- Jan 22, 2008
that's the same thing except one is a single stage, the other a push-pull.
I don't remember this. Those German engineers were fascinating.DaveP said:
The combined resistance of the actual grid leak resistors, and the release resistor in the side chain, form the total grid leak path.DaveP said:If the cap is direct coupled to the grid, do the release caps serve as the grid leaks?
If the cathodes are directly grounded then you'd need to add a bit of permanent DC bias to the side chain, of course. Or you can add cathode resistors instead (LEDs may be better these days). Whatever suits you.Only the Federal and the Altec 436 have cathodes grounded all the rest have cathode resistors.
I think there was just a little detail missing in PRR's comment: " If the cap is direct coupled DC-wise to the grid,".DaveP said:If the cap is direct coupled to the grid, do the release caps serve as the grid leaks? Only the Federal and the Altec 436 have cathodes grounded all the rest have cathode resistors.
I think there was just a little detail missing in PRR's comment: " If the cap is direct coupled DC-wise to the grid,".
If it was actually connected to the grid, the AC signal would be shunted to ground
Via a xfmr is ok. Via a resistor is not. If you want to superimpose the input signal, this will interact with the DC grid voltage in a detrimental way.DaveP said:So it would be OK to feed the DC to the grid via a transformer or say a 1M-10M resistor. The release resistors would be in parallel to the grid resistor, so it needs to be a high enough value to only affect them marginally.
That may work but both the input voltage and the control voltage would need to be increased. That may be not too difficult for the input voltage but not too easy for the control voltage.DaveP said:
OK, I've got that, thanks.Small ones. Just enough to avoid having to add the small initial bias which Merlin mentions. In many cases VERY small, because the useful grid-swing at maximum gain is necessarily small.
You mean the thump in the first tube would cancel the thump in the second tube? I can see that working better in theory than in practice.DaveP said:I'm not thinking of a matched pair in push-pull, I'm thinking about a pair in series with two small gain reductions on each tube.
Then neither will partition noise be a problem.Microphonics will not be such a problem at line level.
It was a big problem with the bunch of 6AS6 tubes I have.