Class A operation (push-pull output)

[bruno]

I've been reading a bit more about this... I haven't found a good source for it yet, but in all honesty I also haven't searched properly, but I intend to. From what I've understood, class A operation, in a regular push pull amp, is achievable, if the output tubes are working at max dissipation wattage at low plate voltages. I also read that the output transformer primary impedance should be higher than normal, didn't really understand why.

But I'd like to understand the math involved, using a 6v6GT, the max plate voltage ought to be around 350 volts, and the maximum dissipation, should be around 14watts. So idle current at plate, needs to be around 40ma? Will this be enought to give me an output stage operation at class A.

Also the transformer output primary impedance for two 6V6 is usually around 6.6k to 8k, is it necessary to go higher?

[PRR]

> if the output tubes are working at max dissipation wattage at low plate voltages.

Not at all necessary.

The long-tail driver on late Fenders is fully Class A, does not work the tube at full Pd nor at low voltage.

But when you get to a POWER stage, bigger tubes cost more. So the best-value Class A amp "must" work the tubes at FULL plate rating to get the most from the tube dollar.

Low voltage is not needed. High voltage is not better. Power is limited by dissipation, not voltage or current. And low voltage leads to low impedance which is usually a slightly cheaper OT, and low voltage is often a cheaper PT and filter-caps.

The key criteria is: does current cut-off at any point in the wave? All Class AB amps are Class A up to a point, then they cut-off one side while the other side's current soars. So the test case for "is it Class A?" is FULL power. Bring it up to clipping then back-off to just-not-clipped. 'Scope each cathode current. Is the current going to zero?

To make the answer "No", you need a standing current at least twice your peak current. You determine that by load impedance and voltage. Basic amp design.

For more fun: tubes cut-off gradually. If current goes to 10%-5% of idle and never quite gets to zero, is it "cut off"? Perhaps not, but it is so very low that it isn't helping the push-pull. This is where practical men must disagree with theorists who assume straight-line perfect devices to make arbitrary categories.

And anyway.... why the heck do you care? Class A does not sound better to the ear, no matter how much bumph they put in the ads.

[navdave]

So whats the difference between class AB1 and AB2? Which one puts out more power?

[bruno]

Hi PRR, thanks for the answer... I actually do not care for it in guitar tube amps, but I'd like to understand it, because I guess it's basic tube circuitry design.

So from what I understood, low voltage is only used because it helps to keep the costs down in terms of output transformer and filtering.

So let's say I want to design a two 6V6GT class A stage, and keep the costs down. I'll have to scope the cathode current to determine whether or not it is cuting off, but where do I start? I'll have to use transformers, that allow me to set up a test bed and get some values to read. So what plate voltage specs should I be looking for?

When I change the cathode resistor value, how do I know where to go in order to get no current cut off?

[FYL]

So whats the difference between class AB1 and AB2? Which one puts out more power?

AB1 : grids are kept at negative potential, no grid current. AB2 : grids can go positive, thus sucking current from the preceding stage. AB2 is more efficient and can put out more power, at the expense of added complexity.

[Fresh_Start]

bruno - I still don't completely "get" load lines, but that's what you need to work with.  These links might help:

http://diyparadise.com/tubeloadline/tubeloadlines.html

http://www.freewebs.com/valvewizard1/pp.html

What is your design goal?  Or is this just an intellectual exploration (worthy itself IMHO)?  I'm asking because a relatively low B+, mostly Class A output stage seems to be part of the mojo of the 5E3 Deluxe.

Cheers,

Chip

[PRR]

> AB2 is more efficient and can put out more power

Not necessarily.

You flog the grids positive when you want more current than the tube can deliver at safe plate (and G2) voltages. When the tube does not conduct well enough to reach its power dissipation limit with safe voltages. (Safe for the tube.)

The first good 1-Watt home loudspeaker amplifier ran 400V because the '10 tube doesn't conduct well. The report says that higher conductance (lower plate resistance) tubes should be developed. within a few years low-Rp triodes were doing 1 Watt at 250V, and then pentodes were doing it with 110V.

The alternative, used in many 833 designs, is to slam the G1 grid positive and suck more current off the cathode. But ~~half that added cathode current goes to the grid and must be absorbed by the driver. Now the driver can't be a little '76 or 12AX7, it must be a small power tube (Gates used 807 which is a sturdy 6L6). And there's bad impedance between driver plate-cathode and power-tube grid-cathode. And in most cases the grid current does not flow over the whole wave, causing a kink where grid current starts.

Early 6L6 datasheets show an AB2 condition. You keep the G2 voltage at the safe 250V, reduce the load impedance for more power, then slam G1 positive to suck the extra current needed to actually get more power in the low load impedance.

For various reasons (cost), AB2 became less popular. Instead new higher-conductance tubes were developed to get more power in AB1. EL34 and 6550/KT88 serve well in audio. Later, TV set H-sweeps needed big power at low voltages and MINIMUM cost. These can dump more current than their plates can dissipate in linear operation (V-sweep is an on/off job), and can be used in audio withough thinking about AB2 operation.

We must not forget class A2. Plate conducts all the time and grid conducts current. Since class A is mostly for small low-cost power, it "does not make sense" to add another small power tube to drive the grid current. Yet that is the only practical way to drive the 833, because its plate current at zero grid (no grid current) is awful small.

Oh, and about 1934 there was a power triode scaled for A2 mode with an internal cathode follower driver so the radio designer did not have to see grid current. This was real popular for a couple years until power pentodes killed such tricks.

Efficiency and power are not functions of AB1 versus AB2. It is about the devices we can get.

[bruno]

Hi, fresh start, this is an "intellectual exploration" as you put it... I think class ab amps sound really nice, so I'm not looking to build or design a class A amp... I would like to understand how I'm supposed to think in terms of how tubes react with the circuit and load values, in order to be able to design this kind of working class.

I'll take a look at the links, that you very much guys.

[tubesornothing]

I pulled this chart from an amp that claims to be "class A".  This is two 6l6GC running at 400V B+ cathode bias with the grid at 35VDC.

Anyway I can tell from the load lines if this is really class A? how?

[PRR]

Which loadLINE?

You can draw an infinite number of loadlineS. Some may be A, some B, some C, and some just aint right.

ASSuming the one with an X, and that X marks the bias point....

Arrrrrgh.... I have a strong feeling NONE of these three lines are plotted correctly. What is the actual plate-plate load? 5.6K? 11.2K? Is there a power-output claim?

[tubesornothing]

5.6k plate load.  They claim 30W.

[Merlin]

5.6k plate load.  They claim 30W.

That's sort of true. Assuming the HT doesn't sag, then that load line does remain in class A up to 30W output.

However, the line you have drawn assumes that 400V is across the valve, but you said it was cathode biased. After subtracting the cathode voltage you might have only 370V across the valve, which would shift the whole load line to the left (you would also need to redraw the characteristics for 370V screen voltage). If you also allow for some power supply sag then it might well stay in class A even up to max output.

[jojokeo]

Oh, and about 1934 there was a power triode scaled for A2 mode with an internal cathode follower driver so the radio designer did not have to see grid current. This was real popular for a couple years until power pentodes killed such tricks.

PRR, I'm not only impressed immensely over your tube knowledge prowess, but your memory is amazing. How do you remember this stuff the way you do? 

[tubesornothing]

That's sort of true. Assuming the HT doesn't sag, then that load line does remain in class A up to 30W output.

However, the line you have drawn assumes that 400V is across the valve, but you said it was cathode biased. After subtracting the cathode voltage you might have only 370V across the valve, which would shift the whole load line to the left (you would also need to redraw the characteristics for 370V screen voltage). If you also allow for some power supply sag then it might well stay in class A even up to max output.

Thanks Merlin.  I have re-drawn the chart, but the upload folder it full at the moment.   So is there anyway to tell from the chart if an amp is class A?  Or does it have to be via measurement?

[tubesornothing]

Here is the re-drawn chart:

[Merlin]

   So is there anyway to tell from the chart if an amp is class A?  Or does it have to be via measurement?

The fact that there is a kink in the load line indicates class AB. A class A load line would be straight (in other words, as you shove the 1/2Raa line upwards, it will pass above the 1/4Raa line)

*But* that is not allowing for PSU and screen sag, which could force things into pure class A at full drive.