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!
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.
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.
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?
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?
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.mm7 said:
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?
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?
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You've measured grid current across a 1 K Ohm resistor? Quite a trick.
What DF96 has already tried to point out is that this is an area where simple general models break down. Is there a compelling reason to operate so close to zero bias? If not, then you should probably don't.
All good fortune,
Chris
What DF96 has already tried to point out is that this is an area where simple general models break down. Is there a compelling reason to operate so close to zero bias? If not, then you should probably don't.
All good fortune,
Chris
LTSpice does not show current through a wire. only through a resistor.
Well, I've put another 1uOhm resistor from grid to earth, LTSpice shows same 1nA current through it. So I assume it is grid current.
No, there is no reason to operate close to 0 bias.
I just found it by tweaking, tries and errors, trying to find a point of lowest THD. I can make bias higher, but THD will be higher.
Well, I've put another 1uOhm resistor from grid to earth, LTSpice shows same 1nA current through it. So I assume it is grid current.
No, there is no reason to operate close to 0 bias.
I just found it by tweaking, tries and errors, trying to find a point of lowest THD. I can make bias higher, but THD will be higher.
Grid current at small negative voltages is mainly electrons which have been emitted from the cathode with sufficient thermal energy to overcome the repulsion of the cathode space charge and the grid potential. Essentially, the grid-cathode is acting as a tiny thermionic generator. SPICE models vary in how well they approximate this, but as I said it varies considerably from one valve sample to another so you can't rely on a simulation - except to warn you that it might be taking place. Some SPICE models omit grid current altogether, or perhaps only offer the much larger current when the grid is forward biased. You need to check the valve model you are using.
Measuring grid current is not always easy. Best to measure the voltage across the grid leak resistor, but remember to include the parallel resistance of your DMM too. At around -200mV grid bias I would expect at least several uA of grid current, so giving an impedance in the 100's of k region. As this is non-linear it can do harm unless your source impedance is much lower.
Measuring grid current is not always easy. Best to measure the voltage across the grid leak resistor, but remember to include the parallel resistance of your DMM too. At around -200mV grid bias I would expect at least several uA of grid current, so giving an impedance in the 100's of k region. As this is non-linear it can do harm unless your source impedance is much lower.
Running the first stage with as least distortion as possible is always the quest. Cathode biasing is where the battle ground usually resides. Cathode bypassing or just plain resistors have their pros and cons. Ideally we all want nothing to be in the way of things.
What are your guys knowledge on the 5755 tube? The phono dude design boasts this tube at the first stage for its qualities. It is capable of running close to zero volts bias? How acceptable is this?
PhonoDude PCB Version
What are your guys knowledge on the 5755 tube? The phono dude design boasts this tube at the first stage for its qualities. It is capable of running close to zero volts bias? How acceptable is this?
PhonoDude PCB Version
actually, look at this page:
PhonoDude Hardwired Version
Scroll to the bottom section "Measurements". He measures 51.3dB of gain over all (without an input transformer.) He also records that the hum measurement is -62dB versus 1 Volt of signal. Or
"max output of the PhonoDude is roughly 3 Volt, so a meaningful Humm to signal ratio would then be ~72dB, which is a very good spec for a Phono amplifier !!!"
Is it possible to compare his data with the white cathode design we have here? (especially with the 5755) Improvements?
PhonoDude Hardwired Version
Scroll to the bottom section "Measurements". He measures 51.3dB of gain over all (without an input transformer.) He also records that the hum measurement is -62dB versus 1 Volt of signal. Or
"max output of the PhonoDude is roughly 3 Volt, so a meaningful Humm to signal ratio would then be ~72dB, which is a very good spec for a Phono amplifier !!!"
Is it possible to compare his data with the white cathode design we have here? (especially with the 5755) Improvements?
Alternate way of getting grid current numbers, in case your DVM causes too much measurement error or makes a tube oscillate when touching the grid pin (common for high gm tubes): Use a CCS on the plate, measure plate voltage at two or three different grid resistance values. Divide by mu and you have the grid voltage change.
The SPICE model I am using is Koren improved model that is described at Improved vacuum tube models for SPICE, Part 1 chapter "New Equations". From which, I believe, that Koren paid big attention to this problem.
From graphs there I see that GC should be around 2mA at Vp=125V and Vg=-200mV. And with Vg=-1.5mV and more it will be close to 0.
I am just amazed why SPICE model shows different result. I'll double check that it is really new model is used.
So, from all of you guys wrote and from Koren's article, I see that it would be better to rise grid bias to something like -1.5VDC - just to be on a safe side when using different tube exemplars.
From graphs there I see that GC should be around 2mA at Vp=125V and Vg=-200mV. And with Vg=-1.5mV and more it will be close to 0.
I am just amazed why SPICE model shows different result. I'll double check that it is really new model is used.
So, from all of you guys wrote and from Koren's article, I see that it would be better to rise grid bias to something like -1.5VDC - just to be on a safe side when using different tube exemplars.
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