Audio Asylum Thread Printer
Get a view of an entire thread on one page
|For Sale Ads|
In Reply to: RE: Boy howdy,that really made a difference! posted by Tre' on February 15, 2019 at 07:47:40
You know that is one of the more interesting things I have run into. JC Morrison, in a article in SP magazine a while back, said that he had come to the conclusion that the smallest coupling capacitor he could use was the best. Just for the heck of it I have tried using the 0.1uf Russian silver mica on each amp I have breadboarded over the last couple of years. I have a pretty good collection of other types so I would have my own little shoot out each time. Invariably I liked the sound of the 0.1 silver mica best each time. I don't know if it's my particular taste or if there is something else going on there but I am now of the opinion that JC had it right. I can't tell you why, but it sure works for me. You can still get them on the 'bay, you ought to give it a try.
Forgot to mention that I did try a 0.5 cap and did not like it as well. The other thing is I am getting really good bass, especially on the 813. I haven't tried the low organ notes yet, but I hardly listen to that music anyway.
Had to add an edit.... went back and tried the 0.5uf cap again after reading your post since I have made a few changes to the amp since I tried it the first time. It sounded pretty good this time, and I noticed the bass seemed to be a bit tighter than with the 0.1uf cap. I will have to do some more listening before I know for sure, but it's showing promise.
Please understand it not the value of the cap alone.
If you had a 270k ohm grid resistor (don't try that with the 813, it won't be happy with that large of a grid to ground resistor) then you could use a .1uf cap without causing frequency response problems.
One more time, the coupling cap, in conjunction with the value of the grid resistor, forms a high pass (blocks the lows at some point) filter.
Here's the math. The 3db down point of the low pass filter equals 159155 divided by (the value of the cap (in microfarads) times the resistance value of the grid resistor)
To make sure the frequency response is flat all the way down to 20Hz and to make sure the phase response is not altered all the way down to 20Hz we want the -3db point of the filter to be 2Hz. (it takes a full decade from the -3db point for the frequency response to get to flat and the phase shift to go away)
Most people say this it not real critical and settle for a -3db point of 5Hz or a little less.
BTW, I agree with you. Smaller coupling caps sound better. So I make my grid resistor values as large as I can without pissing off the tube.
The following is a subject for a different day,
All tubes exhibit what is called "grid leakage current". Some more than others. It is the grid leakage current that dictates the "max permissible grid resistor value".
With the 813 in triode mode, I don't think you would get away with a higher value grid resistor than the 60k one you are using so you have to use a larger value (1uf or .47uf) coupling cap if you want the low bass to not be filtered out and/or phase shifted.
Have Fun and Enjoy the Music
"Still Working the Problem"
Post a Message!
This post is made possible by the generous support of people like you and our sponsors: