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Output Stage Questions


Guitarnut

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This is a portion of the Epi BC30 schematic that shows the output stage. The amp has a switch that switches from triode to pentode... though the tube appears to be tetrode. Maybe they're showing the screen grid (pin 4) and not the suppressor grid since it takes it's power from the cathode supply and isn't connected directly to a pin?

 

My first question is about the switch. In triode position, each tube's screen grid is connected to the "B" tap of the power supply. In pentode, they're connected to the plates. How does this alter the power output?

 

Second question: All the other tube stages have load (anode) resistors but the plates of the 5881s don't. Why not? Is the output Tx the load in this case? And I guess since the Tx won't pass the DC component of the power stage, no coupling cap is needed to block it?

 

output.jpg

 

Peace,

Mark

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Since posting this, I've found out the following.

 

What happens is that when in Pentode mode, the screens are connected to the "B" node of the PS. This gives a steady voltage to the screen placing it in it's intended role witin the Pentode.

 

With the switch in the Triode mode, the screens are connected to the plates and act as secondary anodes making the tube act more like a triode.

 

Fascinating...

 

Peace,

Mark

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