Operating Points and Datasheets

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simon
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#1 Operating Points and Datasheets

Post by simon »

You know how you take something for granted that you understand it? And then when you actually think about it you're not sure any more?

Below is the plate characteristics for a 2A3, and the usual OP as given on the datasheet is 250V, 60mA and -45 grid volts. But 250V and 60mA intersect -43.5V. So I thought, the difference is pretty much half the 2.5V heater voltage. But surely that's only true if the filaments are DC heated, and I would assume that the datasheet assumes AC filaments, so why the difference? Can't help thinking this is something I've forgotten about over the years...

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IslandPink
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#2

Post by IslandPink »

I can't off-hand remember how filament voltage is handled on those old datasheets ; but in any case, doesn't 45 = 43.5 in the real word of valves :D

( by which I mean typical variations of transconductance ) .
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#3

Post by simon »

Yeah, there's not many times in an output valve's life that 1.5V makes a big difference :-). Just trying to work out why the difference on the RCA datasheet. Mental block time.
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Paul Barker
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#4

Post by Paul Barker »

Might just be an oddity.
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#5

Post by simon »

Yeah, could be. That would ruin my vaunted opinion of these old time masters though :-(
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#6

Post by simon »

Hmm, just looked at the 45 data sheet and it similarly is 1.5V lower on both the RC and Radiotron graphs.
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Nick
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#7

Post by Nick »

Good find, I dont think it makes any difference if the heater is AC or DC, its just easier to understand with DC. Look at the 211, 10v Heater. Chart says 1000v @ 70ma is 60v on the grid. Look at the curves, its at 65v. Half the heater voltage again.
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#8

Post by pre65 »

So, does the filament voltage need to be subtracted from the cathode voltage when calculating the cathode resistor value ?

As an example, on the GM70/GK-71 this could be 20v, 10v or 0v depending on where the connection is made.
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#9

Post by Mike H »

There will unavoidably be variations in manufacture, so assume the specs are nominal. Real world behaviour of the actual device used is likely to make the above discrepancy trivial IMHO.
 
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#10

Post by simon »

Nick wrote:Good find, I dont think it makes any difference if the heater is AC or DC, its just easier to understand with DC.
Yes, struggling to get my head round this too. Presumably you need to bias the valve differently if DC rather than AC - taking Philip's example of GM70/GK71 as it's more extreme there could be 10V difference between AC and DC bias, depending on how it's implemented obviously. More thinking required.
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#11

Post by simon »

pre65 wrote:So, does the filament voltage need to be subtracted from the cathode voltage when calculating the cathode resistor value ?

As an example, on the GM70/GK-71 this could be 20v, 10v or 0v depending on where the connection is made.
Guess it depends on how the amp's built. For 2A3 and 45 filaments I've always used AC (so far) and rather than use a humbucker pot use two resistors and connect the cathode resistor to the junction of the two for a centre point. In theory there could be a better hum null-point but I've never had a problem.

20V filaments are a different kettle of fish. Notwithstanding complicated high frequency AC supplies I'd always chicken out and go DC. I have used DC with the two resistor to cathode resistor arrangement, and it works (short term at least) but I can't see that it's the right thing to do as things re going to be fighting each other. This means that there is, as you say, a 20V difference across the filament which theoretically could be accounted for in your design. In reality valves are so tolerant that it would probably make little difference.
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#12

Post by simon »

Mike H wrote:There will unavoidably be variations in manufacture, so assume the specs are nominal. Real world behaviour of the actual device used is likely to make the above discrepancy trivial IMHO.
Agreed, sample to sample variation will no doubt swamp the difference, but why state -45V rather than -43.5V? These guys were smart cookies and would have done it for a reason I'm sure. Just wonder what it was.
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#13

Post by simon »

Is it as simple as ensuring that the valve remains in A1, i.e. never drops below 0 grid volts? If a 2A3 were biased at -43.5V then in theory at full output it could draw grid current up to +1.25V grid volts for half of its 50/60 cycles a second?
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#14

Post by Mike H »

I also suspect that the graphs are sometimes 'tidied up', in other words grid lines may be smoothed out so that they look better. Rather than a true 'join the dots' as it were.

I think all you can do is bias as necessary to get the anode current you want. Don't think this question can be resolved really so I wouldn't worry about it too much, just do whatever works.

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