I was thinking that the resistor would be very high in value, causing a grid-leak situation, keeping the output grid negative, with the cathode of the 1st stage modulating that negative bias. However, at 20K I can't see the grid developing a negative bias, but, rather, is probably biased positive. Interesting. Not sure how it can amplify in a linear manner, but it does sound good!
I have read about this tube elsewhere, and remember reading that the output grid does in fact run positive. I've never built and/or run across an amplifier like this where the two triodes are separate tubes, so I would not be able to measure the voltage personally.
At any rate, if you were ever curious as to the resistor's value, that's what it is. I read on another site that some tubes call for a 30K resistor.
T.
The broken tube has a good resistor. Wonder how many tubes in use have open resistors? Too bad we can't check them until a tube is broken.
You are right. The tube does operate with output grid going positive. Some radios use a 6P5/6J5 driving a 6AC5. These two tubes are similar to 6N6 or 6B5.
Both 6B5 and 6N6 can be sub'd. A 41/42 will work as 6B5 and 6F6/6K6/6V6 as a 6N6 by adding a resistor in series with cathode. Resistor should be bypassed by a 10-25mf cap.
Norm
:A LONG time ago I bought some 6N6G tubes, and one broke in the mail. I've always been curious as to what the value of the resistor inside is, and today I decided to actually measure it. It reads exactly 20K ohms.
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:I was thinking that the resistor would be very high in value, causing a grid-leak situation, keeping the output grid negative, with the cathode of the 1st stage modulating that negative bias. However, at 20K I can't see the grid developing a negative bias, but, rather, is probably biased positive. Interesting. Not sure how it can amplify in a linear manner, but it does sound good!
:
:I have read about this tube elsewhere, and remember reading that the output grid does in fact run positive. I've never built and/or run across an amplifier like this where the two triodes are separate tubes, so I would not be able to measure the voltage personally.
:
:At any rate, if you were ever curious as to the resistor's value, that's what it is. I read on another site that some tubes call for a 30K resistor.
:
:T.
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However, I have used the 6N6 in all of my radios using the 6F6/6K6/6V6. I do like its sound a lot. One radio I actually shorted across the cathode bias resistor so that it could permanently use a 6N6, though if someone wants to change the tube back to its original number, the wire can be lifted. The radio was too mid-rangy even with negative feedback, and I hate to re-cone/change speakers when the original speaker is perfect, so I changed to the 6N6, which is much deeper and warmer and more natural sounding. A long time ago I built an amplifier using a 57, 2A3, and 80, which had negative feedback from the start. Never have I heard such wonderful sound! ...So I do like triodes a lot, though I'm not sure if the 6N6 can perform like a 2A3 because of the strange positive grid configuration, but still I like how it sounds.
T.
The only thing I don't like about these tubes, can't measure that internal resistor. A tube will check ok with the resistor open but over time, with leakage, may run away drawing too much current.
Norm
:I only have one radio that uses the 6B5, and though I had it working, it had a very low impedance output transformer (not at all right for the radio). Currently the chassis is in pieces (being cleaned), and the cabinet needs to be completely refinished and restored (water damage, etc.).
:
:However, I have used the 6N6 in all of my radios using the 6F6/6K6/6V6. I do like its sound a lot. One radio I actually shorted across the cathode bias resistor so that it could permanently use a 6N6, though if someone wants to change the tube back to its original number, the wire can be lifted. The radio was too mid-rangy even with negative feedback, and I hate to re-cone/change speakers when the original speaker is perfect, so I changed to the 6N6, which is much deeper and warmer and more natural sounding. A long time ago I built an amplifier using a 57, 2A3, and 80, which had negative feedback from the start. Never have I heard such wonderful sound! ...So I do like triodes a lot, though I'm not sure if the 6N6 can perform like a 2A3 because of the strange positive grid configuration, but still I like how it sounds.
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:T.
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Here is something I bring up once in awhile. A 6AD7G can be used to replace 6F6G if pin #1 and 6 aren't used in the socket. Tubes have the same electrical spec, look the same and 6AD7G is less expensive. It does draw 150 ma more filament current.
Norm
: I never thought about going from 6F6 to 6N6 but it will work. Probably sounds ok even with the extra cathode resistor.
:
: The only thing I don't like about these tubes, can't measure that internal resistor. A tube will check ok with the resistor open but over time, with leakage, may run away drawing too much current.
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:Norm
:
::I only have one radio that uses the 6B5, and though I had it working, it had a very low impedance output transformer (not at all right for the radio). Currently the chassis is in pieces (being cleaned), and the cabinet needs to be completely refinished and restored (water damage, etc.).
::
::However, I have used the 6N6 in all of my radios using the 6F6/6K6/6V6. I do like its sound a lot. One radio I actually shorted across the cathode bias resistor so that it could permanently use a 6N6, though if someone wants to change the tube back to its original number, the wire can be lifted. The radio was too mid-rangy even with negative feedback, and I hate to re-cone/change speakers when the original speaker is perfect, so I changed to the 6N6, which is much deeper and warmer and more natural sounding. A long time ago I built an amplifier using a 57, 2A3, and 80, which had negative feedback from the start. Never have I heard such wonderful sound! ...So I do like triodes a lot, though I'm not sure if the 6N6 can perform like a 2A3 because of the strange positive grid configuration, but still I like how it sounds.
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::T.
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Regarding the 6N6, it does sound good in radios designed for the 6F6 or 6K6. For a while I had two of them in my Crosley Super 11 (1117), and it sounded very nice. The only problem with not shorting out the cathode bias resistor is reduced output and increased distortion at higher volumes, but it does sound good.
Regarding the internal resistor, I have read elsewhere that it was only to keep current from running away when the tube first warmed up. Otherwise supposedly it isn't needed. I can't confirm this. The only way to confirm this would be to build the circuit using two separate triodes and then measure voltages with and without the resistor. There is a complete circuit within the tube without the resistor, as it would be from the output cathode to the output grid to the input cathode to the input plate, but there being differences in how each section warms up, as well as space charge variables, etc., I can see how things might not be stable without the resistor. I actually was thinking that the output grid might saturate negatively without the resistor, unless the input was conducting, but I guess I'm wrong. I don't see why both sections couldn't work in grid-leak mode, however, if proper resistances were used.