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Thread: Power pentodes as preamp tubes?

  1. #36
    Noodle of Reality Steve Conner's Avatar
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    Nice! I didn't know about the 2N5087. There is a surface mount version, the MMBT5087.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  2. #37
    Old Timer J M Fahey's Avatar
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    Being in an "European Standards" Country I use BC559C. Cheap, available and *very* good.
    The PNP has less noise than the equivalent NPN.
    Juan Manuel Fahey

  3. #38
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    Quote Originally Posted by Steve Conner View Post
    Nice! I didn't know about the 2N5087. There is a surface mount version, the MMBT5087.
    http://www.onsemi.com/pub_link/Colla...T5087LT1-D.PDF

    Go to Digikey, they are cheap. $0.22 ea.

    http://www.digikey.com/product-detai...SCT-ND/1139821
    Last edited by Alan0354; 08-22-2012 at 09:06 PM.

  4. #39
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    Quote Originally Posted by Alan0354 View Post
    You know the two you point out are medium power transistor, I went and search the data sheets in digikey and there is no noise information on either one. They are not meant to be low noise front end application.
    Yes, there are very few transistors that provide good noise information. The types I mentioned have been found to be suitable for low noise circuits through the experience and research of various manufacturers, not because it says so on the data sheet. But there are lots of electronic devices that are suitable for applications other than suggested by their data sheets.

  5. #40
    Noodle of Reality Steve Conner's Avatar
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    I went through a load of transistor datasheets looking for noise data. I just didn't happen to find the 2N5087. (Thanks Alan! ) The 2N4403 used to have noise data, but it was removed in the latest version. I don't think the BCxxx series are available in surface mount, and I want to avoid through-hole for this project.

    Merlin, my point is that if you use it for something that's not on the datasheet, you can't complain to the manufacturer when it stops working. For instance, every transistor I ever tried, the base-emitter junction makes a nice 7.5V Zener. But if I used them for that purpose in production, I'd soon be in trouble.

    Anyway, enough of our little three-legged fuses. Erm, friends rather.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  6. #41
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    Quote Originally Posted by Merlinb View Post
    Yes, there are very few transistors that provide good noise information. The types I mentioned have been found to be suitable for low noise circuits through the experience and research of various manufacturers, not because it says so on the data sheet. But there are lots of electronic devices that are suitable for applications other than suggested by their data sheets.
    The concern is at audio frequency the major problem is not thermal noise from the base spread resistance, nor even the shot noise from the current. It's the 1/f noise that is not predictable. It does not show up on every device. For example, the carbon comp resistors. The thermal noise is EXACTLY the same as the metal film. What make the carbon comp noisy is the 1/f noise. It is unpredictable, you can test a 100 device and don't see any noise and then you design into the circuit. Then it'll come and bite you late. You can pick and find individual carbon comp resistor as quiet and metal film, but you don't design calling carbon comp on your design that require low noise.

    You look at the general noise graphs, they rise at the low end, that's 1/f noise. Unless the data sheet specified for that frequency, don't take for granted even you did characterization on a few devices. Yes, if you are willing to hand pick device, it will work, if it does not have 1/f noise, then that device is good. But short of hand picking, don't count on it.

    There are plenty of transistor providing all the noise data you need to do all the designs. The two I provide have everything you need to design the low noise front end. What yours are equivalent to 12AT7 or 12AU7 in the tubes, they are medium power transistors that are not meant for low noise front end.

    It is not obvious which one to pick, it all depends on the operating conditions, your input impedance, power requirement and all. That's what the noise figure contour graph is for, showing you the noise figure at different impedance.

    If it is not because of 1/f noise, all the FETs and the group III and V advanced semi-conductor will out perform BJT hands down, it's not even close. For one, FET has very low shot noise for very obvious reason, they don't draw gate current!!! Problem is their 1/f noise is so high that it is hard to use it in audio application. The Group III and V type has 1/f noise well into MHz. But it works for RF because you use small coupling cap to block the low frequency noise. As 1/f stated, noise is inverse proportional to frequency, noisiest at low frequency.
    Last edited by Alan0354; 08-22-2012 at 09:30 PM.

  7. #42
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    Quote Originally Posted by Steve Conner View Post
    I went through a load of transistor datasheets looking for noise data. I just didn't happen to find the 2N5087. (Thanks Alan! ) The 2N4403 used to have noise data, but it was removed in the latest version. I don't think the BCxxx series are available in surface mount, and I want to avoid through-hole for this project.

