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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: RE: Boy howdy,that really made a difference! posted by vinnie2 on February 15, 2019 at 11:57:20:
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'
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Follow Ups
- RE: Boy howdy,that really made a difference! - Tre' 14:49:11 02/15/19 (1)
- RE: Boy howdy,that really made a difference! - vinnie2 15:07:00 02/15/19 (0)
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