Are tube PA systems still being made?

[Dave Grafe]

Thanks for checking my assumptions, Brad. I took the numbers from an old cross-reference book on my tech shelf, and I have heard engineers with far more knowledge than me say the same thing at various times, in discussing their preference for several other 12-volt-filament dual triode audio tubes for exacting high fidelity applications.

The published response table I have looks like this:



I am certain that there is a way to get flat response to 30kHz out of it, but that is beyond my pay grade.

[Brad Sarno]

Interesting chart. It must be based on some old RCA or standard application circuit.

In a lot of circuits, since the 12AX7 is high-Z, there's gonna be a lot of stuff that does easily kill the top end, and in hi-fi applications there are ways to do a circuit that preserve a very flat response beyond 30kHz. In a guitar amp, we actually want to dump that stuff, so all the long wire runs, the stray and miller capacitances and the basic plate driven gain stage is definitely prone to rolling the highs off.

B

[Richard Sinkler]

Can some briefly explain how a tube works in simple terms. One of the great mysteries of life for me, is how certain electronic components work, or recommend a website for dummies and not engineers. I look stuff up and and end of needing a double dose of Prozac.

[Les Cargill]

The British call then "valves". It's a better term.

For a triode:
When you apply voltage to the grid, that voltage causes current to flow from the cathode to the anode. Pardons if I got that backwards, but I'm pretty sure the anode is the "output".

So it's analogous to turning on and off a faucet.

Beyond that, the "how" gets pretty hairy. One way to think if it is adding electrons to the grid changes the conductance ( like a potentiometer only a lot scarier ) of the path through from the cathode to the anode. Electrons jump the gap.

It's a lot an accident that tubes came first. It came about because of the way light bulbs were. I think solid state is easier to understand because the semiconductor is at least *there*.

But I do not understand the basic physics of tubes much at all, beyond that it's electrons moving. You can understand solid state with a little physical chemistry; tubes are a bit more magical.

[Stephen Cowell]

The 'valve' was discovered when someone put an extra element inside a light bulb and determined that a current could flow between the filament circuit and the extra element... was that Lee DeForest? No! It was Edison...DeForest added the third element (the grid) and created the possibility of amplification. Fleming created the true diode radio detector circuit, taking the place of the cat's whisker crystal.

Edison discovered the eponymously-named Edison Effect when he investigated why his light bulbs always burned out near the positive end... he put an extra thing inside the bulb (the 'plate') and determined that a current could flow between the hot filament and plate, but only if the plate was connected to the positive end of the (DC) filament supply. We now know that this was because the hot filament was able to 'donate' 'charge carriers', electrons, to the evacuated atmosphere inside the lamp, and positive charges draw the electrons while negative charges repel them.

DeForest put the grid in there and was able to control the current between plate and filament... if the grid is negative, it inhibits the current flow, and if the grid is positive, it accentuates it. Only later did we come up with indirectly-heated cathodes (requiring chemistry magic) to isolate the filament voltages from the rest of the circuit. The directly-heated cathode is still the most efficient way to run a tube, and is found in the largest transmitting tubes (coated with magic Thorium chemistry stuff too).

Materials science is working toward true 'cold-cathode' tubes, where the cathode is coated with Buckytubes, or diamond, or some other electron emitter. Previous CC tubes are not near efficient enough for power amplification.

You can read all this on Wikipedia... it's actually time to donate now, they're doing their once a year begging. I have no problem giving them money, I use the thing many times a day, every day, at work and at play.

[Craig Baker]

Stephen,
Great post. The Radio Corporation of America (RCA) "Radiola III" that my father bought new in 1923 uses two DeForest "WD-11 Audions" After all of these years, the tube filaments still light. Wish I could afford the B and C batteries to see if it would pick up anything. Still have the original headphones also. Last time it was turned on, I think Lindbergh had just landed. Here is a Photo of the DeForest WD-11




Brad,
Years ago I built a 6 meter transmitter using a 12AX7. One half was the overtone oscillator, the other half was the final amplifier. It put out half a watt on 50.4 Mhz. There have been articles regarding using the 12AX7 at 144 Mhz.

That's higher than anything my dog can hear.

Craig