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In Reply to: RE: confused again posted by vinnie2 on February 13, 2019 at 15:24:44
Why what?
Follow Ups:
Why a coupling capacitor. I have built other direct coupled amps that did not have a coupling capacitor.
It will be a lot less confusing if you have a coupling cap. The grid of the output tube will be at 0V DC, then you can bias up the cathode accordingly.
Direct coupling means that you are relying on the plate voltage of the driver stage to be predictable so that the output stage operates properly. This isn't always the easiest thing to do. You're also going to run the same output tube current through a much higher value biasing resistor, so heat will become quite an issue.
You also don't seem to have enough B+ to properly run a directly coupled circuit, at least with what you've been discussing.
Learned a new trick today I guess. I did like you suggested and put a .1uf Russkie cap and 1k grid leak resistor in front to the output tube and a 60k grid stop resistor going to ground from the cap. The plate volts jumped way up to 480, the grid volts went up to 48.1 and the plate current went to 65mA. Looks like I need to do some tweaking now, but at least I am in the ball park now. It's sounding better too. Can't wait to hear it when I get things dialed in. Thanks for the tip; this one will go in my notebook.
Vinnie, you have the terms reversed. The 1K in series with the grid is the grid stopper (it's there to damp or "stop" oscillations) while the 60K to ground is the grid "leak" ( it leaks off any charge that may accumulate on the grid).
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Opps! I believe you are correct. Thanks for the heads up.
That .1uf coupling cap will not let all the bass through to the output tube's grid.
(.1uf into the 60k ohm grid resistor creates a high pass filter with a -3db of 26Hz.)
Try a 1uf cap.
(1uf into the 60k ohm grid resistor creates a high pass filter with a -3db point of 2.6Hz)
A .5uf (.47uf) would also be just fine with a -3db point of just over 5Hz.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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.
Edits: 02/15/19 02/15/19 02/15/19 02/15/19 02/15/19 02/15/19
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)
159155/(.1x60,000)= 26.52Hz
159155/(1x60,000)=2.65Hz
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.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Thanks for the explanation. This one goes in my notebook!
Ok, thanks. I think I understand.
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