Grid leak bias on lower MU tubes? pentodes? other grid leak bias questions? - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: August 03, 2022, 11:22:39 am
content: I still don't quite understand how grid leak bias works, some articles say it's the control grid being heated by the cathode and emitting some electrons that makes it negative.... but I'd think it would do the opposite; make it positive.others talk about secondary emissions and stuff like that, that seems more promising.of course a popular explanation is that grid current is what makes the grid more negative... I don't think this is quite so either.., since the operating point would constantly change a lot, and I just don't see that happening in grid leak biased stages. and if this were true, that would have the same effect on lower MU triodes, it would always settle at a point where grid current is just starting to appear.and I read on many articles about this that only high MU tubes can be grid leak biased, with MU above around 60... why would MU impact the grid leak biasing effectiveness? unless it is secondary emissions that pull the grid low, I just don't see how this could be true.what about pentodes? and power tubes? why is it when you don't have a ground reference to an output tube, it red plates instead of going into cutoff?oh and of course I know there's better biasing options for tubes.

author: trobbins
author_rank: Frequent Contributor
timestamp: August 03, 2022, 12:03:50 pm
content: There are a few identified processes that lead to grid current - some counteract others - some vary depending on grid voltage - so it is a net outcome. If you were really keen on exploring details then I'd recommend working your way through the 1962 RCA book titled Electron Tube Design (eg. athttps://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_16/Sec_51/4396_Electron_Tube_Design.pdf).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 03, 2022, 12:54:24 pm
content: Mind that several of your questions are angled towards tube design, which isn't going to be very applicable to circuits built with already-designed tubes.I don't know that grid leak biasing has been of any importance in recent history (as little history as that covers: i.e. the few manufacturers in guitar and hi-fi, and few remaining industrial uses). AFAIK, the main effect is it saves one or two components, and may provide some signal compression (which might be of interest in some circuits, but mainly introduces distortion).It basically means slightly less than a zero-bias condition, which will draw uncomfortably high power in anything but a high-mu type. There are zero-bias class-B transmitter types, for example, which tend to have fairly high mu to achieve that.It's not like MOSFETs, where the threshold voltage can be adjusted by gate metal choice, or ion implantation. Tubes just are what they are. The easiest way to adjust the grid cutoff voltage is by varying mu (grid wire spacing).Physically, the effect is electron capture by the grid. The space charge has a somewhat lower potential than the cathode, because electrons are emitted with extra thermal energy. And this can be sensed by measuring the voltage on a floating grid, particularly while carrying beam current (screen/plate biased up).Grid emission does indeed result in a positive voltage there, causing a runaway condition particularly when the grid-leak resistor is very high. So it's a prime failure mode of this approach.Since grid rectification also biases the grid, it can be used as a "DC restore" function with an input signal. This causes bias current to drop with rising signal amplitude, reducing average gm and thus causing compression (and distortion, particularly as it moves deeper into cutoff with still higher input amplitude).This can indeed be required operation, for TV video amps and sync separators for example. Or it might not be of any consequence: the compression and harmonic distortion are mostly irrelevant to tuned RF stages for example, where the input amplitude may be consistent already (AGC), or indeed where compression is desired (FM IF stages).Mostly, I imagine it's a cost saving measure (muntzing).Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 03, 2022, 01:17:08 pm
content: Grid leak bias (whatever its underlying mechanism) results in electron flow to grid which is <<< Ia.Actual data here for grid leak biased 12AX7/ECC83 : Va = 90 V , Ia = 0.45mA Rg = 3.3M, Vg = -0.9 V, so Ig_leak =272 nAIt must have been stable because that method was commonly used.The grid AC signal varies +/- about 0.9 V so this method should be OK for signal level up to -40 dBu or more.If using cathode bias, combined with a high DC impedance from grid to ground, the bias will be the sum of cathode + grid leak.When the load line is correct, the triode has a usable linear range of dIa/dVg.However the class A pentode is not so linear, that is, the dIa/dVg varies between the + and - signal excursions.For pentode this could be partially corrected by cathode feedback of an undecoupled cathode bias resistor.Alternatively, for 6AQ5 Class A into a speaker transfomer, sometimes feedback was from the transformer load side.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 03, 2022, 01:55:56 pm
content: Indeed, I recall once testing a 6V6, probably at 100 or 200V supply, with grid open; came out to -2 or -3V, something like that? So, it was getting pretty toasty. Only did that long enough to get the measurement.Tim

author: TimFox
author_rank: Super Contributor
timestamp: August 03, 2022, 01:58:55 pm
content: There are two different applications called "grid-leak" bias. Both involve charging the input capacitor due to grid current.1. In your small-signal case, with a high-mu triode and grounded cathode, if you connect the grid to ground through a high resistance and couple the input through a capacitor, the natural grid current due to the electrons boiled off the cathode having a substantial kinetic energy will flow through that resistor and produce a negative voltage on the grid. The resulting DC voltage will reach equilibrium when it is sufficiently negative to repel further electrons, at very roughly -1 V, corresponding to the data you quoted. This works for high-mu triodes, which require low bias voltages. The cut-off grid voltage for a triode is approximately -(plate voltage)/(mu).2. In large-signal cases, including oscillators, the large AC voltage on the grid is sufficient to draw large grid current through the grid-cathode diode, which flows through the grid resistor to make a large DC voltage. The grid voltage then swings from slightly positive (to make a large grid current) down to very negative. This is used in class-C circuits, which are useless for audio, but good for oscillators such as the local oscillator in a superheterodyne receiver.Incidentally, when designing audio amplifiers you normally need sufficient negative grid voltage on each stage to minimize grid current since it is a non-linear load on the previous tube or input. Consult the usual textbooks about grid current, which is a sum of (1) the above current due to energetic electrons from the cathode, (2) diode current due to positive grid bias, (3) current due to ions produced in the vacuum, and (4) grid photoelectrons from soft x rays produced in the tube.

author: TimFox
author_rank: Super Contributor
timestamp: August 03, 2022, 03:46:28 pm
content: Quote from: ELS122 on August 03, 2022, 03:18:52 pm(1) refers to operation (like your example) with the grid connected to ground through a resistor. Since the cathode is very hot, electrons emitted thermally flow to the grid.(2) refers to biasing the grid positive so that it conducts like a normal diode, not resulting from the cathode temperature and the electron kinetic energy.In the pentode case, you can approximate the cathode, grid 1, and screen as a triode. Grounding the screen then cuts off the cathode current, but a positive grid 1 voltage will still conduct cathode current to the grid. There is a finite mu for that screen triode.For an ideal triode (where the field at the cathode is uniform, not location-sensitive), the Child-Langmuir law for space-charge limited cathode current isI=Gx [(Vp+muVg)/(mu+1)]3/2Note that this implies my statement about cut-off voltage, whereI=0 forVg= -Vp/(mu).In real triodes, mu is not exactly constant. In remote-cutoff tubes, it varies with grid voltage by design, using a grid with a non-constant winding pitch.For the space-charge limited diodeI=Gx (Vp)3/2The constantsG(perveance) and mu (amplification factor) are functions of the tube geometry (areas, spacing, grid shielding of cathode, etc.) and are constant for ideal devices so long as the cathode temperature is high enough and the plate voltage allows a space charge limit to happen.

author: TimFox
author_rank: Super Contributor
timestamp: August 03, 2022, 04:08:41 pm
content: An example of measured grid current vs. grid-cathode voltage can be found inhttps://www.researchgate.net/figure/Measured-grid-current-g-I-of-12AX7-in-wide-range-of-grid-to-cathode-voltages-gk-V-in_fig4_290534843

