has evaded common knowledge, I'd still love to hear something more detailed than utter hearsay.
 
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Stu Blue

Member
Messages
3,167
Don't know about tubes, but to me 8Hz is a DC offset as much as it is a "signal". If you waste all your headroom amplifying DC, you clip the hell out of any audio that's riding on top of that. And at 8Hz, the "kerplunk" that goes with plugging in a gutar or something like that can be massive. So it's just a bad idea in general to have that there unless you really need those frequencies.
Billy Joe probably has nailed the substance of it. BTW my father says it was 1961 not 1965, and he probably has the paperwork stashed away somewhere, but he's 92 and quote "I might find it before I die..." He says the cliff face filter was designed for use in the video circuits of TVs and the audio use to filter sub-audio was a side issue/benefit.

From what others have said here, instruement valve amps have lots of high pass filters which would effectively block/miniumize 8Hz from the output valves anyway...

BTW the Tunguska 1908 event was a airborn thermal explosion caused by a meteor... (we had a near miss one just a while back)
 

Stu Blue

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3,167
So, in case anyone's still interested, I asked a couple of well respected guitar amp designers about this 8Hz thing and they said almost the exact same thing.



As a rule, most power tubes don't do too well under 10 Hz, and most output transformers can't begin to pass anything in that range.

Very sharp drop-off filters always create a huge amount of phase "mess" for a few octaves above and below their cut-off frequency.


There are low-frequency limiters on every gain stage, and high-frequency limiters on almost every gain stage. As a result of many, carefully tuned filters... over 60 dB down at 8 Hz by the time it reaches the grid of the power tube.


Sorry if I got anyone worried about this... but now I know why there are no cliff-face type filters in guitar amps...
 
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