Hi
Still enjoying my little amp. I was wondering about the this mod above. We changed the shared cathode bias resistor/cap of each pair of power tubes to individual resistor/cap per power tube…
So i saw a video with a guy who has the same amp and had considered this mod but then after some calculations he decided it wasn’t a good idea because it would increase the Milliamp current draw from the power transformer and make it run even hotter than it already does. Your thoughts?
I’ve been using it fine all this time with the mod I was just wondering if this will eventually lead to power transformer failure. Thx
maddmaxxdrum,
Example of a Parallel Single Ended pair of output tubes:
Two tubes cathodes are tied together to a 270 Ohm self bias resistor to ground.
Say the common self bias voltage is 2.7V.
The current is 2.7V / 270 Ohms = 0.010Amp (10mA).
That is an Average current of 5 mA per tube.
270 Ohms x 2 = 540 Ohms
Break the connection between the cathodes from each other.
Now, connect one cathode to a 540 Ohm resistor to ground; and connect the other cathode to a 540 Ohm resistor to ground.
Suppose the self bias voltage on each individual cathode is 2.7V
2.7V / 540 Ohm = 0.050Amp (5mA). 5mA for each tube.
The tubes are well matched, and both dissipate the same plate watts.
But if the tubes are not well matched, one cathode might be at 3V.
3V / 540 Ohm = 0.00555Amp (5.55mA)
And the other cathode might be at 2.4V
2.4V / 540 Ohm = 0.00444A (4.44mA)
5.55mA + 4.44mA = 9.99mA. That is the 10mA current that you had with the 270 Ohm common self bias resistor.
One mismatched tube always was drawing more current than the other, and so it was dissipating more power in the plate, than the lower current tube.
You could not know that with the common self bias resistor of 270 Ohms.
But with individual 540 Ohm self bias resistors you can easily see the un-equal tube currents.
Admittedly, those currents are more like a pair of parallel input tubes.
So, to illustrate a parallel output tube pair, just keep the resistors at 270 Ohm or two 540 Ohm resistors.
. . . But change the self bias voltages to 27V, 30V, and 24Volts, instead of 2.7V, 3.0V, and 2.4V.
The currents are 100mA, (50mA per tube); and 55.5mA and 44.4mA for the unmatched tubes.
Example of a Parallel Single Ended pair of output tubes:
Two tubes cathodes are tied together to a 270 Ohm self bias resistor to ground.
Say the common self bias voltage is 2.7V.
The current is 2.7V / 270 Ohms = 0.010Amp (10mA).
That is an Average current of 5 mA per tube.
270 Ohms x 2 = 540 Ohms
Break the connection between the cathodes from each other.
Now, connect one cathode to a 540 Ohm resistor to ground; and connect the other cathode to a 540 Ohm resistor to ground.
Suppose the self bias voltage on each individual cathode is 2.7V
2.7V / 540 Ohm = 0.050Amp (5mA). 5mA for each tube.
The tubes are well matched, and both dissipate the same plate watts.
But if the tubes are not well matched, one cathode might be at 3V.
3V / 540 Ohm = 0.00555Amp (5.55mA)
And the other cathode might be at 2.4V
2.4V / 540 Ohm = 0.00444A (4.44mA)
5.55mA + 4.44mA = 9.99mA. That is the 10mA current that you had with the 270 Ohm common self bias resistor.
One mismatched tube always was drawing more current than the other, and so it was dissipating more power in the plate, than the lower current tube.
You could not know that with the common self bias resistor of 270 Ohms.
But with individual 540 Ohm self bias resistors you can easily see the un-equal tube currents.
Admittedly, those currents are more like a pair of parallel input tubes.
So, to illustrate a parallel output tube pair, just keep the resistors at 270 Ohm or two 540 Ohm resistors.
. . . But change the self bias voltages to 27V, 30V, and 24Volts, instead of 2.7V, 3.0V, and 2.4V.
The currents are 100mA, (50mA per tube); and 55.5mA and 44.4mA for the unmatched tubes.