    Merlin, my point is that if you use it for something that's not on the datasheet, you can't complain to the manufacturer when it stops working. For instance, every transistor I ever tried, the base-emitter junction makes a nice 7.5V Zener. But if I used them for that purpose in production, I'd soon be in trouble.

    Anyway, enough of our little three-legged fuses. Erm, friends rather.
    The BC560 that JM Fashey might be interesting. It does not provide graphs, but it specified 2dB from 30Hz to high frequency. The thing I like is the beta stay at 100 even down to Ic=0.1mA.

    http://www.fairchildsemi.com/ds/BC/BC556.pdf

    Transistors sure are more friendly to me, don't you notice the tubes are kicking my behind right now!!!?

    I agree with you 1000% even on the post before that unless it is specified in the data sheet, don't take for granted unless you are willing to hand pick device.
    Last edited by Alan0354; 08-22-2012 at 09:47 PM.

  8. #43
    Old Timer J M Fahey's Avatar
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    BC559/560 *are* very good and alive and kicking.
    The exact same die packaged in SOT23 is BC859/860
    Go to Datasheet & application note database, pdf, circuits, cross reference, data sheet, datasheets | Datasheet Archive and in the search window paste "bc859"
    Out of the many datasheets offered, a good one is:
    >>>
    BC859 Fairchild Semiconductor PNP Epitaxial Silicon Transistor
    ri

    5 pages, 58.94 Kb
    Original
    <<<
    Covers both, 859 and 860.
    I use them to build potted mini custom gain blocks ("hybrid ICs" if you wish)
    Juan Manuel Fahey

  9. #44
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    Quote Originally Posted by J M Fahey View Post
    BC559/560 *are* very good and alive and kicking.
    The exact same die packaged in SOT23 is BC859/860
    Go to Datasheet & application note database, pdf, circuits, cross reference, data sheet, datasheets | Datasheet Archive and in the search window paste "bc859"
    Out of the many datasheets offered, a good one is:
    >>>
    BC859 Fairchild Semiconductor PNP Epitaxial Silicon Transistor
    ri

    5 pages, 58.94 Kb
    Original
    <<<
    Covers both, 859 and 860.
    I use them to build potted mini custom gain blocks ("hybrid ICs" if you wish)
    I think the complementary part is BC550 which is used in EC boost. I have used this and I don't hear any difference in noise performance compare to MPSA18 in the circuit I designed. BC560 and 2N5087 is going to have a good shoot out!!!

  10. #45
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    Quote Originally Posted by Alan0354 View Post
    You look at the general noise graphs, they rise at the low end, that's 1/f noise.
    Thank you Alan, I am familiar with noise theory. I was just adding to the bulk of suggestions, not trying to out-do yours.

  11. #46
    Old Timer J M Fahey's Avatar
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    Same here
    Think of this as a collaborative effort
    Love this Forum.
    Austin likes this.
    Juan Manuel Fahey

  12. #47
    Senior Member Austin's Avatar
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    So say I configure my input pentodes as triodes by connecting the screens to the plates with 100 ohm resistors just to get it working, if I add screen supply afterwards would I then maybe have too much gain?

  13. #48
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    Quote Originally Posted by Merlinb View Post
    Thank you Alan, I am familiar with noise theory. I was just adding to the bulk of suggestions, not trying to out-do yours.
    Sorry I came out wrong. I talked about this mainly because you specifically mention base spread resistance which has nothing to do with 1/f noise that is the main concern at audio frequency.
    Last edited by Alan0354; 08-23-2012 at 05:50 PM.

  14. #49
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Austin View Post
    So say I configure my input pentodes as triodes by connecting the screens to the plates with 100 ohm resistors just to get it working, if I add screen supply afterwards would I then maybe have too much gain?
    Dear Austin.
    If it's a "Metal" amp, there is *never* such a thing as "too much gain".
    Juan Manuel Fahey

  15. #50
    Senior Member Austin's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Dear Austin.
    If it's a "Metal" amp, there is *never* such a thing as "too much gain".
    So I should stick with triode mode. Just kidding I like metal sometimes when I hit "play all" and the "random" button is ticked... I crank it up a little bit just to test out my system before hitting the "next" button.
    Last edited by Austin; 08-24-2012 at 12:42 PM.

