Equivalent gain - triodes in parallel

[Sir Cuitous]

I think someone on this forum can help me with this. If I have a single triode stage(1/2 12AX7):

Grid Load Resistor = 220k
Plate Load Resistor = 100k
Plate Voltage = 170v
Cathode Resistor = 820
Cathode Capacitor = 25uf

What would those values be for two triodes in parallel to achieve the equivalent gain? What I want to do is replace the gain make-up stage for a Fender reverb circuit with a single 12AX7 in parallel instead of half of a 12AX7.

[Dave H]

I'd just change the plate resistor to 51k and the cathode resistor to 430R.

Dave H

[Sir Cuitous]

Thanks Dave. I'm not good with the math (theory yes, application no). I'm sure there are ways to determine this outcome using math. Thankfully, there are only a few parameters here. With the benefit of the experience on the forum here I have learned a lot. I'm really starting to dawn on this whole plate vs. cathode relationship.

I would have guessed just dropping the cathode resistor to about half - 430 being a good choice - would keep the parallel stage biased about the same as a single triode. Are you sure that the plate resistor needs to be halved as well?

[Enzo]

Why not just try it? It is not like a "wrong" value will harm anything. Get a couple little trim pots - a 100k and a 1k, and tack them in the circuit in place of the 100k and 820 ohms resistors. Now you can dial in any value you like to try. Once you setle on some setting, remove the pots and measure the resistance at that setting. then solder in a resistor with a standard value close to the pot setting.

[Dave H]

Are you sure that the plate resistor needs to be halved as well?

It will still work with 100k but you really need to halve both the cathode and plate resistors to keep the parallel stage biased the same as a single triode. With the triodes in parallel the single plate resistor has the current of both triodes through it so its value needs to be half that of a single triode to drop the same voltage.

Dave H.

[Sir Cuitous]

I should just try it. I have two designs for it but it may be a few months before I get a real implementation.

Dave, I believe you are correct. The bias will be equivalent by halving the values of both the cathode and plate resistors. I am forgetting, however, that two triodes in parallel have less gain than two triodes that cascade.

So when comparing the gain of a single triode vs. the gain of two parallel triodes using the same values (no resisotrs changed) the gain of two parallel triodes will be only 2 - 3 dB higher than the single triode. To achieve the same gain charateristic as a single triode, my guess is that I shouldn't drop the resistor values by as much as half.

This just drives home Enzo's point. I really need to setup a test circuit and try it out.

[booj]

I'm wondering if you should increase the value of the cathode cap to keep the rc constant the same

[BackwardsBoB]

I'm wondering if you should increase the value of the cathode cap to keep the rc constant the same

If you want to maintain the same frequency response, yes, you should double the capacitance, but if you leave it at 25uF, the -3dB point is at about 15Hz, and lower than that is just silly.

[Sir Cuitous]

Thanks for reminding me of the frequency response. I was thinking only in terms of gain.