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Old 17th October 2010, 05:08 PM   #111
JohanB is offline JohanB  Sweden
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Quote:
Originally Posted by piano3 View Post
That is tremendous work JohanB! Many thanks for that and I hope many members read it. I had estimated by ear on 75 microsecond RIAA stage that the values for these triode connected pentodes had to be in this region though I had not suspected quite as high as 5.6 pF!


your last post was while I was writing this-yes that seems more believable!

Yes, the g1 to anode capacitance is only some 30-50 micro-pico-farads when the shildings are grounded, so anode and g3 should not be measured together with g2 for capacitance. It is the stray capacitance between the shildings inside that is left floating, that makes the higher readings when the anodes (and g3) are included.

By the way, do you have problems with microphonics with these tubes?

Many people say that they are, but I suspect there in many cases are VHF self-oscillation, that's causing the exaggerated microphonic rumors......
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Old 17th October 2010, 05:18 PM   #112
Salas is offline Salas  Greece
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Thanks for the work Johan. They need Rg stopper in most cases, which works against best noise in a head amp to a point. Kinda prone to oscillation sometimes, yes. When cranked for gain with much Miller at play you know how it goes when stray inductances are meeting the grid.
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Old 17th October 2010, 06:28 PM   #113
piano3 is offline piano3  United Kingdom
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I used trioded E180F in a mic preamp and you would certainly get a nice silvery shimmer if you banged the case but in actual use there was never a problem. I am currently using trioded E810F(and I can drop in E180F,E280F or EF184 as I have switchable capacitance in the HF EQ stage) in the second stage of my phono amp without any problems whatsoever-you have to really hit the case near the valves to get any ringing at all. I always use grid stoppers with these kind of valves.
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Old 13th December 2013, 02:52 AM   #114
popilin is offline popilin  Argentina
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Hi DF96

Amazing thread by the way.

Quote:
Originally Posted by DF96 View Post
No, given the same valve and bias a cascode has exactly the same gm as common-cathode. The difference in gain comes from the different output arrangements. The cascode largely isolates the lower valve from anode feedback, so the effective mu is raised to pentode-like figures. The raw signal current and noise current generated by the lower triode are the same as for common-cathode, but there is no anode feedback to then reduce them. The signal-noise ratio stays the same, except for a little extra noise from the upper triode - usually negligible.
As usual I'm a bit confused about noise, especially that produced by the upper triode of the cascode, since its common grid connection, I'm confused about how to refered noise, triode noise usualy is refered to grid.
The gain of the upper triode is

G2=(μ+1)RL/(rp+RL)

For the bottom triode, I have an expresion that is unwritable, because of index notation (Herr Vogel), but clearly

│G2│>│G1│

So, what about upper triode's noise ?
Why is usually negligible ?

I'm struggling with a hybrid cascode, and can not find answers, neither Morgan Jones says anything about it.

Thanks in advance.
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Old 13th December 2013, 07:54 AM   #115
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by popilin View Post
So, what about upper triode's noise ?
Why is usually negligible ?

I'm struggling with a hybrid cascode, and can not find answers, neither
It is easier to solve noise problems like this by considering the noise current generated in the tube, rather than trying to convert it to EIN and then working out different voltage gains and adding etc etc... Having said that, the noise formulae for a cascode are fairly involved whichever way you do it, so simplify:

Assuming the two device both identical and generate the same noise current in the anode, for the upper device to contribute negligibly to the total, the lower triode needs a voltage gain of perhaps x3 or more. Provided this is the case, the lower triode contributes three times more (or more!) noise than the upper. And we all know that when adding uncorrelated noise sources, the dominant sounce is always VERY dominant:
sqrt(32 + 12) = 3.16 (i.e. only 0.5dB worse)

The EIN is therefore roughly equal to the EIN of the lower triode on its own. Of course, if the lower triode happens to have less than x3 gain then you can't make this simplification any more, and things get rather awkward.

The lower triode works into the internal cathode impedance of the upper triode, which is
rk = (Ra+ra) / mu
You can use this figure to work out the voltage gain of the lower triode using the usual formula for a common cathode gain stage.

With a hybrid cascode it gets even more tricky, because the two devices are unlikely to generate equal noise currents. If the lower device is a FET then it will probably generate less noise than the upper (triode), in which case the total noise may well end up worse for the cascode than if the FET were arranged as a common-source amplifier with plenty of gain. For example, if the lower device generates 4 times less noise current than the upper, then it will need to acheive a voltage gain of 4 x 3 = 12 to dominate the total noise. Possible with a BJT, but perhaps not with a JFET?

Last edited by Merlinb; 13th December 2013 at 08:03 AM.
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Old 13th December 2013, 01:15 PM   #116
Salas is offline Salas  Greece
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Merlinb, excellent summary.
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Old 13th December 2013, 01:45 PM   #117
popilin is offline popilin  Argentina
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Hi Merlinb, thanks for your reply !

For more clarity, unifying notation, lets asume that the upper triode is 2 and the lower triode is 1, asuming also both are identical, for the unbypassed version of the cascode, voltage gains are

G2=(μ+1)Ra/(ra+Ra)

G1= - μ rk/[ra+rk+(μ+1)Rk]

With

rk=(ra+Ra)/(μ+1)

Clearly

│G2│>│G1│

So I don't understand your numbers │G2│=1, │G1│=3

Please bear with me, I think slowly...
Thanks again.
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Old 13th December 2013, 07:36 PM   #118
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by popilin View Post
G2=(μ+1)Ra/(ra+Ra)

G1= - μ rk/[ra+rk+(μ+1)Rk]

With

rk=(ra+Ra)/(μ+1)

Clearly

│G2│>│G1│

So I don't understand your numbers │G2│=1, │G1│=3
The numbers I wrote are not the gains of the triodes, it was just meant to illustrate that if one noise source is three times larger than another, then the total is only 0.5dB worse than with the noisiest source alone.

Another way to think of it is to imagine that the EIN the triodes is, say, 1uV each. If the lower triode has a gain of 3 then its EIN is amplified to produce 3uV of noise at its anode. The upper triode then adds its 1uV to the mix, making 3.16uV (basically the same as the lower triode alone).
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Old 13th December 2013, 07:42 PM   #119
Salas is offline Salas  Greece
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Also at least 1/3 the overall gain must be coming from its head amp to dominate any full preamp's noise performance.
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Old 13th December 2013, 07:47 PM   #120
popilin is offline popilin  Argentina
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Quote:
Originally Posted by Merlinb View Post
The numbers I wrote are not the gains of the triodes, it was just meant to illustrate that if one noise source is three times larger than another, then the total is only 0.5dB worse than with the noisiest source alone.

Another way to think of it is to imagine that the EIN the triodes is, say, 1uV each. If the lower triode has a gain of 3 then its EIN is amplified to produce 3uV of noise at its anode. The upper triode then adds its 1uV to the mix, making 3.16uV (basically the same as the lower triode alone).
Ahh, now I understand (I hope), thanks for clarify.
That means the common grid stage only contributes its own noise regardless its gain.(?)

In addition that I think slowly, I'm a dumb.

Thanks a lot
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