"The bypass cap controls degenerative feedback through the cathode resistor.
The incoming signal causes the plate current to fluctuate. The bias voltage,
the voltage drop across the cathode resistor, will also fluctuate.
This condition is not desirable because it reduces the amplification available."
Unless you are tailoring the gain of the stage based on frequency, then this is desirable. The bypass cap gives a nice way to roll off low end rumble or mud simply by reducing the gain of the lower frequencies. One cap to roll off the lows and it is done without introducing any parts in the signal path. Pretty cool in my opinion.
Tom:
Fixed bias in a preamp tube is the same as a power tube: ground the cathode and apply a negative voltage to the grid. HiFi guys sometimes use batteries for that. And yes, you still have to use coupling caps between stages.
D
There are no stupid questions, just stupid people.......
jjman wrote:HiFi guys are using LED biasing on preamp tubes. A red one provides about 1.6volts and no cap is needed. They run a few in series when more bias voltage is needed, or a different color. Those guys are adverse to caps in general so they know how to avoid them. If you don't want the drawbacks of a cathode cap, I'd check into it.
I use small signal diodes quite a bit for preamp cathode bias, mostly 1N4148s. they work really well, 2 in series gives a steady approx. 1.2V and it can give you some very noticeable change in tone and tighten things up it seems. They generally need no bypass either, hence the tonal difference. This probably works better in higher gain applications, but tastes vary and you should try it sometime so you know what you have in your tone arsenal. another 2 cents...
