dynosoar1 wrote: The first (and favorite) was simply to change the grid leaks on the power tubes from 470k to 220k. This will allow the power section breath and bounce a little better.
tkroeger wrote:Why is this??? Wouldn't this significantly drop the the output/volume on the amp?? Aren't you in effect bleeding more to ground by reducing this resistor?
Merlinb wrote:It doesn't affect the power section at all. It loads down the phase inverter more, reducing it's gain and output swing, so it can't drive the output stage as hard or as soon.
Actually, all of you are right. In terms of where the amp breaks up in pure loudness terms from the speaker, nothing really changes. So if you need an amp that's louder when clean, the 36W power stage helps buy you that.Merlinb wrote:Well, yeah, if you turn down the volume, the volume goes down... The power stage hasn't changed though. It's still capable of 18 watts if you drive it fully.
However, another frequent observation made about Marshall 18W style amps is that they start to break up at around 3 or 4 on the volume knob and after that point, don't get a lot louder, just get dirtier and more compressed. Changing the power stage grid reference resistors from 470k to a Vox-like 220k, scales back some of the drive to the EL84s, so that you get more clean range (ie later breakup) on the amp's volume knob. At max volume, you still get plenty of distortion and grind, although not quite as dirty and compressed as with the regular 470ks. This also means that the EL84s operate a bit more in their more linear zone, so that like dynasoar1 says, they "breathe and bounce a little better".
So to some people, reduced value grid reference resistors (220k or compromise values of 330k or 270k) on the power valves make more sense. I once tried the 220ks, and then out the 470ks back - as many folks know, I personally think too much clean tone is a waste of a good tube amp!
HTH