6V6 MOSFET driver stage failures

[billings]

I've got an amp that happily had MOSFET followers in front of its power tube grids for quite a while. One day, I got a bug in me to swap out the tubes from EL34s to those JJ 6V6s that can take such a beating and all. Something blew and the tubes started redplating.

A postmortem revealed that my crude old bias circuit had died due to overvoltage on the gate, in the process, had caused a grid to cathode short in the 6V6s

I took the opportunity to gut the bias circuit, redesign it, and install a new one. I've attached a rough schematic - I don't have all the specific parts I used in LTSpice, but these are close enough.

The bias circuit appears to be working fine now, even the old faulty tubes didn't faze it, but when I crank it all the way it still pops the 500mA fuse between the power tube cathodes and ground. I need to stop it from doing this, because whenever it does the amplifier stops working.

I have two ideas as to what might be going wrong, but I wanted to get some input here because I know that some people here have messed around with class A2/AB2 before and know much more than I do about it. I think it is possible that I'm pulling too much grid current and causing the power tube to fail; in that case, my guess is that the appropriate fix would be to limit positive signal to no more than some number of volts above ground with a zener somewhere or another. My other idea is that there is something non obvious about throwing a JJ 6V6S into a circuit meant for an EL34 that I am missing completely.

Thanks for anyone who can help me out here - maybe this should go into troubleshooting, but that forum seemed more concerned with production amplifiers than tinkering, so I posted here instead.

[Enzo]

Instead of assuming, scope the grids and SEE if they are going positive.


And if the grids were drawing current, what is the current rating of your FETs? 100ma of grid current ought not to overtax a 5 amp FET.

I am not up to speed on MOSFETs, any chance yuor problem is one of polarity shifts? That is if the grid rises to a point where the MOSFET sees it as a reverse polarity voltage somewhere it doesn't want to see one?

[Alex R]

B+ too high for (even JJ) 6V6?
Primary impedance too low ditto?

Sorry if I'm stating the obvious...

[Steve Conner]

Try adding grid resistors on the power tubes, maybe 1k. That will limit the grid current to a safer level, and stop parasitic oscillations if there are any. Heavy grid current can ruin tubes with gold-plated grids that weren't designed for it, by depositing cathode material on the grid IIRC.

Also look inside the tubes to see if your control grids are glowing red hot when cranked!

BTW, it's standard practice to include a 12v zener between gate and source of any MOSFET in a circuit like this. If you didn't have it already, you should.

[billings]

Thanks for all the responses. Here's what I've got:

1) B+ is 440V. I'm told JJs can take this, but you may be right. Primary impedance on the OT is spec'd at 3200; the amp is switched to 4ohm impedance and is hooked up to an 8ohm speaker load, so it should be seeing 6400.

2) Unfortunately I don't have a scope, so I can't verify that they're being driven positive. I can say for sure that this problem only occurs when the output tubes are being heavily overdriven, and it tends to occur more easily the hotter they are biased. In this case, the amp blew with the master volume wide open and the phase inverter in fairly heavy distortion at a bias of -30V. This is a marshall style LTPI with the negative feedback line cut.

3) 1K grid resistors were already soldered to the grid pins when the failure occurred, as were 12V zeners from gate to source on the MOSFETs. In fact, having just poked around again, it looks like the grid resistor on one of the tubes BLEW - 1/4W metal film resistor, looks like it just popped open on one side. Bad part?

[Steve Conner]

No, your MOSFET driver is probably quite capable of pushing enough current - and the grids capable of sinking enough as you try to pull them positive - to burn out those grid resistors. Once they have opened, the tube grid will float to zero bias, and the tube will conduct heavily and blow the fuse.

Just be thankful the resistors blow before the grids melt, and while you're at it, ponder what will happen to the clipping behaviour when you increase the resistor values to stop them frying, and then ponder why so few amp designs attempt to drive grid current.

The only ones that come to mind are the Fender 300PS and 400PS, and they had GE make them special 6550A tubes. The Ampeg SVT doesn't: its cathode followers are crippled by large grid stopper resistors, and a diode clipper upstream of the PI limits power amp overdrive even more.

[billings]

I guess I'm slightly incredulous that the driver is pushing enough *voltage* to blow the grid resistor, but it's hard to argue with the blown resistor right in front of me. I guess an increase in grid resistance is what is called for here. This has certainly been a learning experience.

Thanks so much, it can be very difficult to get help with silicon support circuitry in guitar amps. I appreciate it a lot.

[Bruce / Mission Amps]

...
Thanks for anyone who can help me out here - maybe this should go into troubleshooting, but that forum seemed more concerned with production amplifiers than tinkering, so I posted here instead.

How do you open that .asc file?

[Steve Conner]

ASC is a LTSpice schematic file. You need SwitcherCad/LTSpice from Linear Technology to open it.

[billings]

GIF attached.