  16. #51
    Old Timer J M Fahey's Avatar
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    Because of your nickname and Avatar, I *never* thought you were a "metal" guy
    Juan Manuel Fahey

  17. #52
    Member jbefumo's Avatar
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    A bit late to this thread, but I encountered it by wondering if I could use a 6BM8 as a preamp tube. Not trying just to be different. the 6BM8 is, to my knowledge, the only pentode/triode back in current production. I'm using a pair of them in a lunchbox amp with the triode sections (5751 Equiv, I believe) for phase inverter, and the pentode sections (EL82 equiv) in push pull, fixed bias, for around 12W power section. Have used EF86s with good results in several builds (following Merlin's guidelines closely), but obviously, pentode and triode in a single socket would be an advantage. Hence, using the 6BM8 would be advantageous if it sounds good, and can be configured so as not to stress the power supply too much. Were it not for the space issue, I would be inclined to use them in multiple spots, i.e., in the first stage for the touch sensitivity, in the OD for its driven tone.

    Joe

  18. #53
    Member jbefumo's Avatar
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    I actually have a reason for investigating this question, other than just trying to be different 8^) I've used a variation on the Dumble OD scheme (i.e., a 2-stage bypassable overdrive circuit), in which the second stage is an EF86 (configured for relatively low gain), and like the sound. A bunch of guys in another forum do something similar with triode/small-signal-pentode tubes, to reduce part count. Unfortunately, there are no such tubes in current production, and I prefer to avoid designing to obsolete components. The only current production triode-pentode I know of is the 6BM8, which is a high-mu triode and a power pentode (they work great as a 1-tube reverb-driver/recovery, btw). I guess the only way to judge the tone will be to go ahead and try, but would be interested in any contraindications before wasting time ....

  19. #54
    Senior Member Enzo's Avatar
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    Tube circuits are SOOOO lacking in criticality. Try it, even if it doesn't "work" on paper, it may well work fine in your application. If my mom liked driving around in a cement mixer, she could use one to go shopping, even if it didn't "make sense". We have working circuits with 12v on triode plates, we have TV sweep tubes in hifi amps, you can "abuse" tubes all over the place and they still function. it either sounds pleasing or it doesn't. Give yourself permission to screw around with it.

    As to obsolete components, there are plenty of small pentodes NOS that will be in large supply for years to come. Certain ones commonly used in amplifiers may have been snatched up already, but unless you are going into production, a box of say 10 of some odd type ought to be a lifetime supply for a one-off amp, obsolete or not.

    I haven't bought tubes in a while, apparently Sovtek gave up on the 7199. triode-pentode.

    Considering cost of the new 6BM8, I might trade a couple parts counts for saving $20 per tube.
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    Quote Originally Posted by jbefumo View Post
    A bunch of guys in another forum do something similar with triode/small-signal-pentode tubes, to reduce part count.
    Would you be willing to share more details - name of forum, name of thread, or even better, a direct link to relevant thread(s)? It would be much appreciated.

    I have been tinkering with 6JW8 (triode pentodes) in guitar preamps, with promising results, but there are still some issues to iron out. It would be interesting to see if anyone else is using the same valve, and if so, what sort of circuits they're building around it.

    Quote Originally Posted by jbefumo View Post
    Unfortunately, there are no such tubes in current production
    In the USA you can still get some NOS triode-pentodes for $1 USD each. Often they have much-sought-after brand names on them. But these valves were usually originally intended for use in (valve) TVs, so the Hi-Fi and guitarist crowds don't want them.

    But a small-signal triode is a small-signal triode; it doesn't matter what it was originally designed to do, it will amplify audio! And a properly used preamp tube will probably last a couple of decades, so it's pretty likely that if I have one or two in stock, they will outlast me.

    Another way to look at it: buy twenty NOS $1 triode-pentodes, and you can have twenty times as much fun experimenting as you would get from one new-production $20 valve!

    -Gnobuddy
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  21. #56
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    The 6BM8 works fine configured as a small signal amplifier. I've used this tube a fair bit in hi-fi builds as a preamp/driver tube. There's also a commercial amp from the 60s, the Telewatt V-112, that used the tube as the final stage of a preamp (pentode side) and PI (triode side).

    The real bargains are the TV versions of tubes. Here in the UK the 6BM8 is known as an ECL82. The same tube with 16v heater voltage is PCL82 and are way cheaper for Mullard/Brimar NOS.

