small signal pentodes, no Ck, Cg2 connection???

[jimboyogi]

Hey,

A question of theory. When connecting a small signal pentode without a cathode bypass cap, where should the screen bypass cap connect to? Screen voltage supply is via dropping resistor from HT only, not using a voltage divider.

To my limited understanding, there are 3 factors to think about with this -

1. reduce screen nfb for max gain,
2. bypass any PS noise to ground to avoid noise being amplified,
3. to get the appropriate fidelity of signal amplification.

I ask this because in the pentode circuit I have been trialling, I have bypassed the screen direct to the cathode, and then bypassed the cathode to ground. But I have maybe too much gain, so I tried removing the cathode bypass cap. I lost gain (nearly 50% reduction), but there seemed to be more hum without the cathode bypass cap than with the cap

[Gingertube]

If there is no bypass cap on the cathode then the screen bypass cap MUST go to the cathode - unless you are deliberately trying to introduce feedback. Even if the cathode resistor is bypassed it is still preferable to connect the screen bypass cap to the cathode to avoid having the cathode bypass cap effectively in series with the screen bypass cap as far a as AC signals are concerned.

Often it is instructive to examine the extreme conditions to get your head around things like this.
Consider a Pentode Cathode follower, unless the screen is connected AC wise to the cathode then it is the triode characteristics of the tube which will apply and all benefits of using a pentode for the cathode follower follower will disappear.

The additional hum is typical when not bypassing the cathode and it happens with triodes too. It is noise coupled from the heater which is no longer being shunted to 0V by the bypass cap (because the bypass cap has been removed). The tubes "drive" signal is the voltage between grid 1 and cathode so that noise effectively becomes part of the input signal and an amplified version of it appears on the output. Make sure you have the heater wiring balanced by connecting the centre tap to 0V or by creating a "false centre tap" with a pair of resistors if the heater winding does not have a CT. Some benefit can also be gained by elevating the heaters to say +20 or +30 volts, that is, connect the real or false CT to a +30'ish V supply. That supply doesn't need to supply any current so divide it off B+ from a well filtered node.

Cheers,
Ian

[jimboyogi]

Thanks for the response Ian. That clears things up well for me.

Also logical about the extra hum when not using a Ck. I then though about lowering gain by removing Cg2 instead of Ck, but then I guess similar hum problems will occur, because PS hum would then be introduced to the screen.

I'm going to start a seperate thread for advice on minimising noise in the pentode gain stage, if I can keep the noise down then all of the pentode gain can be a boon, and then I can happily bypass both screen and cathode!