Newbie question: best way to reduce gain in gain stage

[Rafael]

Say I'm looking at a standard cathode follower gain stage with too much gain: what is the best way to reduce the gain? Between throwing a voltage divider on the output side and changing the value of the grid/cathode resistors, it would seem that the voltage follower route would decrease the S/N ratio but altering the grid/cathode resistor values would be more work to calculate.

Is there an easy way to calculate the change in gain from varying the grid/cathode resistors or does this throw the whole thing off too much to make it easy? Thanks in advance!!!

[Andy Le Blanc]

theres a triode gain model at adams amps
www.zugster.net
that you can fiddle with with out getting too complicated

[Phil_S]

Raise the value of Rk. Put a pot in there temporarily and tweak to taste. Then find the R value. The relieves the need to calc and allows you to use your ears.

[Firestorm]

Say I'm looking at a standard cathode follower gain stage with too much gain: what is the best way to reduce the gain? Between throwing a voltage divider on the output side and changing the value of the grid/cathode resistors, it would seem that the voltage follower route would decrease the S/N ratio but altering the grid/cathode resistor values would be more work to calculate.

Is there an easy way to calculate the change in gain from varying the grid/cathode resistors or does this throw the whole thing off too much to make it easy? Thanks in advance!!!

Most of these you'll see in a production amp have an unbypassed cathode resistor on the stage that feeds the cathode follower stage, in which case, fiddling with Rk or Ra will alter the gain. If Rk is fully bypassed though, it doesn't really have any effect on gain. In that case you need to reduce Ra. Cutting it from 100K to 50K will knock off 3 or 4dB. It'll also significantly reduce the plate output impedance. If the cathode is unbypassed, you can reduce Ra, increase Rk, or both but when you do, you also change the available voltage swing in the tube which can lead to clipping if the signal is big. Changing Rk also moves the bias point of the tube more or less positive, which can also lead to clipping (of either the top or bottom of the waveform). That's the principal reason Ra and Rk tend to get changed together -- to keep the signal "centered."

[ted01]

Hey Firestorm,

I want to thank you for your reply. I just learned more useful information in three minutes than I have in two weeks of reading TUT, Morgan Jones, the Mullard manual and richard Kuehnel's book Guitar Amplifier Preamps (the latter lost me on about page 7). Not intending to be negative, there's a TON of great info in all those books, particularly KO'C's TUT, but truthfully, I just don't have the math skills to follow along. What you just said helps me understand some of the things I've read and is JUST the kind of real world info I was hoping to find in all those books.

I'm a new-comer to this forum and I really appreciate the amount of knowledge and cool people here.

Ted