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In Reply to: RE: Potentiometer - Volume Pot posted by Eli Duttman on January 04, 2017 at 15:55:07
But if his source impedance is low - like even as high as 10K ohms, putting a 100K across a 250K pot is irrelevant.If he'e coming out of a plate coupled tube preamp, then there might be some issues
Edit: I am of course assuming the volume pot is directly across the input jack. If it's between stages within the amp's design, then disregard my
prior statements. In that case we do need to see the schematic.
Edits: 01/04/17Follow Ups:
Pot is directly across the input jack.
What tube is in the 1st stage? If it's a pentode, like the EF86 or 6SJ7, 250 Kohms is perfectly fine. OTOH, a 12AX7 section will be in trouble. Consider the low pass filter formed by the 1st tube's C Miller and the I/P resistance. Good phase shift performance considerations suggest that the LPF "corner" above 100 KHz.
Eli D.
Edits: 01/04/17
Exactly Eli. I am wanting 100K pot feeding a single section of a 6CG7. BTW- I have a 50K pot. Is 50K better for a 6CG7?
Edits: 01/04/17
As long as the source feeding the pot is happy with 50k.
The lower you can go (without causing problems for your source) the happier the 6cg7 will be. (lower impedance feeding the Miller capacitance of the 6cg7 = "better" highs)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
But again if he's looking back into a CD player with a output impedance of 100ohms via an OPAMP, the 250K input pot with a 100K shunt is irrelevant.How much shunt capacitance do you need to attenuate 20khz by even a tenth of a db at 100 ohms?
Edits: 01/04/17
It's not the output impedance of what's feeding the pot that matters, it's the output impedance of the pot and that will be determined by the position of the pot.At the -6db point (equal resistance either way from the wiper) a 250k ohm pot will have an output impedance of 62.5k ohms and that assumes the source impedance feeding the pot is zero ohms. (a 100 ohm source feeding the pot will increase that number but only by a tiny fraction)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/04/17
That's correct. My point is that with an OPAMP drive on the source, the input impedance of the tube power amp first stage is not that important.Now if he has a Dynaco PAS-3, then we might have a problem!
Edits: 01/04/17
Now I get what you're saying.
You're saying he could use a 10k pot and the source would still be happy and the Miller of the first tube in the amp would be more than satisfied.
Sorry I mis-read you.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
An op-amp would destroy the sound of the amp.
What is your source? it's it's a typical CD player, you most likely have an OPAMP driving your new amp like it or not!Now if you are using a turntable with a tube preamp, different story altogether.
And seriously, modern OPAMPs designed for audio are practically transparent with noise floors below any source material you can find.
Tubes are fun but let's not discount our vast technology advances since then. But then there are plenty of people here who like to implement tube I/V stages in their CD players and outboard DACs. So if you really hate OPAMPs, you can still have tubes with your CDs!
Edits: 01/04/17 01/04/17 01/04/17
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