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Dave the bass
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#16

Post by Dave the bass »

andrew Ivimey wrote:Monday might be the high spot of the week; cool fresh morning of brightest summer and al :D l
Yeah.... but no ....but...you didn't skate yer legs off for 4 1/2 hours in the blazing hot sun then attempt to quench the unquenchable thirst at the new Witheredpoons in Dirtford for another 2 hours afterwards didja eh :lol:

Deffo not 'freshest of the brightest' here pal! :lol: :lol:

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

Post by Nick »

Dave the bass wrote:OK. Swotting again.

Now then, this Transconductance measured in 'mhos', I've seen that detailed in some Data sheets. If a unit of resistance is 'negative ohms' it becomes an indication of 'gain' or amplification yes?

Just checking I've got this bit right before going on.

Not quite as bright as Sunday what with it being a Monday .....

DTB
No, its not a measure of gain, but a measure of the transconductance of the valve (you could consider it the current gain of the valve). It can be used to calculate the voltage gain of the valve, but only if you know the anode resistance (ohms law again).

gm = µ / ra

So

µ = gm * ra

Remember that we measure the gain of the valve as the ratio of signal voltage in to signal voltage out. The fact the valve is a transconductance device means it converts a change in voltage to a change in current. To convert current to voltage we need to pass it through a resistance (ohms law again), so the above equation shows how the gain of the valve is a result of the gm multiplied by the anode resistance.

For example, the 5687 has a reported gain of 18.5, and a transconductance of 11500 µmhos (micro mhos) So we can find its anode resistance by

ra = µ / gm

ra = 18.5 / 11500e-6

ra = 1608 ohm.

So to bring it on topic, if we have a 50H choke, how will that play with a 5687?

The -3dB point of a LR filter is f-3dB = R / L * 2 * pi, so its 5Hz. So 50H is ok, but more might be better. 220H gives 1.2Hz

Though remember that we hen we talk about the gain of the valve, its the internal gain, the actual gain in a amplifier will be less than µ as we will be seeing the effect of the anode resistance in parallel with the load resistance, so as the load resistance gets larger the gain of the circuit approaches the µ of the valve.

Also remember that the gm of a valve will vary with the op point, so the figures given are only valid at one set of current and voltage conditions. The less a valves gm varies with current the more linear it is.
Little known fact, coherent thought can destructively interfere with itself leaving no thought at all, that’s why I prefer incoherent thought.
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#18

Post by Dave the bass »

Hi, back again.

Right, lunchtime trying to get to grips with 'big things' listed yesterday and this morning.

I'll start from the last point in Nicks last post. Ah! I see now why my little 'H' 100H chokes stifled the bass now. It's part of a filter network, i didn't realise that. That big 'R' in Ra of 6EM7 working with a smidgey little choke is asking for trouble. Pick a lower Ra a tube and there's more chance of it working and lowering the -3dB point yes? i'd need a much bigger 'H' to get a lower -3dB point with 6EM7 hence others suggestion of 2 in series (which is i didn't have 2 duff 'uns from our Tim I could've, Grrrr. They were cheap i s'pose.)

I'm such a dullard, i didn't know the term 'mu' and Transconductance were the same thing, gm. Derrr.

Right, I'm gonna pick a valve out of a list and try and work out what the gm is from the gain and Ra. Its working things like equations and the like that confuse me. I'm OK if I can put them in triangle like I do with Ohms law, its the only way I can remember it, looks like I can with gm, u and Ra yes? that'd make it easier to work things out for me.

Back at tea time, gotta do boring work now.

DTB
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#19

Post by Nick »

Yep, they can be used as a set of three in just the same way as ohms law
I'm such a dullard, i didn't know the term 'mu' and Transconductance were the same thing, gm. Derrr.
They aren't gm and transconductance are the same thing, but mu is just a ascii way of saying µ, which IS the gain of the valve

gm = µ / ra
µ = gm * ra
ra = µ / gm

Though gm should be g with m as a subscript, the m being for mutual. And the r of ra being a small r to indicate its a dynamic resistance and not a real fixed resistance like Ra which is the load resistance.
Little known fact, coherent thought can destructively interfere with itself leaving no thought at all, that’s why I prefer incoherent thought.
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#20

Post by Dave the bass »

Nick wrote:Yep, they can be used as a set of three in just the same way as ohms law
I'm such a dullard, i didn't know the term 'mu' and Transconductance were the same thing, gm. Derrr.
They aren't gm and transconductance are the same thing, but mu is just a ascii way of saying µ, which IS the gain of the valve

gm = µ / ra
µ = gm * ra
ra = µ / gm

Though gm should be g with m as a subscript, the m being for mutual. And the r of ra being a small r to indicate its a dynamic resistance and not a real fixed resistance like Ra which is the load resistance.
Some times I just think Electronics isn't for me :) Back to the drawing board.

I had to look up what ascii is. OK, backtracking to 1st stage of 6EM7, Ra refers the physical anode load resitance eg in this case it WAS a 120K resistor. whereas ra is actually about 40K, it being the dynamic resitance of the valve yes? MrI over a pre Eggy meet dwinky mentioned a rough guide of 3 x ra normally being the Ra, have I got that right?

DTB
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#21

Post by Nick »

Dave the bass wrote:
Nick wrote:Yep, they can be used as a set of three in just the same way as ohms law
I'm such a dullard, i didn't know the term 'mu' and Transconductance were the same thing, gm. Derrr.
They aren't gm and transconductance are the same thing, but mu is just a ascii way of saying µ, which IS the gain of the valve

gm = µ / ra
µ = gm * ra
ra = µ / gm

Though gm should be g with m as a subscript, the m being for mutual. And the r of ra being a small r to indicate its a dynamic resistance and not a real fixed resistance like Ra which is the load resistance.
Some times I just think Electronics isn't for me :) Back to the drawing board.

