Metropoulos Forum

as we know they are 82k and 100k...........

why different? is this because the plates in the driver are not usually balanced?

if you had a tube w/ balanced plates, could you then use two 100ks?

or is it the "nature of the beast" when you split the signal in two? thanks

for any enlightenment!!!

lifer wrote:as we know they are 82k and 100k...........

why different? is this because the plates in the driver are not usually balanced?

if you had a tube w/ balanced plates, could you then use two 100ks?

or is it the "nature of the beast" when you split the signal in two? thanks

for any enlightenment!!!

The plate resistors are purposely mismatched to give more juice to one triode than the other. The plate voltage in one half will be greater than the other, resulting in a different amount of drive to one side of the output circuit than the other. The difference is slight but noticeable to the human ear. In fact, because you don't get an identical signal going to both sides of the output section, the difference gives you some extra harmonic detail. It is conceivable that you could get a PI tube mismatched just right so that the tube draws less current from the 82K on one side and more current from the 100K on the other side, and actually end up giving you an identical signal strength going out of the PI and into the output tubes. But that's pretty outlandish... You'll find this mismatched PI quite popular. Values may change; some Dumbles use 100k on one side and 110k on the other side, for example. The PI in our Marshalls takes part of the output signal and brings it back in in the form of NFB. The NFB goes in the 100K (weaker plate, non-inverted) side of the PI, and the preamp signal goes into the 82K (stronger plate, inverted) side of the PI. The fact that one side gets the NFB also was a reason behind giving one side the larger plate resistor, as the NFB signal will add a little something extra to that side, bringing the output back closer to being matched. Either way getting a perfectly balanced phase inverter is practically impossible. Paying for matched triodes in a PI tube is probably not necessary.

hmmmm....interesting

so in reality an actual "balanced" signal might not be as dynamic?

lifer wrote:hmmmm....interesting

so in reality an actual "balanced" signal might not be as dynamic?

Well, in a high fidelity sound reproduction amplifier, the goal is to get your push-pull circuit fairly balanced.

But in a guitar amplifier, which is a sound production device (I think of them as instruments in a way), the mismatch gives you a desirable effect. Going from a matched PI in a guitar amp to a purposely mismatched one might be described as going from a "more sterile sound" to a "more complex sound." So in terms of being more tonally dynamic, yes, but not in being more dynamic volume (softness/loudness) wise.

Best explanation I've ever read of this "shirts". Thanks.