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Old 12th June 2013, 04:21 PM   #161
mm7 is offline mm7  Canada
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Quote:
Originally Posted by DF96 View Post
The impedance you need to worry about is not R19 but the grid input impedance of the valve. This is because you are running well inside the grid current regime. Unlike R19 this impedance is nonlinear. It is conceivable that your low distortion achieved in simulation partly arises from cancellation of 2nd arising from two sources: normal valve curvature, and grid current distortion (these pull in opposite directions, but have quite different shapes). If so, you will see a rapid rise in higher orders as soon as you raise the signal level.
Yes, I suspected that 2-nd is cancelled by the topology. I believe that level I simulated with (3.7mV) is high enough. May be too high. It gives 6VAC p-p output that is even higher than German standard, so I added switching of input transformer to lower ratios 1:5 and 1:2.5 and maybe total bypass of the trannie.
And added output attenuator as well.
Means I expect rather lower input levels than this I simulated with.

I read that input impedance of tubes is virtually infinite, so a resistor should be added that will create some Z and some damping for a mic. (definitely I need t read more about grid impedance.)

Sorry I am kind of newbie in tubes. Could you say what could/should I do to improve the schematic?

Thanks!
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Old 12th June 2013, 04:56 PM   #162
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I have done an amp or two with AC heaters that are elevated center tapped and they appear to be dead silent for the level I was working with. Headphone amp and preamp. The biggest boo-boo was the transformer distance. They were EI transformers and they definitely put out an audible hum field and have to put it at least a foot away from the signal work. I wonder if Toroids are nowhere near as noisy at half the distance.

I was wondering on this Mu + White design why the two stages are DC coupled? is that a potential danger?

I too thought about the input impedance produced by the grid of a tube. I always thought it was the resistors in front of them that defined the load. I too need to do some reading.
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Old 12th June 2013, 05:28 PM   #163
Marik is offline Marik  United States
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Quote:
Originally Posted by mm7 View Post
Ideally all ratios, so I could calculate which is the best for my case.
But for me currently most important is 1:37 or close.
Thanks!
We have 1:36. Pri L~3mH, Sec L~3.8H
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Old 12th June 2013, 05:33 PM   #164
mm7 is offline mm7  Canada
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Quote:
Originally Posted by woodrough View Post
I have done an amp or two with AC heaters that are elevated center tapped and they appear to be dead silent for the level I was working with. Headphone amp and preamp. The biggest boo-boo was the transformer distance. They were EI transformers and they definitely put out an audible hum field and have to put it at least a foot away from the signal work. I wonder if Toroids are nowhere near as noisy at half the distance.

I was wondering on this Mu + White design why the two stages are DC coupled? is that a potential danger?

I too thought about the input impedance produced by the grid of a tube. I always thought it was the resistors in front of them that defined the load. I too need to do some reading.
Hi woodrough!
Nice to hear one more confirmation that AC heating is not noisy.

Stages are DC-ed because, if you look at the last schematic I've attached, you'll see that DC voltage of grid of WCF upper tube is already elevated, basically it is a voltage of cathode of MuF upper tube. And cathode of WCF upper tube is biased accordingly. So the DC voltage between the stages does not require use of a cap. Even opposite, it requires no use of a cap, otherwise WCF upper cathode will be overbiased. It is my understanding.
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Old 12th June 2013, 06:38 PM   #165
mm7 is offline mm7  Canada
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Regarding input capacitance.
to DF96
Given that:
- Total capacitance (C=Cmiller+Cgc) of 12AX7 is about 200pF;
- DC resistance of two secondaries in serial of input transf LL1935 (R) is 1300Ohm,
Then -3dB cutoff should start from

f = 1/(2*Pi*R*C) = 506kHz

even if we add 47pF of RF filter, it will be 400kHz.

Is my calculation correct?

If so, why should we bother about input capacitance when we have so low source resistance?
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Old 12th June 2013, 06:41 PM   #166
DF96 is offline DF96  England
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Quote:
Originally Posted by mm7
I read that input impedance of tubes is virtually infinite, so a resistor should be added that will create some Z and some damping for a mic. (definitely I need t read more about grid impedance.)

Sorry I am kind of newbie in tubes. Could you say what could/should I do to improve the schematic?
Valves only have almost infinite grid input impedance when the grid bias is sufficiently negative to avoid grid current. This typically requires at least -1V, but in some cases it could be as much as -1.5V. RDH4 is about the best reference I have seen. Simply Googling 'tube grid current' just gives a lot of pages about Class AB2 or C so is not very helpful.
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Old 12th June 2013, 07:57 PM   #167
mm7 is offline mm7  Canada
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Quote:
Originally Posted by DF96 View Post
Valves only have almost infinite grid input impedance when the grid bias is sufficiently negative to avoid grid current. This typically requires at least -1V, but in some cases it could be as much as -1.5V. RDH4 is about the best reference I have seen. Simply Googling 'tube grid current' just gives a lot of pages about Class AB2 or C so is not very helpful.
Unfortunately I do not have RDH4 .
If tube U1 cathode has 227mV bias, that means grid is -223mV. Will it be grid current there? Is it a current through cathode resistor?
How to find impedance for this value?
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Old 12th June 2013, 10:02 PM   #168
DF96 is offline DF96  England
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I think RDH4 is available online somewhere. Grid current is current through the grid. You can't calculate it as it varies from one valve to another, and also changes with valve age. You can either measure it or avoid it.
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Old 12th June 2013, 11:33 PM   #169
mm7 is offline mm7  Canada
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Quote:
Originally Posted by DF96 View Post
I think RDH4 is available online somewhere. Grid current is current through the grid. You can't calculate it as it varies from one valve to another, and also changes with valve age. You can either measure it or avoid it.
yes, I've found RDH4, thank you for the hint,I read about grid impedance and admittance and understood ... that I understand almost nothing.
Though I understand what it is about, and can visualize "flows of electrons" flying between electrodes, and resistances and reactances, still it is very hard to grasp all these formulas.
I just hope all this complexity is included in SPICE models of tubes, and I can rely on results of simulations.

>Grid current is current through the grid.
This is confusing. Is it current from grid to cathode? or opposite? or to anode? or to external circuit? Or current of electrons flying from cathode to anode through grid?
I understood it as a current from cathode to grid. (well, opposite. electrons from C to G, current from G to C). Am I right?
But, if grid is -230mV below the cathode how electrons come to it? Are these electrons that were attracted by anode and hit into grid wires on their way? Or these are some fastest electrons that fly high on top of e-cloud? Or actually both?
If so, yes, this will somewhat reduce resistance between cathode and grid. But how much? Is it magnitude of kOhms?
I believe it should be a function between Vp, Vg, cathode temperature and distances between anode, grid and cathode. Latter two should be constants (well, almost) from a tube model. Vg and Vp are known from schematic. So we can calculate this conductance for an average 12AX7. But may be it is already done by SPICE?
Is this conductance included into tube SPICE model?
Is "grid current" equal to current through grid leak resistor?

Last edited by mm7; 12th June 2013 at 11:39 PM.
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Old 13th June 2013, 12:08 AM   #170
mm7 is offline mm7  Canada
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From cathode voltage minus grid voltage, 694mV, and current via closest resistor R20, 1nA, I've found that grid resistance is 691.4MegOhm.
It is 532000 times larger secondary of input transf DCR that is 1.3K.
Is my calculation correct?
If yes should I bother about this grid current?
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