I had to look up what ascii is. OK, backtracking to 1st stage of 6EM7, Ra refers the physical anode load resitance eg in this case it WAS a 120K resistor. whereas ra is actually about 40K, it being the dynamic resitance of the valve yes? MrI over a pre Eggy meet dwinky mentioned a rough guide of 3 x ra normally being the Ra, have I got that right?

DTB
No, you are doing ok, its just that resistance is a thing you can "touch", its a value that a resistor has. Gain is also something you can find in the real world, how much more comes out than goes in. gm is a bit trickier, you can only see it by what it does. Though you could say a valve is to transconductance what a resistor is to resistance.

You are spot on with the Ra and ra. Big R is always a real (static) resistance, where little r is only a dynamic resistance. And yes, the three times rule is a good rule of thumb for power output stages. For voltage amplifiers then the bigger the better.
Little known fact, coherent thought can destructively interfere with itself leaving no thought at all, that’s why I prefer incoherent thought.
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#22

Post by Dave the bass »

Right trying to apply the transconductance equations to real life using the
data sheet for a 76. http://www.nj7p.org/Tube4.php?tube=76

The data sheet says u=13.8 and that the Transconductance is
1450u(upsidedown omega) and ra is 9500Ohm

Supposing I wanted to work out the ra. Using my triangle i cover up
ra and that shows me the formula is u/gm.

So thats 13.8/1450u(upsidedown omega) which I first worked out as 0.0095172
ohms...

You'll like this next bit, my boss (having seen me with a calculator and studious expression during tea-break) explained that as the data sheet shows
the value written in 'micro' the value is actually 0.001450. Ok, so i stick that in my calculator, 13.8/0.001450 and the answer is 9517.2214ohms. Wicked! It works, data sheet says 9500, near enough innit?

Big thanks to Nick and my boss :lol:

I'm gonna work out another, I'm doing maths <proud smiley>.

DTB
Last edited by Dave the bass on Mon Jul 13, 2009 4:24 pm, edited 1 time in total.
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#23

Post by Dave the bass »

Watch this :shock:

I wanna find the gain of a 26.

Using my triangle I cover up mu (u) and that shows the formula is gm x ra.

Data sheet http://www.nj7p.org/Tube4.php?tube=26 using the values for 90V anode voltage...

gm is 935u(uypsidedownomega) and ra is 8900ohm.

Input into calculator 0.000935 x 8900= 8.3

Whahey! Bang on the nose :lol:

DTB
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#24

Post by Dave the bass »

...and again...(I'm on a roll!).

Power valve I've actually used this time, I want to work out gm of a 2A3.

http://www.nj7p.org/Tube4.php?tube=2a3

Formula, mu/ra = 4.2/800 ohm= 0.00525(upsideownomega)... now get this... note I didn't say 'u upsidedownomega'. Now then you have to convert the answer don't you? Now its about moving the decimal point isn't it, boss explains it could be described as 5.25milli (upsidedownomegas) but for the sake of continuity we'll call it 5250u(upsidedownomega).

That ok so far?

I need to lay down.

DTB
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Nick
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#25

Post by Nick »

Dave the bass wrote:Right trying to apply the transconductance equations to real life using the
data sheet for a 76. http://www.nj7p.org/Tube4.php?tube=76

You'll like this next bit, my boss (having seen me with a calculator and studious expression during tea-break) explained that as the data sheet shows
the value written in 'micro' the value is actually 0.001450. Ok, so i stick that in my calculator, 13.8/0.001450 and the answer is 9517.2214ohms. Wicked! It works, data sheet says 9500, near enough innit?

Big thanks to Nick and my boss :lol:

I'm gonna work out another, I'm doing maths <proud smiley>.

DTB
Yep, thats it, as you said, just need to get the units right, you will often see gm expressed in milli amp per volt so thats a 1000th or mmho, or micro amp per volt thats a 1000000th as in the case of the 76.

You should also be aware that the values change with the operating point, gm is highest at high current. And gm will fall as the valve ages.

And just for fun, if you connect two valves in parallel like in your amp, the gm is double, the anode resistance is half, and so the gain remains the same.
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#26

Post by Nick »

5.25milli (upsidedownomegas)
Yep, but for sanity sake, you could say 5.25 milimho or 5.25 mS

http://en.wikipedia.org/wiki/Siemens_(unit)
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#27

Post by Nick »

I can hear the sound of a dropping penny...
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#28

Post by Dave the bass »

Yesssssss!

Now then, instead of writing uupsidedownomega I should write umho yes?

mmoh <doing!> penny drop moment, that little 'm' is milli, duh!

Therefore 6797umho is = to 6.697mmho

I think.

DTB
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#29

Post by Dave the bass »

Ha! we both posted at the same time :lol:

DTB
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#30

Post by Dave the bass »

Ah! After browsing that link, I'll retype my reply...Check this...

Xmmho = XmS yes?

0.006797S= 6.797mS = 6797uS

..where the 'S' = Siemens the SI unit for conductance...which is the inverse of resistance..... which where we came in this morning innit...

Note Capital S.

If I used a lower case 's' that could be confused with seconds (a unit of time).

Valve monkey is learning :lol:

DTB
"The fat bourgeois and his doppelganger"
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