|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
75.82.52.72
In Reply to: RE: It dawns on me that the grid resistor does lower the source impedance but... posted by Tre' on June 25, 2012 at 14:55:16
I use a 5687 as a driver for my 6b4g SET.The 5687 has a plate resistance of about 2500 ohms.
I use a CCS instead of a plate resistor so the output impedance (at the plate) is 2500 ohms.
I use the mu output of the CCS. It's output impedance is about 500 ohms.
This is without considering the grid resistor of the 6b4g.
If I use a 10k ohm grid resistor that will change the drive impedance since it is in parallel with the output impedance of the CCS.
10k//500ohms=476 ohms
If I use a 470k ohm grid resistor the drive impedance will be 499 ohms.
If I take the output off the plate and use a 10k grid resistor the output impedance would be 2.5k//10k=2k and the load line for the 5687 would be more vertical and I would lose gain and the harmonic distortion would increase.
a 470k grid resistor give us 2486 ohms and the load line would stay pretty horizontal.
With any one of those, the Miller is fully driven and the -3db point is more than a decade out of the audio band.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 06/25/12Follow Ups:
Note that the OP asked about the first stage of an amplifier. If that stage happens to be a 6SL7 or 12AX7, Miller rears its ugly head in a way that can be difficult to deal with. Many commercial designs from past decades resorted to relatively simple, add-on RC compensation to flatten the response. I see nothing wrong with that so long as the implementation uses good quality parts and doesn't incur a noise penalty.
--------------------------
Buy Chinese. Bury freedom.
"Note that the OP asked about the first stage of an amplifier. If that stage happens to be a 6SL7 or 12AX7, Miller rears its ugly head in a way that can be difficult to deal with."Yes. I looked at his system and I see that he uses a 4kn8 choke loaded line stage.
I couldn't find a data sheet for that tube but I did read that the transconductance is higher than a 6dj8.
If I assume that the plate resistance is the same as the 6dj8 or no higher then the output impedance will be the plate resistance with the plate choke in there (at least through the midband and lower everywhere else).
If I assume 3k for the output impedance I don't think I'm too far off.
The Miller of a 12ax7, assuming a in circuit mu of 65, will be about 112pf. Let's call it 120pf.
3k (assuming a very high value grid resistor on the 12ax7) driving 120pf has a -3db point of 442kHz.
If the 12ax7 is actively loaded and the in circuit mu is the mu of the tube then we have 1.7pf (grid to plate) X 101 + 1.8pf (grid to cathode capacitance) for a total of 173.5pf
The -3db point would now be 305kHz.
I don't see any reason to load down the preamp tube with a low value grid resistor on the first stage of the power amp (even if it's a 12ax7) when you have a 3k drive impedance to start with.
Unless there is a lot of cable capacitance HF roll off shouldn't be a problem.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 06/26/12 06/26/12
Thank you all for the replies, I caught the confusion and was not confused by it. Tre, its a 4kn8 loaded with that cheapo Hammond 150h choke, 3 foot interconnect driving a 12hg7,, 100k grid resistor. The 4kn8 draws more current in the same socket as 6dj8 so it's rp is lower eh? JH
That should be fine. The 100K grid resistor on the 12hg7 defines the load line on the 4kn8.
100k on a 6dj8 is close to horizontal.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: