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In Reply to: RE: ? How to Calc optimum grid Leak R? posted by kellymon on November 02, 2009 at 12:13:30
it's really called "grid grounding resistor." a grid leak resistor is the kind that makes use of grid leak to bias the tube so no cathode resistor is needed. those are in the vicinity of 10-50 megohms.
a small leak and a small grid-resistance-to-ground resistor makes the grid set to ground, essentially. hence a grid grounding resistor.
a large leak and a large grid-resistance-to-ground resistor makes the grid more negative than ground, setting up a Vgk bias with respect to grounded cathode. the negative potential that floats there is caused by electrons hitting the grid and not drained away by a little leak, and so voltage starts to settle to a steady DC point until leakage current out grid equals Vg/Rg, and there makes the steady DC bias condition.
to "see" the current flow one must remember that electron flow direction is the opposite of standard positive current flow. So electrons hit grid and leak down grid-to-ground. But the positive direction current flow diagram for a grid leak biased triode is Ip from plate to cathode meeting up with Ig from grid to cathode = total cathode out current to ground.
just had to nit pick.
-kurt
I think it's a misuse of the term unless the tube is being biased by the grid resistor. If it is then it's a 'grid leak resistor'. If it's not then it's a 'grid resistor'.
I have looked at some of the old books and I've been shown to be wrong.
But I still think 'grid resistor' is the proper term. I don't know anyone using "grid leak bias" in modern audio designs.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
We're not using grid leak bias in these designs, so it's not a grid leak resistor. It's just a grid resistor. Asking someone to use the correct terminology when describing a part is not nit-picking.
I want the term "grid grounding resistor" to be used to distinguish itself from both "grid leak resistor" and "grid stopping resistor".
just "grid resistor" is very general. sure it's most commonly used because it is usually not accompanied with a grid stopping resistor.
-kurt
the last tube stage is a grid leak biased one. With Rk=0, minimum Zout is available for drive to the next stage. For it to be good, the CCS is needed for better bias stability.
-kurt
Kurt,
The idea of grid leak biasing the 2nd gain block of a phono preamp is intimately associated with Thorsten Lösch. The rationale for the arrangement is to ease loading of the EQ network and extend bass response.
Search the archives and you'll find a tweaked version of the "classic" RCA circuit I'm associated with. It works fine, without CCS loading the 2nd gain block. MOSFET buffering of the 2nd gain block probably helps in the stability dept., along with improving drive capability.
Eli D.
then i designed it into my phono and Gary copied it, except his CCS is used. That phono stage was designed by many contributors.
-kurt
Yep. I was following Kurt's phono stage development online. Thorsten joined in and suggested using grid leak bias on the output stage.
When I decided to do a phono stage I started with Kurt's design and added my changes which consisted of using the battery bias CCS and using the MU outputs to drive the RIAA network and parafeed output transformers. I wanted to use the drive capabilities of the MU output so the the impedance of the RIAA network could be scaled down ~10x. Lowering the impedance of the RIAA should result in slightly better noise performance and also allowed the use of the Russian Teflon capacitors.
Another change made was change the input tube to the 6FH5. I had more gain than I needed with the Grado cart I'm using. The 6FH5 seemed to clean up the sound overall. I suspect that with the higher mu of the 6ER5 the signal level presented to the 6GK5 output tube was too high for the grid lead bias.
Gary
"I suspect that with the higher mu of the 6ER5 the signal level presented to the 6GK5 output tube was too high for the grid lead bias."
Yes, partially correct there. I measured the overload characteristics of the 6GK5 grid leak stage on the anode. Grid leak bias has two places it operates under: linear, then overload to cause a charge-up on the coupling cap to the grid making it more negative and shifting operating points to a "peak hold" until it discharges back slowly, maintaining still class A linear opertion. This means a very soft overload characteristic and a possible strange shift in sound as it had moved in operating points and continues in another class A area with that operating point drifting back slowly.
If designed correctly, only a short quick overload peak will be nonlinear. But unstable operating points is not the road to hi-fi and it's really important not to overload the 6GK5. One other solution I found was to increase Vgk and bias more deeply by increasing the grid leak resistance, to about 60 Megohms. Then the CCS is the same, but the plate voltage drops into a wider swing area. Vgk is more voltage now that tells you it ought to be in a wider dynamic range area. But you can't measure Vgk in a 60 Megohm circuit. You can only measure the plate voltage here and see that it has dropped, implying Vgk is more negative than before from the plate curves. This was my final solution to that problem.
-kurt
Yes, I read the archives and your and others concerns regarding the term "grid leak resisitor".....
I suggest that in true terms "grid ground reference resistor" (GGRR)is most accurate, since that is what it is doing.....
That is why I clarified in my original post exactly the position I meant:)
Regarding the grid choke, I had previously used it with a hi mu driver, 6n2P-EV (12AX7 equiv) I think, and I really didn't care for it. Too much high end, lost the sweet low bass, no soul.
I don't think that is a result of the choke quality, just my application.
High rp, other issues....
Now I have re-designed several aspects of this SET amp, and the GGRR is one of the last to grasp.....
I find it hard to believe that in the DIY tube group, something as potentially significant as the GGRR hasn't been more fully explored?
I mean this sincerely, with no disrespect intended, as many of you have helped me gain the limited knowedge on this subject that I currently have...... and the ear will always have the final evaluation of "Best"....but I seek the technical background on this also....
peace,
robert
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