Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Other Topics => Topic started by: spacelabstudio on September 07, 2010, 09:00:49 pm
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Tomorrow evening I'm giving a presentation about building tube guitar amplifiers to a club called Dorkbot. My slides are here, if anyone wants to take a look. As I'm pretty much a dilettante in this field, I may very well be lying about something and it would be nice to get called on it.
http://spacelabstudio.com/tube_amp_presentation/
This version differs from the version I posted a few hours ago: I have removed one usage on one slide of the f-bomb, which I used in a sentence emphasizing that working on tube amplifiers can be dangerous and even lethal. It was the one and only usage of any four letter words.
sluckey, I hope this version is more acceptable.
I did receive a notification that someone responded to the earlier topic, but it had been removed before I could read it. Feel free to repost if there aren't any f-bombs. ;)
Thanks all,
Chris
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The presentation might be a bit long, depends upon the amount of time your have.
Perhaps a slide on why tube is better than SS or DSP for guitars.
Is it too technical for the audience? Will they understand the electronics fairly quickly? Going from ohms law to tube bias to bypass casp in a short span might loose some people in the audience.
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Slides are not numbered (but text is selectable; how's that work??).
On slide "Impedance" the image does not display for me.
http://spacelabstudio.com/tube_amp_presentation/Complex_Impedance.svg
I suspect I dunno what a "SVG" image is.
If it works for you, OK; but if you will run this one someone else's PC, better check (or just use GIF).
"Impedance is frequency dependent."
I would generalize further. If it isn't a PERFECT resistor (same R at any freqency or voltage), we fudge with the word "impedance". This reminds us that while it may be "about 26 ohms in this condition", we must remember that it will be different for another condition.
On a triode etc with a separate cathode, "heater" is more common than "filament". Similar but not the same. I agree the distinction is fading.
Thank you for skipping tetrodes.
A pentode screen is not always near plate voltage. It is a positive and (usually) steady voltage which indeed "helps to accelerate electrons". It also shields the control grid from plate voltage variation, possibly increasing voltage gain.
"a bit fuzzy on why you might want any of this"
Because a POWER stage must suck its plate to as low a voltage as possible to get the most power possible. In a triode, the low plate voltage stops sucking electrons, limiting peak current and useful voltage swing. A pentode can pull its plate down 80% of supply voltage with trivial reduction of current (because the screen keeps sucking, yet most electrons miss the screen wires and fly to the plate).
All of which may be too-much for an entertaining talk.
I've only got as far as Cathode Bias, but I endorse it as a general introduction to tube amplification which many people should read. (The slide-show format may or may-not be best for reviewing without you talking; my click-finger is tired.)
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I think its a great presentation.
you've elegantly captured the basic concepts of what is involved.
these power point type presentations are designed to do just that. enough information to be dangerous.
those who are interested will delve further the rest it will slip past them.
I did not see any blatant misconceptions or erroneous facts.
Present it with pride and confidence.
OH! nice AC you built there, I mean after the plate of spaghetti was pulled out of it :grin:
Ray
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Slides are not numbered (but text is selectable; how's that work??).
On slide "Impedance" the image does not display for me.
http://spacelabstudio.com/tube_amp_presentation/Complex_Impedance.svg
I suspect I dunno what a "SVG" image is.
The latest version of any major browser should be able to display that. It's a vector image, meaning it stores information about lines and shapes, rather than pixels, like in a bitmap image--ie everything you're used to. Anyway, if this was for a general audience I would convert, but it's only really important that it work on mine.
On a triode etc with a separate cathode, "heater" is more common than "filament". Similar but not the same. I agree the distinction is fading.
Ah, ok. I don't understand the distinction, but a simple search and replace should fix that one.
A pentode screen is not always near plate voltage. It is a positive and (usually) steady voltage which indeed "helps to accelerate electrons". It also shields the control grid from plate voltage variation, possibly increasing voltage gain.
"a bit fuzzy on why you might want any of this"
Because a POWER stage must suck its plate to as low a voltage as possible to get the most power possible. In a triode, the low plate voltage stops sucking electrons, limiting peak current and useful voltage swing. A pentode can pull its plate down 80% of supply voltage with trivial reduction of current (because the screen keeps sucking, yet most electrons miss the screen wires and fly to the plate).
All of which may be too-much for an entertaining talk.
Ah, I actually understand that. Thanks!
I've only got as far as Cathode Bias, but I endorse it as a general introduction to tube amplification which many people should read. (The slide-show format may or may-not be best for reviewing without you talking; my click-finger is tired.)
Yeah, I allegedly have an hour to kill, so I'm trying to come in around that with some time left over for questions and cranking up the amp.
Thanks!
Chris
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>I don't understand the distinction,...
The word heater is typically used to describe the heating element found in indirectly-heated cathodes, i.e. an independent wire not part of the amplification circuit that heats up the cathode for electron emission.
The word filament describes a directly-heated cathode, or the heater and cathode are one and the same. Think 'light bulb'.
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What jhadhar said.
The earliest tubes used the "filament" from an incandescent lamp. Electrons fly off the surface of a thin white-hot wire. (Tommy Edison tinkered with bamboo filaments. The technology didn't catch on but the name stuck.)
There are a few improved types, thoriated and oxide coated, but much the same. Electrons fly off the surface of a self-heated wire.
In filament tubes, the heater power and the cathode-current ae in the same part. In a multi-stage amp you would like one heater-supply for all tubes, but that means all your cathodes are essentially directly connected together. That can work, but there's neat-stuff (like cathode bias) which needs separate cathodes.
And if you heat with AC, with a filament that AC voltage is right ON your cathode. Cathode is an input too, not as sensitive as a grid but heater power tends to be far higher than cathode-plate power. We can sometimes heat many-Watt filament tubes (2A3, 300B) with AC, but not for low-level stages.
Improved oxides emit better, even at red-heat. So they separated the heater-function from the electron emitting function. The electrons come off an oxide-coated sleeve. A wire like the filament (but different and now called "heater") is jammed up inside to heat the sleeve. Some clay between carries heat but blocks current. "Indirectly heated cathodes" cost more but are SO much handier that true filament tubes mostly vanished except in battery-power, instant-heat, rectifiers, and HIGH-voltage work.