I just simulated some parameters with VS version2.G
here were my "inputs"
r plate tube 7000 ohms
r load 990Kohms
L primary xfmr 2500 henries
R primary DCR 4000 ohms
C parafeed .5uf (half a mic)
L choke 250 henries
R choke DCR 1000 ohms
C DrP 1uf (one mic)
R DrP 75000 ohms
and got output peaking of .22db (point 22 db) at about 5 or so hertz.
My Strategy:
on purpose I used a plate choke (as opposed to CCS) and kept it's nominal L at about 1/10th the L used in the sim of the grid choke.
For DrP cap size I used Voltsec's guidance of 2 times blocking cap size.
For DrP resistance I used 75K figuring it was more than ten times the value of r sub p.
Another thought: cool as VS calculator is.... and it is a GREAT tool... it still may not capture reality... it may overstate the resonance problem since we are not incorporating the impedance (i.e., losses) of the iron core of the grid choke. So bear this in mind... at least preliminarily.
The issue of resonances is one of them hot-potato issues... and ya want to drop it or get it out of your hand as quick as you can. Yet... another issue (that at least I think relevant) is the question of... taking a horrid resonance say at five hertz (which is two octaves below 20 hertz) we still need to (or should?) ask... how will this resonance be excited? Do we have program material that will excite a 5 hertz resonance? If your playing cd's... rarely will there be much material recorded below what... 40 hertz or so? I, too love to crunch the numbers and VS's calculator is godsend (and thanks also goes to
gianluca and the nice gentleman from NY who got the ball rolling initially)... but... we still need to use these programs with a bit of reserve or caution. Use them as tools and use them to learn the relationships that exist... changing numbers and variables around can give good insight into the workings or tendencies of the circuit taken as a whole... but... for those blessed with good test equip... scoping the circuit and looking at the actual behaviour may yet be the best way to do the final tuning.
and one last general question: would an alternative to DrP be to use a high pass filter on the input of the amp? Or would (could?) the slope of an filter be sufficient to keep the amp from "producing" the resonance???
msl