Pentode vs. Triode....Difference?

[kiteman]

The tone is definitely not as brilliant as in pentode mode and does of course break up earlier. I like jumping back and forth between the two though.

Agreed, I use the triode mode a lot. It sound softer compared to pentode.

[jim p]

I posted earlier what the changes were in the amplifier that I modified to have both pentode and triode mode. An analogy to what the triode vs. pentode mode is like would be dribbling a basket ball with the output transformer and speaker the ball your hand the output tube. In the pentode mode it is like bouncing a well inflated basket ball easy to bounce and will continue to bounce by it’s self for a while when you stop (the bounce being the harmonics or high frequencies you hear). While the triode mode is as if you took air out of the ball, you have to work harder to bounce the ball and it stops quickly if you stop. So in the triode mode more power is lost across the output tube with less power to the speaker and the speaker is stopped quickly by the action of the tube (lower harmonics you hear as less high frequencies).

[P.J.]

I posted earlier what the changes were in the amplifier that I modified to have both pentode and triode mode. An analogy to what the triode vs. pentode mode is like would be dribbling a basket ball with the output transformer and speaker the ball your hand the output tube. In the pentode mode it is like bouncing a well inflated basket ball easy to bounce and will continue to bounce by it’s self for a while when you stop (the bounce being the harmonics or high frequencies you hear). While the triode mode is as if you took air out of the ball, you have to work harder to bounce the ball and it stops quickly if you stop. So in the triode mode more power is lost across the output tube with less power to the speaker and the speaker is stopped quickly by the action of the tube (lower harmonics you hear as less high frequencies).

Very nicely explained!
=-) PJ

[jim p]

Something I just started to look into is the effect of the transformer on frequency response. The low frequency response is set by the reflected impedance at the primary in parallel with the output tube plate impedance then that being in parallel with the primary inductance of the output transformer. Bottom line the greater the primary inductance of the transformer the lower the frequency response so in simple terms a bigger output transformer will give you more bass response. Also if you lower the reflected impedance you will increase the bass if you connect a 4 ohm speaker to the 8 ohm output the reflected impedance will half so 2500 ohms instead of 5000 ohms so if bass response was down to 100Hz it would drop down to 50Hz. What the triode mode does is change the plate impedance to 2k ohms from say 50k ohms so with 5k ohms reflected in the pentode mode you have 4.5k in parallel with the transformer impedance while in the triode mode it would be 1.4k ohms. So if you’re bass response was down to 100Hz in the pentode mode it would drop to 30Hz in the triode mode. So this is another reason the triode mode will sound darker.
Another way to increase bass response is adding feedback to the amplifier. The triode mode is self inflected feedback because the control grid is not isolated from seeing the output plate as it would be in the pentode mode.

Hope this does not muddy the waters of understanding what’s going on in your tube amp.