6EM7-6DN7-6GL7 Familly

If they glow, this is the place to be
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Dave the bass
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#16

Post by Dave the bass »

Nick wrote:Nice graphs, the -10v line on the big triode looks a tad suspect, but other than that just what I would have hoped for.
The dotted bit is my 'artists' impression, the little PSU couldn't supply anymore current so I went Jazz. The odd curve is down to me only having a 3 plot points hence the odd curve, the others are fairly close.
Nick wrote:
All you need to remember is the valve only knows about the voltages and current on its pins, it has no way of knowing anything else.

Its always a potential difference, there is no official 0v, (ok earth in your mains is a good candidate), it sall in the difference
If I can get that last bit to go into my head I'm sure it'll make sense. Am I right in thinking that when a resistor is placed on the end of the cathode the valve 'sees' the grid differently, like the voltage difference between 'looks' -ve and that biases the valve to a point -ve of 0V?
Nick wrote:If you were hanging off a live 500kv power line, that wire would be your 0v, and nothing bad would happen unless you went near another point at a different potential. Of course the problem you face on the live wire, is getting on and off. I think they use helicopters and wear special conductive clothing (yes, they actually do that).
The power line example came up when I was at college in the early 80's and was explained in the same way. (Me hanging on the HT line)...as I've got no other reference I don't know where Ov/Earth/Ground is so it doesn't matter (untill yer mate gets the ladder to help you down said the tutor).
Nick wrote:So if the grid is at -10v and the cathode 0v, the grid regards itself as being 10v negative of the cathode. If the cathode is at 10000v and the grid at 9990v the grid still loogs at -10 with respect to the cathode. So (and this is the point), if the grid is at 0v and the cathode at +10v, the grid is still -10v with respect to the cathode.
This I think I've got, it's the PD the valve sees on it's pins that matters to the valve. But, what still trips me up is looking at a cct diagram ...say ...take the PB/K 6EM7 cct diagram. Pin 6, the cathode of the first triode, thats sitting at 1.2v above the cct's 0V line BUT the valve doesn't know where 0V is (I think) so how does the valve get it's bias to conduct at a point -Ve of 0V? I can see the grid has got a signal on it but that is referenced to 0V via the pot but i cant see how the Valve is biased.
Image

Sorry for the long sentences, I'm sure it's much easier to explain down a pub than over the web.

Ta for your patience BTW.

DTB
"The fat bourgeois and his doppelganger"
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Nick
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#17

Post by Nick »

This I think I've got, it's the PD the valve sees on it's pins that matters to the valve. But, what still trips me up is looking at a cct diagram ...say ...take the PB/K 6EM7 cct diagram. Pin 6, the cathode of the first triode, thats sitting at 1.2v above the cct's 0V line BUT the valve doesn't know where 0V is (I think) so how does the valve get it's bias to conduct at a point -Ve of 0V? I can see the grid has got a signal on it but that is referenced to 0V via the pot but i cant see how the Valve is biased.
Coz, if the cathode is +1.2v above the 0v line, and the grid is connected to the 0v line. (Ok its connected via a resistor, but as there is no current, both ends of the resistor are at the same voltage, ie 0v). So the grid is at 0v, and the cathode is at 1.2v. As we have said before, thats exactly the same as the grid being at -1.2v and the cathode at 0v. So the grid is negative with respect to the cathode, so its negativly biased .
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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