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Oscillation in tube amps
Oscillation in tube amps
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Old 26th June 2016, 08:12 PM   #121
poptart is offline poptart  Canada
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Quote:
Originally Posted by Gnobuddy View Post
Assuming that scaling relationship is reasonably correct, the 330 uF caps I use for B+ filtering in my preamp might be expected, very roughly, to have a resonant frequency in the range of 100 kHz. That is a factor of 4000 lower than the 400MHz capabilities of the valve itself.

It does not make any plausible engineering sense to me, to suggest that a capacitor that stops being a capacitor at 100 kHz, provides sufficient high-frequency bypassing for a 400 MHz-capable valve.

-Gnobuddy
If the capacitor is useless in your mind above 100kHz then it's also useless below 100kHz. The impedance rises in BOTH directions from that minimum. Fortunately for us the impedance rise is very gradual and the exact value of the impedance at any particular frequency is largely irrelevant as long as it's low compared to the impedance of the circuit it's supplying. To a 10 MHz signal the impedance of your 330uF cap would likely be in the 1ohm range, so very very low compared to the high impedances found in tube circuits. If you consider the capacitor useful in the audio band, decades below it's resonant frequency, then you must consider it equally useful decades above it's resonant frequency. It would be completely illogical to think otherwise.

It's also illogical to say that simply because a tube is capable of 400MHz operation that you should be designing audio circuits based on that bandwidth. That would be absolutely terrible practice. If your circuit has gain at 400MHz there's no point in worrying about capacitor minutiae, the design needs to go in the garbage.
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Old 27th June 2016, 10:00 AM   #122
DF96 is offline DF96  England
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Originally Posted by Gnobuddy
Add in the 400 MHz capability of the 6AG5 valve I'm using, and, once again, I am forced to the conclusion that an electrolytic mounted, at a minimum, over an inch away, is not going to provide adequate supply decoupling to guarantee stability for my circuit.
On the contrary, the lossy nature of an electrolytic at 400MHz may help guarantee stability for your circuit. Put a low loss cap across it, and you have created a UHF resonator with the wires; the electrolytic loss will be swamped and no longer effective in providing HF damping for the circuit.

People add low value bypasses when they want to amplify RF (or oscillate at RF). For audio you may be better off with a nice lossy supply rail (at RF frequencies).

We deliberately put lossy elements elsewhere - grid stoppers. Why remove lossy elements too?
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Old 27th June 2016, 11:50 AM   #123
Johan Potgieter is offline Johan Potgieter  South Africa
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Exactly, as DF96 said.

I had this problem with a s.s. design; kept getting x-MHz oscillations until I purposely removed poli-caps from the power supply rails. This seemed somehow to be wrong as simulation did not show any problems, until I hit on the thoughts by DF96. (As said elsewhere, simulation does not show resistance/inductance of p.c. tracks and intertrack/component capacitance, however small. Though only audio, one has to keep some r.f. savvy in the mind!)
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Old 27th June 2016, 01:19 PM   #124
DF96 is offline DF96  England
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Perhaps I should also have said that if you want to build audio circuits with UHF valves then it helps if you know how to build UHF circuits with UHF valves, and also know the difference between an audio circuit and a UHF circuit.

The thing which people forget is that the valve is not very intelligent; it can't tell which frequency range you wanted it to amplify. It doesn't know whether you intended to build an amplifier or an oscillator, but simply behaves according to its nature in the electrical environment in which you placed it.
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Old 27th June 2016, 06:54 PM   #125
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by poptart View Post
Fortunately for us the impedance rise is very gradual
It's a first-order filter either way, either plus or minus 6dB/octave.

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Originally Posted by poptart View Post
the exact value of the impedance at any particular frequency is largely irrelevant as long as it's low compared to the impedance of the circuit it's supplying.
Agreed, the object is to make the B+ rail effectively a signal ground.

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Originally Posted by poptart View Post
It's also illogical to say that simply because a tube is capable of 400MHz operation that you should be designing audio circuits based on that bandwidth.
I believe we have a miscommunication here. UHF-capable devices are capable of misbehaving at UHF frequencies, even if you intend to use them at audio frequencies; therefore I want the circuitry around them to behave properly even at those frequencies. In particular, we don't want enough stray inductance to show up to create an unintentional UHF oscillator.

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Originally Posted by poptart View Post
If your circuit has gain at 400MHz there's no point in worrying about capacitor minutiae, the design needs to go in the garbage.
If you use a device capable of 400 MHz, you DO have gain at
400 MHz, whether you want it or not.

And if you're assuming I've done nothing externally (outside the device in question) to control bandwidth, you're making the wrong assumption.

Nevertheless, I want the B+ rail to behave itself - provide a low AC impedance - over the entire frequency range where the device itself is capable of amplification.

