American Circuits

[Nowhere-Man]

A few observations on American circuits. I have taken a pic of a Cathode follower from my American HAM theory book as it had me puzzled. I thought I would take a photo from the page of my book and see if it would upload. It seems that it has uploaded. I always find I can upload the first pic I take with my digital camera.

Here is what had me baffled: If you study the pic of the cathode follower doesn't it seem to you that the signal is incoming between cathode and grid? Apparently not. It states the signal is between the anode and the grid. It adds that there will be some negligible impedance in the batteries.

After trying to sort all of this out, I finally concluded that the big factor here is that, in the cathode-follower circuit, you can see a resistor between cathode and ground, yet there is no resistor directly between anode and ground. In most other circuits I've looked at there is always an anode resistor similar to the other illustration above.

Thus, it seems to me the cathode follower in this diagram allows the signal to bypass the cathode due to the cathode resistor (I can't recall its value) and it passes to the anode instead. Finally it seems the AF is built across R in the lower illustration. I'll go over it again tonight maybe.

In any case, I find a few differences in American circuits. One thing you notice is they have a tendency to use grid bias batteries. This may be the case with output tubes. I've seen American circuits where a battery is used to create a fixed cut-off bias. Not sure why they didn't always just use a GB resistor.

[ppppenguin]

I don't like the way that US scheamtics are drawn.

One point which you have hinted at is that the power supplies have negligible impedance. If everything is ideal the anode is at the same SIGNAL potential as the ground or 0V rail. The DC potential is different, that's called HT Or B+ in US speak.

[Nowhere-Man]

I don't like the way that US scheamtics are drawn.

One point which you have hinted at is that the power supplies have negligible impedance. If everything is ideal the anode is at the same SIGNAL potential as the ground or 0V rail. The DC potential is different, that's called HT Or B+ in US speak.

I'll go over some of this again tonight. Does it seem to you the cathode resistor here is being used to simply push the signal directly to the anode? That's how it seemed to me as at first I was confused and thought the signal was between the grid and cathode.

They were quite keen on grid leaks which they call "plate detectors". In fact, lots of American circuits show triodes being used with signals being taken to the grid. Not so many DDT's.

[AlanBeckett]

I don't like American Drawings either, but that's life.

The secret is in the last sentence. To make sense of it you have to add the Input and Output.

Putting it into English gives:-

   

Alan

[ppppenguin]

The cathode resistor is mainly there to provide a DC path for the current through the valve. In an ideal world it would have infinite resistance but then no current would flow at all.

The input signal is between grid and 0V (ground). Not between grid and cathode. As far as signals are concerned, in an ideal world, the anode is at the same potential as ground so you could say the input signal is between grid and anode. But that's not a very helpful way of describing the cathode follower.

What's more interesting is that both input and output signals are referenced to 0V. This is unlike the common cathode configuration we normally see. Here the input (grid) is referenced to 0V while the output (anode) is referenced to HT+. I've said that the anode is at the same signal potential as 0V but this is only true in an ideal world. In practice the decoupling capacitors between HT+ and 0V are not perfect and neither is the HT+ supply. This imperfection gives rise to all sorts of practical problems in amplifiers including hum and instability.

[Terry]

The cathode resistor in the cathode follower also provides the bias as the cathode being positive wrt ground means that grid, as it is also normally tied to ground via a grid leak, must be negative wrt to the cathode!

So what's the point of the grid bias battery, then? It is totally redundant!

[AlanBeckett]

It means you can vary the Operating Point without changing the Cathode Resistor.
You may want a particular Cathode Resistor value for other reasons.

Alan

[Alan Douglas]

Alan's drawing is a bit easier to understand.

Not many American sets used battery bias, only a few around 1940, usually portable radios.

I agree the British schematic convention is better, at least for the positioning of the HT rail, but it's 90 years too late to change it.

You might look for a 3rd edition of the Radiotron Designer's Handbook, originally from Australia. The 4th is much larger and is the one everybody wants, but the 3rd has all the same tube theory.

[Murphyv310]

On a practical viewpoint a lot of cathode followers will have the grid directly connected to the preceeding valves anode, the anode of the follower wiill be connected to HT via a very small value resistor and may or may not be decoupled, the output signal developed from the cathode across the load resistor will be virtually the same as the preceeding valves anode with the exception of being at a considerably lower impedance.

[Nowhere-Man]

I don't like the way that US scheamtics are drawn.

One point which you have hinted at is that the power supplies have negligible impedance. If everything is ideal the anode is at the same SIGNAL potential as the ground or 0V rail. The DC potential is different, that's called HT Or B+ in US speak.

I had another look at it last night. I'm so used to seeing the signal between grid and cathode that the cathode follower had me a bit puzzled.

The way I begin to understand it now is there is mostly one electrode on a tube that forms part of the input and the output as well. I know this will be understood already by most but I'm putting it into my own lingo, so to speak (just to help me clarify my following post). So, with regard to the cathode follower the signal still is directed to the grid but the grid shares this signal with the anode. And the anode also is included in the AF output along with the cathode. Every writer says the anode is "common" to the circuit but that always confused me a bit.

I take the point about 0 reference as I have a couple of car radios that use the chassis as positive ground.

Here is something interesting, though: In cases where a resistor is between cathode and chassis, there is a DC voltage drop. Someone already just explained that correctly. And, yes, one part of the GB resistor is more positive and the chassis part can be used for grid bias. When an AC signal is in series with this resistor it can flow through the DC part of the circuit and create negative feedback. So, the usual 0.1 mF capacitor can be shunted across that same resistor and prevents any AC drop at that part of the circuit. There are one or two diagrams that compare negative feedback circuits with filtered circuits where the AC isn't allowed to influence the resistor drop.

In closing, I note there is a schematic for a Simplex Super X receiver. This had me baffled but I note a few people online have built one. What throws me is the tubes are shown exploded so, for example, the mixer is shown in two halves, upper and lower. The L.O. part of the tube is lower down with all the HT voltages. I really prefer complete diagrams but maybe I can work it all out one rainy day.

It would be fun to build one of these one day.

Tonight I'll have a look at why cathode followers are used and what advantages they offer.