  22. #57
    Senior Member Old Tele man's Avatar
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    ANY multi-grid tube can be used as a preamp as long as it produces gain, but there are reasons why some tubes are better than others:

    1) Multi-grid tubes tend to generate more odd-order harmonics as the number of grids increase; so triodes are preferred for lowest distortion.
    2) Multi-grid tubes can be configured to provide "adjustable" gain, triodes can't; so for amplification flexibility a multi-grid tube is preferred.
    3) Multi-grid tubes have much higher dynamic plate load values than triodes and thus present LESS loading to subsequent circuits.
    4) Multi-grid tubes behave much like constant-current sources making them easier to interface to / from.
    5) Multi-grid tubes can utilize much more of their Eb-Ib curve and thus are more efficient at power utilization.
    6) Multi-grid tubes require more support components than triodes; so triodes are simpler to use, circuit-component wise.
    Last edited by Old Tele man; 10-14-2017 at 04:19 PM.
    ...and the Devil said: "...yes, but it's a DRY heat!"

  23. #58
    Member jbefumo's Avatar
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    Quote Originally Posted by Enzo View Post
    Tube circuits are SOOOO lacking in criticality. Try it, even if it doesn't "work" on paper, it may well work fine in your application. If my mom liked driving around in a cement mixer, she could use one to go shopping, even if it didn't "make sense". We have working circuits with 12v on triode plates, we have TV sweep tubes in hifi amps, you can "abuse" tubes all over the place and they still function. it either sounds pleasing or it doesn't. Give yourself permission to screw around with it.

    As to obsolete components, there are plenty of small pentodes NOS that will be in large supply for years to come. Certain ones commonly used in amplifiers may have been snatched up already, but unless you are going into production, a box of say 10 of some odd type ought to be a lifetime supply for a one-off amp, obsolete or not.

    I haven't bought tubes in a while, apparently Sovtek gave up on the 7199. triode-pentode.

    Considering cost of the new 6BM8, I might trade a couple parts counts for saving $20 per tube.
    Well, I decided to try it in MultiSim first. I took the basic Dumble OD Special schematic for the overdrive section as a baseline (green), then repeated the same circuit, using a 6V6 for the second stage (don't have an EL82 or EL84 model, which would be closer to the 6BM8 pentode section I plan on using). The test circuit plot is shown in red. I adjusted using the load value from the 6V6 datasheet, and the various other components more or less seat-of-the-pants. From this I conclude that trying it with real components is probably justified.

    Joe

    6v6_preamp_test.jpg

  24. #59
    Old Timer J M Fahey's Avatar
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    Just as a side note: during WW2 here in Argentina we had *terrible* supply problems on a lot of stuff.
    NO car/truck tires because Japanese grabbed Far east rubber plantations, all of Brazilian production was bought in bulk by USA and UK, for "some reason" we had no access to German synthetic rubber (BUNA) made at their advanced plant in Auschwitz , so in emergency, some cars and trucks even run on wood or steel wheels, go figure.

    For the same reason, NO tubes available, neither European nor USA nor Japanese, all were fighting and had none to spare, so we being resourceful people used *any* tube available. anywhere as needed.
    I remember yellowing Electronics magazines saying so, specially that any tube could be used, except rectifiers.
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  25. #60
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    I guess at a pinch an output tube could also be pressed into service as a rectifier. You'd need two for full-wave, though one of my contacts has some dual 30W triodes that would do nicely.
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  26. #61
    Old Timer J M Fahey's Avatar
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    Going further with the thread derailing, this is what WW2 soldiers in their foxholes and even concentration camp inmates used when needing an "audio diode" and Mouser was still 40 years into the future: enter the mighty razorblade_and_pencil diode



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    Juan Manuel Fahey

  27. #62
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    Quote Originally Posted by Old Tele man View Post
    <snip>
    3) Multi-grid tubes have much higher dynamic plate load values than triodes and thus present LESS loading to subsequent circuits.
    That should be the other way around - they are more (not less) heavily loaded by subsequent circuits (because, as you pointed out, they have higher internal anode resistance, and behave more like current sources).

    -Gnobuddy

  28. #63
    Senior Member Old Tele man's Avatar
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    Parallel a 100KΩ input load with a 2KΩ triode output and you get something less than 2KΩ upon the input load.

    Parallel that same 100KΩ input load with a 33KΩ pentode output and you get about 25KΩ...more than 10-times LESS loading upon the input load.

