Fender Protube Twin Q5 j113 FET?

[AndrewM]

Hi,

Can someone shed some light on what the Q5 switching circuit on the Fender Protube Twin is for? its directly before the pre-amp out / power amp in jacks and I cant figure out what its doing? It seems to just sit with -15V on the Gate and never change state.

I've attached the schematic hoping that someone can help explain its purpose.

Any advice you guys can provide would be great.

[Jazz P Bass]

Fender Pro Tube Eff Loop

My first impression is that the circuit is a current buffer.
Tubeless way of keeping the effects out from loading down the signal.

[martin manning]

My guess is it grounds the signal until the supply voltages stabilize, and the back-to-back Zeners across the source and drain limit the signal amplitude to 30V p-p.

[g1]

Haven't looked at the schematic but from what has been said it sounds like a muting circuit for the power amp so you don't hear a turn on thump. The -15 on the gate probably doesn't show up for the first couple seconds the amp is turned on.

[Enzo]

What they all said.

Q5 is a power on mute. It is a JFET, which means it is ON until turned off with a voltage at its gate. SO it is niormal to see voltage on the gate, and the transistor not doing anything.

If -16v supply is present, Q4 charges C37 through the 1 meg resistor. That takes a moment. Once the charge in C37 rises enough, the gate of Q5 turns it off.

MY take on the zeners is they protect Q5. Q5 connects directly to the real world through the preamp out jack. SO it is subject to static charges and who knows what. The zeners clamp it to no more than 15v across, one polarity or the other. Yes, it would also clip the signal path at anything over 15v, but that would be a huge signal coming out the preamp out. DOn't think it would get that large very often, and even if it did, it would be no threat to the tube circuits to follow.

[AndrewM]

I couldnt work out what it was for because it appeared the FET was always off. It makes perfect sense now!

Thanks guys!