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 03, 2022, 04:46:46 pm
content: Quote from: TimFox on August 03, 2022, 03:46:28 pmWell, same thing. You can't have thermionic conduction without, well, thermionic emission, and the effect of the temperature difference is simply that there's a built-in voltage there. The energy spectrum means it's never one or the other, it's always a combination of both.Keep in mind, C-L itself is wrong, in that it assumes initial zero energy. The effect of nonzero energy is to smear out the transfer function (whether that be in terms of anode or grid voltages), I suppose more or less the convolution of C-L with the thermal spectrum? In any case, an assumption very suspect indeed, at these voltages and currents!The direct consequence is that, as with all other known transisting devices -- there is an exponential tail, ultimately arising from Maxwell-Boltzmann statistics, i.e. these devices all depend on random motion of hot (for varying degrees of "hot"!) charge carriers. Whereas the BJT is pretty much exponential up to high-level injection, and the FET goes quadratic at high levels, the tube goes 3/2 power at high levels. (Which I believe is a consequence of the ballistic transport? So can potentially be seen in certain semiconductor materials under suitable conditions.)Here's a model of 6AL5 based on my own measurements:Note the (anomalously?) high N factor, more or less corresponding to cathode temperature; and the built-in offset. The nonlinear source models C-L, with a somewhat different exponent in this case, presumably from the cylindrical geometry.Tim

author: TimFox
author_rank: Super Contributor
timestamp: August 03, 2022, 05:02:30 pm
content: I only separated (1) and (2) due to their different dependence on voltages and cathode temperature.Another detail: the grid voltage in the "grid-leak-biased" case is sometimes called "contact potential", but that is a misnomer.From the abstract of an old Proc. IRE paper (1936): "The term "contact potential," when used in connection with thermionic vacuum tubes, has come to include certain combinations of spurious voltages which affect the operation of the tube. The combination which is effective depends upon the method of measurement used or upon the application to which the tube is put. The combinations which are effective are grouped under four heads: (1) floating element potential, (2) floating, shunted element potential, (3) effective current cutoff, and (4) calculated correction potential. The first three are points upon the current-voltage characteristic of the element while the fourth is the correction term appearing in the equation giving plate current as a function of the applied voltages. The true nature of contact potential is also pointed out."In modern usage, contact potential is the potential generated by the contact of two dissimilar materials in air or in a vacuum, due to solid-state physics of the two materials..

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 03, 2022, 05:04:03 pm
content: I can only contribute this educative linkhttp://www.john-a-harper.com/tubes201/

author: TimFox
author_rank: Super Contributor
timestamp: August 03, 2022, 08:11:11 pm
content: There won't be much space charge, but the grid current caused by thermally-emitted electrons leaving the cathode (with finite kinetic energy) would still flow.Space charge is one of the (limiting) operating conditions of a vacuum tube (independent of cathode temperature in that limit), the other common limit being "saturated emission" (where all electrons emitted flow immediately to the plate and no space charge cloud forms) where the current is a function of the cathode temperature and plate voltage.The space charge is a limit to the current, but is not necessary for current flow.As pointed out above, the Child-Langmuir law, which assumes zero kinetic energy at the cathode and a full space charge cloud that reduces the field at the cathode to zero, is not exact, but it is very useful approximation, especially when the effect of a partially-shielding grid is added. I refer you to the textbooks to see the interesting derivation of the 3/2 power in the law.Your example is equivalent to a 6AL5 vacuum diode with full operating cathode temperature and a resistor between plate and grid: there will be a measurable plate-cathode voltage from the emitted electrons.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 03, 2022, 09:11:02 pm
content: Heh... it should indeed be possible to construct an oscillator, on heater power alone.Since the currents are pitiful, the impedances will need to be quite high, megohms perhaps; but this should be achievable with a plain old Hartley oscillator say, with a ferrite core and a fair number of turns.Analogous to those thermocouple generators using JFET oscillator for step-up, but putting the junction into the amplifying device itself.Tim

author: trobbins
author_rank: Frequent Contributor
timestamp: August 04, 2022, 12:01:09 am
content: Nov 1955 Radiotronics is another referencehttps://frank.pocnet.net/other/AWV_Radiotronics/index.html