Jcm800 Downscale

[Chuck H]

But tubes also have a current limit. Am I missing something? I didn't know the current ratings could be ignored in the presence of lower voltages. Sure, they can be fudged. But, for example, and el84 with 350Vp @ 30mA making 10.5 watts is common. The same tube with 40Vp @ 265mA is dissapating the same watts, yes. But that's four times current limit for the tube. As noted, the swing voltage (actual useful AC) would be pretty pathetic.

[soundguruman]

Current Limit?
But that's what we are always exceeding, to make the tubes scream for holy mercy...
Bless me James Marshall, for I have sinned.

[Chuck H]

Using the above example... A tube with a 65mA current limit may be pushed to, oh I dunno, 100mA with lowish voltage? But 265mA at very low voltage? I'm thinking that even tubes have a melting point!

[Steve Conner]

Well, you can force a pentode or beam tetrode to pass excessive current at low plate voltage, by applying too much screen voltage, which was effectively what Marshall did. But a triode doesn't have a screen grid to play that trick with.

[Austin]

Using the above example... A tube with a 65mA current limit may be pushed to, oh I dunno, 100mA with lowish voltage? But 265mA at very low voltage? I'm thinking that even tubes have a melting point!

The way I understand it and somebody help me if I am missing something, tubes are limited by the plate resistance how much current they could pass, the el84 you gave for an example could never pass that much current at that low a voltage because it has too high of an internal resistance. El84 triode connected is about 2k ohms I think, and one of these 6080 is about 200 ohms per plate, so 100 ohms in parallel. So using ohms law you can see a 6080 could pass 400 ma at 40 volts and an el84 could only pass about 20ma. The internal resistance of a tube changes with voltage though, it decreases a bit with lower voltage so an el84 running on only 40 volts will have somewhat lower than the normal 2k resistance and may actually be able to pass a little bit more current than 20ma. You could figure out the plate resistance at that voltage though with a voltmeter.

Well, you can force a pentode or beam tetrode to pass excessive current at low plate voltage, by applying too much screen voltage, which was effectively what Marshall did. But a triode doesn't have a screen grid to play that trick with.

I think the control grid functions as a screen where positively biased and that this is the reason why a2 increases efficiency, the grid becomes a screen by accelerating the flow to the plate when it has a potential above cathode voltage. I guess it is kind of like using screen drive on a pentode where the grid is strapped to the cathode.

[Chuck H]

the el84 you gave for an example could never pass that much current at that low a voltage because it has too high of an internal resistance. El84 triode connected is about 2k ohms I think, and one of these 6080 is about 200 ohms per plate, so 100 ohms in parallel. So using ohms law you can see a 6080 could pass 400 ma at 40 volts and an el84 could only pass about 20ma. The internal resistance of a tube changes with voltage though, it decreases a bit with lower voltage so an el84 running on only 40 volts will have somewhat lower than the normal 2k resistance and may actually be able to pass a little bit more current than 20ma.

Huh... The data sheet says 65mA. Why only 20mA at 40V? Your equation seems incomplete. Why can an el84 only pass 20mA at 40V? I'm not getting the criteria that your basing the results on. Are you using the tubes internal resistance and max dissapation for the quation?

[printer2]

6CD6 (25CD6) in triode?

http://www.classiccmp.org/rtellason/...ta/6CD6-GA.PDF

What transformer impedance in P-P? I am out of my element here.

Class A? not sure if AB helps because we are limited by voltage.

[printer2]

SOB, I thought I had two 25CD6's, one in its box has the top cap broken off. Thought the 230mA average plate current and 60V plate operation might be interesting. I have a pair of 7695's which are ratted at 10W a pair in P-P at 130V but the heaters break the 40V rule, 50V.

[Austin]

SOB, I thought I had two 25CD6's, one in its box has the top cap broken off. Thought the 230mA average plate current and 60V plate operation might be interesting. I have a pair of 7695's which are ratted at 10W a pair in P-P at 130V but the heaters break the 40V rule, 50V.

nice specs, seems like those tubes would do well. The if the op could ask for 50v instead of the 40v limit then he could run the heaters and plate off the same supply.. Of course the heaters at forty volts instead of fifty would still probably work just fine. I have ran 6.3v tubes from the 5v supply several times and there seems to be little to no performance hit.

[J M Fahey]

I think the OP did a big [facepalm] and left the building long ago.
Can't blame him.

[printer2]

That is another question. 40V AC or DC?

[Austin]

I think the OP did a big [facepalm] and left the building long ago.
Can't blame him.

But isn't this thread really about his idea and all the people that may read this thread in the future?

[Austin]

That is another question. 40V AC or DC?

Couldn't an inductor with 40v make nearly 80v ac and be a way to cheat?

[Austin]

Huh... The data sheet says 65mA. Why only 20mA at 40V? Your equation seems incomplete. Why can an el84 only pass 20mA at 40V? I'm not getting the criteria that your basing the results on. Are you using the tubes internal resistance and max dissapation for the quation?

\\

If a el84 has a 2000 ohm plate resistance triode strapped, then with 40 volts 2k ohms could only pass 20ma and .08 watt like any other resistor. I don't know what the plate resistance actually is at that low a voltage though it could be figured out easy enough. A tube is like a variable resistor.

[printer2]

Couldn't an inductor with 40v make nearly 80v ac and be a way to cheat?

What is cheating other than efficient creativity?

Another high current pentode.

http://www.tubebooks.org/tubedata/HB...art_2/6GB5.PDF