    It's easier to generate a voltage GAIN across a larger resistance/impedance than it is across a lower resistance/impedance, because it requires less current to create that GAIN.
    ...and the Devil said: "...yes, but it's a DRY heat!"

  29. #64
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    Quote Originally Posted by Old Tele man View Post
    Parallel a 100KΩ input load with a 2KΩ triode output and you get something less than 2KΩ upon the input load.
    We may have a little terminology confusion going on. Loading is about the output impedance of a driving stage acting as a voltage divider with the input resistance of a subsequent stage. If the voltage is substantially lowered, that's heavy loading. If the voltage is barely affected, that's light loading.

    Parallel a 100k input load with a 2k triode output, and you get 98% of the signal voltage making it through the voltage divider - this is called very little loading (only 2% of the voltage is lost). The load (the subsequent stage) has very little effect on the output voltage of the preceding (triode) stage.

    Quote Originally Posted by Old Tele man View Post
    Parallel that same 100KΩ input load with a 33KΩ pentode output and you get about 25KΩ...more than 10-times LESS loading upon the input load.
    Load a 33k pentode output with a 100k input load (subsequent stage input resistance), and only 75% of the output voltage makes it to the subsequent stage. About 25% of the voltage is lost. This is much heavier loading.

    Quote Originally Posted by Old Tele man View Post
    It's easier to generate a voltage GAIN across a larger resistance/impedance than it is across a lower resistance/impedance, because it requires less current to create that GAIN.
    Quite true, but how is that relevant to the question of loading caused by input and output impedances?

    -Gnobuddy

  30. #65
    Senior Member Old Tele man's Avatar
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    "Following" circuits are not always amplifying tubes, quite often they're RL/RC-tone shaping circuits, which attenuate passing signals QUITE a bit, so more loss means less output to subsequent circuitry, which has to MAKE UP (and usually exceed) those losses. A good designer (employed) uses as few components as necessary because of cost.
    ...and the Devil said: "...yes, but it's a DRY heat!"

  31. #66
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    It seems topsy turvy to describe outputs as loading inputs?

  32. #67
    Senior Member Old Tele man's Avatar
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    Loading is NOT a unilateral event, it goes BOTH directions.
    ...and the Devil said: "...yes, but it's a DRY heat!"

  33. #68
    Old Timer J M Fahey's Avatar
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    Sorry, still not the best description of whatīs going on.
    Parallel a 100KΩ input load with a 2KΩ triode output and you get something less than 2KΩ upon the input load.
    You mean the triode (generator) has 2k internal impedance.
    It is driving the next stage, which has a 100k grid resistor.
    Also applies to passive elements, next stage might have no tube whatsoever but consist of just a 100k resistor (passive load) to ground and nothing else.
    Or a capacitor or inductor or any combination which, at least at that frequency, shows 100k impedance.
    Any of these is loading the triode (generator) which has 2k internal impedance so in principle itīs lightly loaded.

    The 2k is not "loading" anything, itīs the generator internal impedance, and definitely is not loading itself.

    Parallel that same 100KΩ input load with a 33KΩ pentode output and you get about 25KΩ...
    Again, the 25k pentode internal impedance is not loading itself and can not be called "a load".

    To be more precise:
    * "a load" goes from generator output to ground.
    * "internal impedance" is in series with generator output.
    Clearly these two concepts are very different.
    more than 10-times LESS loading upon the input load.
    how can you load a load?
    A load loads a generator.

    If you put 2 loads in parallel (such as 2 speakers in parallel) would you say one is loading the other?
    Last edited by J M Fahey; 10-15-2017 at 05:45 PM.
    Juan Manuel Fahey

  34. #69
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    Quote Originally Posted by Old Tele man View Post
    Loading is NOT a unilateral event, it goes BOTH directions.
    Wow, that’s an unusual take on things🤔
    Is the hill forcing the cyclist to pedal harder, or is the hard pedalling creating the inline?
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  35. #70
    Senior Member Old Tele man's Avatar
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    The control-grid of a triode is 'normally' a HIGH impedance, so it presents a HIGH-Z load to whatever is sourcing/driving it. But, as soon as that grid goes positive enough to draw current (even slightly) it promptly becomes a LOW-Z load back to what is driving it...it has loaded its source beyond what it was originally intended to do.

    As investigators like to say: "...Follow the current (instead of money)..."
    Last edited by Old Tele man; 10-14-2017 at 10:25 PM.
    ...and the Devil said: "...yes, but it's a DRY heat!"

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