dumb question for a newbie..

[Guest]

What do the output coupling caps do?

[Billy Batz]

The same thing all coupling caps do. Coupling is just a term that means connecting or in between. It couples 2 different stage. A stage is where there is gain added. That doesnt mean a boost. Gain can be positive or negative depending on what the stage is doing. But in a tube stage a primary function of a coupling cap (what caps are used for moslty) is to block DC. The tube works in part by applying a DC voltage to the tube plate. The actual audio signal is a much smaller AC voltage which 'rides' on the DC voltage to the plate until it gets to the coupling cap in which the AC signal can pass and the DC doesnt pass. Im not really that knowledgeable about the specifics of that. You can think of the plate as the audio signal 'output' of the tube in most preamp stages for the audio signal the same time the voltage is applied to the plate. Capacitors have an effect on AC audio voltages too. They block bass depending on the value. The higher the value the more bass gets through. YOu can keep lowering the value right up through the mids and tebles until the signal that can get through is so high its not audible. In the preamp Fenders have some large coupler values throughout the amp up to .047uF so many Fenders are woofy loose and flabby. Marsghall has a more 'slim' .022uF in most amps. AC30 top boost channels have something like 500p (.0005uF). Get it?

The output couplers are in the output section in the Phase Inverter. The PI splits the signal to two out-of-phase halves to work in a Push-Pull power section. One half of that tube inverts the signal. The PI is more or less a gain stage. So the same signal needs two couplers for each out-of-phase side of the PI. The most common value is .1 as in many Fenders, Marshalls and Voxs, then .022 in many Fdnders and Marshalls and other amps. you could use .047, .065, .01 etc...