5D8 tweed twin hi/low input switching arrangement

[octal]

Can anyone explain how the unconventional input switching arrangement on the 5D8 Tweed twin works? It's very confusing. There's a mixer after each triode half pair, but the summing point is connected back to the grid of one of the input triodes.

[Willabe]

I think that because the output is 180 degrees out of phase with the input it cancel's the input signal, when nothing is plugged into the input?

Looks like Fender tried it and didn't like it as much as grounding the input jack with nothing plugged in.


             Brad     

 
 

[tubeswell]

One 12AY7 does the 'bright' channel, and he other does the 'normal' channel. One triode of each bottle is tip-switched (at its input jack) to the output stage of each pair, so that this has the effect of providing local NFB when the other triode is used as the signal input (because the signs being fed back from the coupling cap is 180 deg out-of-phase with the input signal on the other triode. The channel mixing resistors and vol controls allows for further subtle variation in this regard

[HotBluePlates]

So what happens when ... plugs a mic in one input, my guitar in the other ...

In that case, the normalled connection to the tip in the uppermost jacks in that layout is disconnected. Each triode acts as though it's just any other triode with no special circuitry.