Last Coupling caps rolloff

[naciketas]

Hi,
usually when I want to know the rolloff freq. from one stage to the following, I just put the R - C values in any RC calculator avalaible online then I have the number, but when it comes to the last CC (from PI to Tube's grids) not being there any resistor I simply don't know how to do it (one variable missing).
The obvious question is how to compute the rolloff point of the last coupling "stage"?

Thanks

[PeterS]

Ummm, there must be a resistor from the power tube grids to ground?

[naciketas]

Ok , at this point I can give up and throw everything outta the windows, myself included.
This was a question I had in mind since days, go figure!

I calmed down, went outside, gathered all the stuff (myself included) and see it the positive way: I offered those who viewed the chance to make some laughs .

BUT now I have another two question:

1) put the canonic values .01uF and 470k in the calc and it gives 33.9hz, while .022 and 470k gives the half, being both of those values under the guitar range, I'd say there's no appreciable difference or what am I missing (except different cap chargin' due to grid current during overdrive)?

2) In my case I have an attenuation at that point : CC in series with a 330k which 1= goes to ground through a 100k res (0,76 attenuation) and 2 = is in series with the 8k2 el84 grid stopper; does the CC sees 470k to ground ? In this case the el84 grid leak (the previous "missing" rez) is 100K and no more the classic 470K?

Thanks again

[Dave]

It sounds like you have CC to 330k to 100k to ground with the 8k2 connected to the junction of the 330k and 100k. If that's the case the CC sees 330k+100k or 430k. Your attenuation would be 100/430=0.23. You have calculated it with the 330k and 100k positions reversed i.e. 330/430=0.76. In both cases the CC will see 430k.

Dave H.

[naciketas]

Thanks for reply,
it is as you described, so it's 0.23 attenuation, but , excuse the stupid Q, does it mean that the signal is 23% less (slight attn) than or is 23% (huge attn) of the source signal?

And the grid leak rez is 100k, right?

[Dave]

I think it's calling it attenuation that's confusing. With the 330k at the top and 100k at the bottom it has an 'amplification factor' (or 'transfer function') of 0.23 i.e. output = input x 0.23.

Yes the grid leak is 100k

Dave H.