As I replace the scratchy pots in my son's Fender amp I have resolved to never use the evil things in the audio path again. Two obvious alternatives are the use of G3 or G2 as suppressors.
Obviously the radio tubes such as the 6D6 provide volume control via G3 specifically for this purpose. A 24V power supply with a pot (out of the audio path) could be applied to the third grid to suppress the gm realizing variable gain from about 2 or 3 to over 100 (with proper load). There must be a good reason not to do this. What is it?
We could also use a Variac, pot, or other means to reduce the voltage to the 2nd grid in a normal pentode or tetrode such as the 6L6 on the output side. Driving the V on G2 down from 400V to 20V will suppress the output by far more than an order of magnitude. Again, I do not observe anyone doing this so there is probably a very good reason.
Please teach me...
Obviously the radio tubes such as the 6D6 provide volume control via G3 specifically for this purpose. A 24V power supply with a pot (out of the audio path) could be applied to the third grid to suppress the gm realizing variable gain from about 2 or 3 to over 100 (with proper load). There must be a good reason not to do this. What is it?
We could also use a Variac, pot, or other means to reduce the voltage to the 2nd grid in a normal pentode or tetrode such as the 6L6 on the output side. Driving the V on G2 down from 400V to 20V will suppress the output by far more than an order of magnitude. Again, I do not observe anyone doing this so there is probably a very good reason.
Please teach me...
There are these things ....
IC Volume Control
I've seen them for sale, but have not seen any applications.
IC Volume Control
I've seen them for sale, but have not seen any applications.
In the dark ages they used the heater power supply to controlthe volume...
Using the grid is ok for some regulation but it also changes the internal resistance of the tube and distortion could be an issue (as the load no longer matches).
Other options are available as mentioned...
LDR or (relay switched) resistor networks.
Using the grid is ok for some regulation but it also changes the internal resistance of the tube and distortion could be an issue (as the load no longer matches).
Other options are available as mentioned...
LDR or (relay switched) resistor networks.
Distortion must be impacted by changing G2, but without graphs on which to plot my loadlines (for reduced G2 voltage) I do not know how to determine the magnitude of the matter.
The relationship between load and distortion is much less complicated for triodes, so how about triode-strapping a 6D6 and using G3 for suppression of the triode?
Any variable-mu triode like ECC189 can be used as electronic pot ... but the signal level must be low , below 100-200mV rms , so the signal itself doesn't interfere with the DC control bias and distort .
So it is not "universal" to be implemented commercially , only for a diy amplifier when you know your signal sources .
So it is not "universal" to be implemented commercially , only for a diy amplifier when you know your signal sources .
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That is why I love this forum. I plead "teach me", and you do!
OK, I had never heard of the ecc189 or any of the variable mu triodes. It appears that the gain of the ecc189 is very low
http://www.r-type.org/pdfs/ecc189.pdf
Exaggerated by the low signal level I must cascode them? (As advised for the 6386 which I cannot afford?)
https://frank.pocnet.net/sheets/142/6/6386.pdf
Are there any variable mu triodes with more gain, low price, and in octal or large sockets? (I am too blind and clumsy to solder the miniature ones anymore)
If not, any suggestions for cascode or other series circuits to achieve more usable gain?
OK, I had never heard of the ecc189 or any of the variable mu triodes. It appears that the gain of the ecc189 is very low
http://www.r-type.org/pdfs/ecc189.pdf
Exaggerated by the low signal level I must cascode them? (As advised for the 6386 which I cannot afford?)
https://frank.pocnet.net/sheets/142/6/6386.pdf
Are there any variable mu triodes with more gain, low price, and in octal or large sockets? (I am too blind and clumsy to solder the miniature ones anymore)
If not, any suggestions for cascode or other series circuits to achieve more usable gain?
Returning to the 6D6, as it is a resistance coupled pentode will it be immune to the changes in harmonic distortion as voltage applied to its suppressor grid is changed?
Not immune ...
There are other tubes as well like the 6BJ6
These are used in limiters and compressors.
6BJ6, Tube 6BJ6; Rohre 6BJ6 ID3599, Vacuum Pentode
6BJ6 @ The Valve Museum
There are other tubes as well like the 6BJ6
These are used in limiters and compressors.
6BJ6, Tube 6BJ6; Rohre 6BJ6 ID3599, Vacuum Pentode
6BJ6 @ The Valve Museum
OK, so to minimize variation in harmonic distortion I must use no pentodes. In that case I need two stages in the preamp. A variable mu triode (or triode-strapped pentode) followed by something like a 12AX7 to drive the 6L6 output?
Pentodes are fine. No tube is immune for distortion especially if parameters are changed. It depends on the circuit how much distortion you will get.
A current source as load can give less distortion then a fixed resistor as an example.
A current source as load can give less distortion then a fixed resistor as an example.
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