Ok, so maybe the subject isn't very punny but I'm wondering about the effects of modifying the plate load resistor values in the preamp to change the gain of a preamp tube. Are there any other side effects of doing so?
There are some reports of replacing the 12AX7 in the reverb on my Traynor YGM-3 with a 12AU7 and getting better sounding reverb with a wider usable range. The only problem with doing that is that the other half of the tube is used in the tremolo circuit and with the gain cut so much the tremolo no longer works. Now, if instead the plate load resistor on the reverb regeneration tube half was cut from 470K to something smaller you could cut the amplification and get a similar effect.
With a gain on the 12AX7 of ~100 and on the 12AU7 of ~20 you would need to drop down from 470K to ~100K to get a similar effect since the gain of a tube stage is directly proportional to the plate load resistor value. Would that have other repercussions? Not sure that I'm going to do this but I'm just wanting to explore some possibilities and also work at getting some more understanding of tube amps. Working through possible changes and how to do them is a great way to build up the mental model of how things work.
Thanks.
Pop music is about stealing pocket money from children. - Ian Anderson
If you simply change the plate resistor without changing the cathode resistor, you change the bias point of the tube and may see clipping too soon on one peak. The cathode resistor will usually need to change approximately in proportion to the plate resistor change. Tube manuals like the RCA Receiving Tube Handbook RC-31 usually have a section in the back with typical values of the various resistances for an amp stage using a particular tube type, calculated to all work together, with calculated gain values and also with coupling capacitor values for particular bottom end cutoff frequencies.
If you drop the plate (& cathode) resistance too much without dropping the plate voltage as well, you may exceed the allowable plate dissipation for the tube by increasing plate current too much.
Drop the gain this way and you also drop the maximum magnitude of the input signal that the amp stage can handle before it clips. That can be both good and bad, depending on what you want to do.
"A cheerful heart is good medicine."
huh?
aka Izabella
Mostly I just said that if you change the plate resistor, you've got to change the cathode resistor too, to keep them in about the same ratio of resistances. Otherwise the tube won't be biased right.
"A cheerful heart is good medicine."