-Gnobuddy

Last edited by Gnobuddy; 27th June 2016 at 06:55 PM. Reason: Typo
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Old 27th June 2016, 06:59 PM   #126
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by DF96 View Post
Perhaps I should also have said that if you want to build audio circuits with UHF valves then it helps if you know how to build UHF circuits with UHF valves, and also know the difference between an audio circuit and a UHF circuit.
If you know something about UHF that you're actually trying to convey, you would be much more successful if you stopped being condescending.

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Originally Posted by DF96 View Post
The thing which people forget is that the valve is not very intelligent; it can't tell which frequency range you wanted it to amplify. It doesn't know whether you intended to build an amplifier or an oscillator, but simply behaves according to its nature in the electrical environment in which you placed it.
Which is exactly why I want the power supply to be well-behaved even at the highest frequency the device is capable of.

Which "people" are you speaking of, exactly?

-Gnobuddy
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Old 27th June 2016, 09:57 PM   #127
poptart is offline poptart  Canada
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Originally Posted by Gnobuddy View Post
It's a first-order filter either way, either plus or minus 6dB/octave.
Far from the resonant frequency yes, but there's a broad flat area where resistive elements dominate. See the lower left of page 8 here.

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Originally Posted by Gnobuddy View Post
If you use a device capable of 400 MHz, you DO have gain at
400 MHz, whether you want it or not.
Sitting there on your desk it's gain is zero. The circuit you put it into determines whether there will be gain, and how much at any given frequency. The device may be capable of functioning in an RF amplifier circuit, but that doesn't mean you should be building one for it. Quite the opposite really. You say you're already taking steps to this effect so I probably don't need to drone on about this.

The inductance of an aluminum capacitor is almost entirely from the loop of it's leads, there's no other significant internal inductance. A radial 330uf/450v cap should have about 15nH of series inductance, that's 2 centimetres of wire. The composition of the cap is irrelevant, to do better you'd need to get your circuit's loop area down under 2cm. Given the size of tubes and sockets it seems unlikely you're going to do significantly better. With miniature components maybe you could turn 2 ohms at 10MHz into 1 ohm, but if that tiny difference is make or break for your circuit (or has any effect whatsoever) then it should never have been built.

You should be less worried about this than you are about the impedance rise in the audio band where you have a lot gain, so try to keep this in perspective. At 500Hz your cap already has higher impedance than it does at 10MHz. At 20Hz it's 25ohms. If you're not obsessing about reducing that by 1ohm then it's illogical to do the same for a band your circuit shouldn't even be amplifying.

Last edited by poptart; 27th June 2016 at 10:04 PM.
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Old 28th June 2016, 09:42 AM   #128
DF96 is offline DF96  England
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Quote:
Originally Posted by Gnobuddy
Nevertheless, I want the B+ rail to behave itself - provide a low AC impedance - over the entire frequency range where the device itself is capable of amplification.
No you don't, for reasons I explained earlier. You could get better RF stability by allowing some resistance in the supply rail. If a valve sees mainly resistive terminations then it can't oscillate. For oscillation you need the right mix of capacitance and inductance, relatively undamped by resistance. The circuit wiring and component strays provide the C and L, so you have to ensure that there is enough R to damp things down.

As I said, it is not enough to know how to build a UHF circuit; you also need to know the difference between that and an audio circuit.
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Old 28th June 2016, 09:01 PM   #129
hpeter is offline hpeter  Slovakia
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Originally Posted by Gnobuddy View Post
Nevertheless, I want the B+ rail to behave itself - provide a low AC impedance - over the entire frequency range where the device itself is capable of amplification.

-Gnobuddy
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Old 30th June 2016, 03:46 AM   #130
Johan Potgieter is offline Johan Potgieter  South Africa
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Quote:
Originally Posted by Gnobuddy View Post
If you use a device capable of 400 MHz, you DO have gain at
400 MHz, whether you want it or not.
Not desiring to belabour this, but I reallly have a problem with this statement, with due rerspect.

A device surely is only one part of the capability of a circuit; this has been stated above in different ways. It needs all elements of a circuit to 'come together' at a particular frequency to have gain there. E.g. nothing on earth will make a 400 MHz circuit work at say 100 Hz, although the active device is fully capable of working at that frequency.

In general, I am somewhat puzzled about all the worries about deep-MHz problems in audio circuits. I must have been singularly lucky/gifted not to have really had such problems. Sure, one takes care of matters which can bother outside the intended frequency band, but much of that comes with basic design procedure. If problems still creep in, it should be simple to 'quench' such by sober investigation/action. One can have power anode leads in a loom with others if the basics regarding undesired proximity and length is observed; many amplifiers do this. This should not be thumb suck or other such guesswork.
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