Hello,
I know some things about electronic, even though I didn't practiced much myself. I have a kind of ambitious-ish project of tube amp, and I would like to understand things better before doing too many mistakes 🙂 Those are some question for which I couldn't find answer online.
I wondered some few things about power supply in a tube amp when it involves an LC filter.
Usually, the value of the self is about some few henrys, the capacitor is a bit less than a farad. That's gives an oscillation frequency very low, under the audible spectrum.
We cant here the sound, but can we hear the variation of the volume ? Like a kind of tremolo ?
Is the shoke used to store energy, or because it has some kind of "constant current source" effect ? Increasing the voltage if the current lower. The fact that power supply never include a reverse diode to keep the energy in the self makes me think it's the second hypothesis.
But isn't the primary of the output transformer a big self enough ?
Usually, the tension of a power tube is close to it's upper limit. But with an inductive circuit, this idle tension is a minimum, when current lower, the tension rise. And here we have two inductor in series, the choke, and the output transformer. What makes it safe for tube ? The short duration of those tension spikes ? The fact that those spikes aren't that high ?
What happens when something goes wrong, like what if a power tube disappears for instance. what prevent the rest of the circuit from getting fried by the sudden tension surge ?
What happens during shut down ? The falling temperature of the cathode makes the shutdown smooth enough ?
What about HT switches ?
Summary :
Problem in oscillation of power supply ?
What are chokes for exactly ? Limiting the current before power supply, or keep it constant after ? Bit of both ?
Under normal condition, can it do some harm to tubes, caps, or transistors ?
Under "abnormal" condition (switch off, failure) what prevents it from destroying things ?
Thank you so much 🙂
I know some things about electronic, even though I didn't practiced much myself. I have a kind of ambitious-ish project of tube amp, and I would like to understand things better before doing too many mistakes 🙂 Those are some question for which I couldn't find answer online.
I wondered some few things about power supply in a tube amp when it involves an LC filter.
Usually, the value of the self is about some few henrys, the capacitor is a bit less than a farad. That's gives an oscillation frequency very low, under the audible spectrum.
We cant here the sound, but can we hear the variation of the volume ? Like a kind of tremolo ?
Is the shoke used to store energy, or because it has some kind of "constant current source" effect ? Increasing the voltage if the current lower. The fact that power supply never include a reverse diode to keep the energy in the self makes me think it's the second hypothesis.
But isn't the primary of the output transformer a big self enough ?
Usually, the tension of a power tube is close to it's upper limit. But with an inductive circuit, this idle tension is a minimum, when current lower, the tension rise. And here we have two inductor in series, the choke, and the output transformer. What makes it safe for tube ? The short duration of those tension spikes ? The fact that those spikes aren't that high ?
What happens when something goes wrong, like what if a power tube disappears for instance. what prevent the rest of the circuit from getting fried by the sudden tension surge ?
What happens during shut down ? The falling temperature of the cathode makes the shutdown smooth enough ?
What about HT switches ?
Summary :
Problem in oscillation of power supply ?
What are chokes for exactly ? Limiting the current before power supply, or keep it constant after ? Bit of both ?
Under normal condition, can it do some harm to tubes, caps, or transistors ?
Under "abnormal" condition (switch off, failure) what prevents it from destroying things ?
Thank you so much 🙂
I think you mean "choke". Please make a note of it if using an auto translator.
No, it's never anywhere near a farad. Typically tens of microfarads.
No. The choke does not "oscillate" when working properly. It simply smooths the recitified dc voltage.
It has a high Z at ac and low R at dc. Yes an inductor stores energy.
Not clear what your question is. There is no need to "keep the energy" in the choke.
It's the designer's job to anticipate and mitigate those events. Yes a poorly designed circuit can get fried.
It's the designer's job to make sure it is smooth or it is muted.
I like them. Especially the yellow ones. 🙂
No. The L in conjunction with the C smooths the rectified voltage, i.e. removes the ripple.
The capacitor and choke form a high pass filter, value's are usually chosen so that AC ripple is low. The first capacitor also serves as a reservoir cap.
For most amps a reservoir cap of about 100u or less are used with a choke of a few henry's followed by a bigger value of cap, so a CLC filter. For a first build or just to have a play with solid state rectification just use what you have, EG 220u/470u caps out of a SMPSU and whatever choke you have around. A resistor will do if you don't have a choke, but a voltage drop occurs across it.
Problems can arise with LF oscillation or motorboating where an amplifier resonates at LF with the PSU. Layout is important here, keep wiring short.
You'll learn a lot by building a power supply and not worrying too much about all minutia, there are several good articles online about valve amp power supply design - The Valve Wizard & here - Amplifiers: Power supply here also - Power Supplies, Filter Circuits
Good luck, Andy.
For most amps a reservoir cap of about 100u or less are used with a choke of a few henry's followed by a bigger value of cap, so a CLC filter. For a first build or just to have a play with solid state rectification just use what you have, EG 220u/470u caps out of a SMPSU and whatever choke you have around. A resistor will do if you don't have a choke, but a voltage drop occurs across it.
Problems can arise with LF oscillation or motorboating where an amplifier resonates at LF with the PSU. Layout is important here, keep wiring short.
You'll learn a lot by building a power supply and not worrying too much about all minutia, there are several good articles online about valve amp power supply design - The Valve Wizard & here - Amplifiers: Power supply here also - Power Supplies, Filter Circuits
Good luck, Andy.
Hello, thank you !
Choke and self aren't the same thing ?
For my fist amp im just gonna use a cheap boost converter from ebay. I was just supprised when seeing the classical schematics.
Choke and self aren't the same thing ?
For my fist amp im just gonna use a cheap boost converter from ebay. I was just supprised when seeing the classical schematics.
In what language? I've never heard the term "self" in connection with chokes. Where did you hear it?
Be careful, many of them are not isolated.
Oh... it's in french, it's because of "self inductance" we call them french, as it si an english word I was sure it was an english name. Ok, it's just in french then 😛
Sorry, I though I was writing in english 😛
Surely layout won't affect low frequency behaviour of a power supply? Layout
matters for ground loops and high frequency/RF stuff, and motor boating is usually due to positive feedback of the amplifier at sub-sonic frequencies.
To my mind the loading of the supply is key to damping any resonance, you can think of the amp load as roughly resistive, so you have a damped LCR circuit in effect - make it critically damped and all is well, or ensure the resonant frequency is well below the audio and power supply range.
Loading an LC stage with an RC stage can provide good damping without having
to pull lots of DC current from it by my calculations. The ESR of a choke itself can add significant damping.
Self (inductance) was the common name for a choke in the Radio Days, in both the english and french language. In german it's called "Selbstinduktion", in dutch "smoorspoel" 😉