High Voltages After Headfirst Origin 50 Mod

The formula for transconductance or Gm of a vacuum tube is as follows. Gm=delta Ia ÷ delta Vg. This is the difference in plate current divided by the difference in grid voltage that produced it. So in simpler terms it would be plate current divided by grid voltage. These terms are for RMS plate current & RMS grid voltage. Not DC values. Basically, a 1V signal that causes a 1 milliamp change in plate current is said to be a Gm of 1000
 
67plexi said:
The formula for transconductance or Gm of a vacuum tube is as follows. Gm=delta Ia ÷ delta Vg. This is the difference in plate current divided by the difference in grid voltage that produced it. So in simpler terms it would be plate current divided by grid voltage. These terms are for RMS plate current & RMS grid voltage. Not DC values. Basically, a 1V signal that causes a 1 milliamp change in plate current is said to be a Gm of 1000
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Thanks!!!!!
 
Robert Herndon said:
@ivan H - What do all the numbers mean???
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67plexi said:
The formula for transconductance or Gm of a vacuum tube is as follows. Gm=delta Ia ÷ delta Vg. This is the difference in plate current divided by the difference in grid voltage that produced it. So in simpler terms it would be plate current divided by grid voltage. These terms are for RMS plate current & RMS grid voltage. Not DC values. Basically, a 1V signal that causes a 1 milliamp change in plate current is said to be a Gm of 1000
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Thankyou @67plexi for correctly answering the question.
Amp Mad Scientist said:
What I found was the plate current of the EH fake 6CA7 is looking much higher than a normal real 6CA7.
Therefore the bias may need to be changed to accommodate this...
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Please stop referring to the EHX 6CA7 as a "fake" 6CA7. We have been through this twice before, where I went into detail explaining to you that the fat bottle 6CA7 (beam tetrode) was originally designed by Sylvania to be a suitable EL34 (pentode) substitute, while avoiding paying Philips "pentode" royalties,,,, as such, the EHX 6CA7 is the only current production 6CA7 that is a "true" big bottle beam tetrode 6CA7, as "originally designed" by Sylvania.
Regarding the EH 6CA7's plate current looking much higher than a normal "real" 6CA7, I am imaging that you are comparing the figures shown on the EH 6CA7 boxes that @Robert Herndon posted a pic of, being;
Ip: 66 (plate current in mA)
GM: 6,800 (transconductance in umhos)
to figures for EL34's.

Firstly, plate current test figures can vary depending on exactly which type tester the tubes are tested on (various testers supply different voltages to plate, screen & control grid).
Secondly, these are the plate current & transconductance figures for NOS/NIB Blackburn Mullard xf2 EL34's (as "real" as EL34's get)IMG_20210807_145044.jpgGE branded tubes;
Ip: 82mA/80mA
GM: 11,300/11,200 umhos

Amperex "globe" branded tubes;
Ip: 81mA/79mA
GM: 11,200/11,000 umhos

As you can see, the plate current (& transconductance) figures are quite noticeably higher than those of the EH 6CA7's. I have other NOS Blackburn Mullard EL34 duets & quads & all have Ip & Gm figures in this range. Cheers
Edit:
As originally designed, the USA produced fat bottle beam tetrode 6CA7's had electrical characteristics almost exactly matching those of the British made EL34's (Mullard, Brimar etc etc). Cheers
 
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syscokid said:
Some "maximum operating voltages" that you'll see on datasheets are rated very conservatively. Dont let those numbers scare you, but it's always good to ask around for real experiences.
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Thanks, Amigos...

I know so little about this stuff that I'm asking as many questions as possible, but I'm also trying to self-educate as much as possible.

One day, I will probably venture into some of the kind of work that you, @ivan H and some others are doing, but for now, I'm just trying to learn as much as I possibly can, so I appreciate the replies!!!
 
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