Peavey Fet protection

[CJ]

amps with blown J174 fets keep coming in, don't know if there was an electrical storm when the lot came in or what,

looking for a way to protect the gate, any ideas?  there are relays around the area that could be the perps of this strife and grief

thanks!

Valve King 112 is the amp. same fet circuit used on other Peavey amps. solve this one, solve them all.

 

[JohnRoberts]

The way you truncated the schematic you assume the fault is in the gate drive?  Is the failure coincident with the remote switch operation..?

If it is the gate, a C to ground might eat static charges, but if the fault is coming in through the drain, a cap could make that worse. and  a 2M gate resistor could get too slow with a cap to gnd...

JR

PS: I remember John Fields, he was a junior engineer last century when I knew him...

 

[CJ]

never thought about the other terminals, doh on me here is another slice of the pie>

 

[JohnRoberts]

I am not a tube guy, and that looks like the low voltage end of the tube (I think?) but do any turn on-off transients exceed the JFET breakdown voltage?

The fault is assuredly voltage punch through, so maybe diode clamps to +14V rail at gate/drain?

If it is coming through the gate perhaps change 2M to two 1M with cap at gate. (speculation).

Can you force the failure to occur (probably not)?

JR

 

[PRR]

Without bothering to grok what the kid in the design room was thinking...

_I_ see a JFET gate connected to a 1/4" jack tip. With NO series resistance. (Duh!!)

Maybe 100% of the users ONLY plug the PV-approved control pedal 100% of the time. Maybe bears only poop on Charmin. Maybe the check is in the mail.

But it is far too easy to make an "oops." Small-oh oops, because no smoke and no pop. Even if the mis-connect is noted and corrected at the time, it won't be remembered in the morning. Only that you woke up and the boost does nothing.

Add the dang resistor the kid didn't.

 

[CJ]

thanks for he help!

will add that resistor, we changed the 2.2M to 1M and suck a 47 pf across it ,

that cap probably will not do much, we need a storage scope is what we need.

fets can get damaged in the assembly process, so we were careful to not touch the new one,

put a scope on the gate and switched overdrive on and off, a little spike but not much,  we got a bigger spike when turning the power sw on ad off,