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maddmaxxdrum,
It has been too long since I read through all 83 posts of this thread; or too many posts to remember even if they only covered one week. So I did not check to see which exact modification you were talking about.
Sorry. I was too lazy.
Please refer to the exact Post #(s).
I only answered your question as a general question, about going from a single common self bias cathode resistor to dividing the currents individually with individual cathode resistors.
Post #s 46 and 55?
If you split the cathodes and connect to individual self bias resistors, you can easily see how well matched (or not well matched) the output tube currents are: bias voltage / self bias resistor Ohms = current, in amps.
And, for anything other than a coupled cathode phase splitter, there most normally would need to be individual bypass capacitors across each individual self bias resisitor.
With no bypass capacitor across an individual self bias resistor, the plate impedance increases, and the gain decreases.
We can get specific, if you mention the Post #(s)
It has been too long since I read through all 83 posts of this thread; or too many posts to remember even if they only covered one week. So I did not check to see which exact modification you were talking about.
Sorry. I was too lazy.
Please refer to the exact Post #(s).
I only answered your question as a general question, about going from a single common self bias cathode resistor to dividing the currents individually with individual cathode resistors.
Post #s 46 and 55?
If you split the cathodes and connect to individual self bias resistors, you can easily see how well matched (or not well matched) the output tube currents are: bias voltage / self bias resistor Ohms = current, in amps.
And, for anything other than a coupled cathode phase splitter, there most normally would need to be individual bypass capacitors across each individual self bias resisitor.
With no bypass capacitor across an individual self bias resistor, the plate impedance increases, and the gain decreases.
We can get specific, if you mention the Post #(s)
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Post # 55.
A Parallel Single Ended output stage that has a 270 Ohm self bias resistor with both cathodes tied together . . .
You can use individual self bias resistors of 2 x 270 Ohm = 540 Ohm when you disconnect the 2 cathodes from each other.
The tube currents will tend to be more equal with individual 540 Ohm self bias resistors, one tube can not as easily hog most of the current. Current hogging can easily happen with the cathodes tied together, especially if the 2 tubes are not very well matched.
However, with the common 270 Ohm self bias resistor, a bypass cap is optional, and often best left out.
But with individual 540 Ohm self bias resistors, you should put an individual bypass cap across each 540 Ohm resistor.
I hope that makes it clear.
A Parallel Single Ended output stage that has a 270 Ohm self bias resistor with both cathodes tied together . . .
You can use individual self bias resistors of 2 x 270 Ohm = 540 Ohm when you disconnect the 2 cathodes from each other.
The tube currents will tend to be more equal with individual 540 Ohm self bias resistors, one tube can not as easily hog most of the current. Current hogging can easily happen with the cathodes tied together, especially if the 2 tubes are not very well matched.
However, with the common 270 Ohm self bias resistor, a bypass cap is optional, and often best left out.
But with individual 540 Ohm self bias resistors, you should put an individual bypass cap across each 540 Ohm resistor.
I hope that makes it clear.
we
If I do his mod, does it interfere with your mod?
Thx
hi hope all is well. I’m still working on this amp.. I was going over the mod u explained to me about making the plate to cathode voltages the same for both triodes of the 6SN7. It’s up in your post quoted above… i saw someone else do another mod to the same amp that makes the lower triode work more resulting in less distortion at higher volumes. He explains it in this video…
If I do his mod, does it interfere with your mod?
Thx
Attachments
You are talking about the input stage cascode amplifier, right?
My advice is to do one mod, or to do the other mod, do not do both.
Both mods are intended to set the top triode's cathode voltage (or the bottom triode's plate voltage, depending on how you look at it).
In other words, do not connect a motor output to a Ford transmission input, and then connect the Ford transmission output to a Chevrolet transmission input.
Just my opinions.
My advice is to do one mod, or to do the other mod, do not do both.
Both mods are intended to set the top triode's cathode voltage (or the bottom triode's plate voltage, depending on how you look at it).
In other words, do not connect a motor output to a Ford transmission input, and then connect the Ford transmission output to a Chevrolet transmission input.
Just my opinions.
