Parrallel triodes

[Spookyman]

So I'm part of the way through designing my first amp, struggling to overcome a complete lack of electronics knowledge or experience. Using a training manual from a second hand bookstore and help from people on the interweb I get most of the way through the design when a question occurs.
My design currently has the guitar signal fed to grid of triode. Plate output of that goes to grid of next triode (RC coupled). You will be familiar I am sure with the concept.
But what if I fed to guitar signal to the two grids of the triodes in my (for aguments sake) 12ax7? If I was doing this, would I still have to have two cathode resistors and cathode caps? I am assuming that I would, or if I didn't I would have to double up the value of the cathode caps and reduce the resistor values because those components would essentially be in parallel.
I did see a marshall schematic where the cathode resistors that were in parallel were linked together by a resistor between them. I wonder what that was about?
I am asking this only in theoretical terms - This design has taken too much of my time for me to go changing the bits that I have managed to get done so far. I am just curious. It is an approach I may consider for the next one though.

[MWJB]

To run the 2 triodes of a 12AX7 in parallel, halve plate resistor and cathode resistor values. Indeed, you could experiment with doubling cathode cap value, though often the original cap value is kept.

[Robert M. Martinelli]

By means of the cathode resistor you can set the tube bias, as we' ve already seen, this means you "lift" the cathode from GND to make the grid negative with respect to its cathode.

You can use only one resistor if you want both the triodes' grids biased at the same negative voltage, because you surely remember the cathode is the "reference point" for a tube. If this is the case you' ll need to take into account the sum of the plate currents of both triodes. If the two grids of a 12AX7 need to be biased differently, you' ll need two separate cathode bias resistors ( and take into account each triode' s plate current by itself ).

In both cases, the cap in parallel with the resistor serves as a bypass capacitor, it allows the triode to release the designed gain all over a spectrum of frequencies, without it the frequency response of that tube would be compromised. Very large cathode caps also help keep the cathode voltage constant.

Again, I encourage you to study, or at least read, the basic principles behind this subjects ( tube amp technology, Ohms law, RC coupling and so on ). It' ll make your understanding easier and faster.

Hope this helps

Best regards

Bob

[echuta13]

http://www.diystompboxes.com/pedals/tubedummy.html

A very basic, but decent description of what is going on in the typical triode preamp stage.

You may also want to check out sewatt.com's FAQ section for ideas as well.