Operating Points and Datasheets
-
- Thermionic Monk Status
- Posts: 5648
- Joined: Thu May 24, 2007 11:22 am
- Location: People's Republic of South Yorkshire
#1 Operating Points and Datasheets
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...
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...
- IslandPink
- Amstrad Tower of Power
- Posts: 10041
- Joined: Tue May 29, 2007 7:01 pm
- Location: Denbigh, N.Wales
#2
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
( by which I mean typical variations of transconductance ) .
( by which I mean typical variations of transconductance ) .
"Once you find out ... the Circumstances ; then you can go out"
- Paul Barker
- Social Sevices have been notified
- Posts: 8991
- Joined: Mon May 21, 2007 9:42 pm
#4
Might just be an oddity.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
#7
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.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
- pre65
- Amstrad Tower of Power
- Posts: 21400
- Joined: Wed Aug 22, 2007 11:13 pm
- Location: North Essex/Suffolk border.
#8
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.
As an example, on the GM70/GK-71 this could be 20v, 10v or 0v depending on where the connection is made.
The only thing necessary for the triumph of evil is for good men to do nothing.
Edmund Burke
G-Popz THE easy listening connoisseur. (Philip)
Edmund Burke
G-Popz THE easy listening connoisseur. (Philip)
- Mike H
- Amstrad Tower of Power
- Posts: 20189
- Joined: Sat Oct 04, 2008 5:38 pm
- Location: The Fens
- Contact:
#9
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.
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
-
- Thermionic Monk Status
- Posts: 5648
- Joined: Thu May 24, 2007 11:22 am
- Location: People's Republic of South Yorkshire
#10
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.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.
-
- Thermionic Monk Status
- Posts: 5648
- Joined: Thu May 24, 2007 11:22 am
- Location: People's Republic of South Yorkshire
#11
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.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.
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.
-
- Thermionic Monk Status
- Posts: 5648
- Joined: Thu May 24, 2007 11:22 am
- Location: People's Republic of South Yorkshire
#12
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.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.
-
- Thermionic Monk Status
- Posts: 5648
- Joined: Thu May 24, 2007 11:22 am
- Location: People's Republic of South Yorkshire
#13
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?
- Mike H
- Amstrad Tower of Power
- Posts: 20189
- Joined: Sat Oct 04, 2008 5:38 pm
- Location: The Fens
- Contact:
#14
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.
HTH
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.
HTH
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."