Using Jfets for switching in tube amps - high voltage concerns

[Gaz]

Do I need to be concerned about start-up voltages getting to the fet when the coupling cap is charging? If so, how could I protect it? Would it be as simple as adding a couple zener diodes from the drain to ground? Thanks.

[nickb]

I doubt it as the current is limited by the 100K resistor. In addition the voltage falls slowly and the capacitor will greatly suppress that ( i = C * d(v)/d(t) ).

Of greater concern is that FETs are suited to small signal amplitudes of just a few volts. You can tens out of tube stage.

[Enzo]

100k limits current, but before tube conducts, full B+ voltage is at the plate. But the cap downstream end at the JFET also sees the volume pot. That serves to keep the cap grounded and so the voltage never comes through anyway.

But you are right about signal. What is the JFEt good for? Maybe 25-30v?

[Mick Bailey]

Why not relocate the FET to the grid?

[Gaz]

Why not relocate the FET to the grid?

I can definitely do that, but how would it change anything if the volume control was full up?

[nevetslab]

Why not relocate the FET to the grid?

I agree. The Mesa amps came to mind, where they use a lot of J175 P-Ch Switching Fets, that have -30V DG rating and 30V GS rating. Most of their usage is in the Grid section, though in one location they have that FET following the DC blocking cap on the plate circuit in their Reverb Send circuit. And, I've had to replace that FET on several Lone Star amps. They also make liberal use of VacTEC cells for switching/muting on countless amp designs, which tend to be for tube use, more reliable? They also have small relays everywhere, so there's lots of ways to do the switching.

[Gaz]

I doubt it as the current is limited by the 100K resistor. In addition the voltage falls slowly and the capacitor will greatly suppress that ( i = C * d(v)/d(t) ).

Of greater concern is that FETs are suited to small signal amplitudes of just a few volts. You can tens out of tube stage.

100k limits current, but before tube conducts, full B+ voltage is at the plate. But the cap downstream end at the JFET also sees the volume pot. That serves to keep the cap grounded and so the voltage never comes through anyway.

But you are right about signal. What is the JFEt good for? Maybe 25-30v?

Yes both of you are right about signal voltages and I’ve limited them earlier in the preamp down to reasonable levels.

Was just concerned about the the DC reaching the fet during the charging time. I’m not sure how the pot being grounded keeps the voltage from coming through before the cap charges. Could you explain that a little more?

Also would adding back to back zeners be cheap insurance to block any transients from coming through? I was already going to add some to limit voltage to the fet just in case.

Thanks!

[uneumann]

Also would adding back to back zeners be cheap insurance to block any transients from coming through? I was already going to add some to limit voltage to the fet just in case.
Thanks!

Sure - zeners would help, but the transient coming through the cap is potentially full B+ voltage. Yes, current is limited, but voltage can fry semis at low currents - as with static zaps. There are options that seems better and safer. The DN3545 is a mosfet with 10 ohm on resistance and rated at 450 volts. Or how about solid-state relays - they're rated at 300-400V. I suspect Jfets are attractive because they're cheaper. But if you really want to be sure something is safe and reliable, maybe cheaper isn't the only criteria to consider.

[Gaz]

Sure - zeners would help, but the transient coming through the cap is potentially full B+ voltage. Yes, current is limited, but voltage can fry semis at low currents - as with static zaps. There are options that seems better and safer. The DN3545 is a mosfet with 10 ohm on resistance and rated at 450 volts. Or how about solid-state relays - they're rated at 300-400V. I suspect Jfets are attractive because they're cheaper. But if you really want to be sure something is safe and reliable, maybe cheaper isn't the only criteria to consider.

I'm totally open to using a different device if it allows me more flexibility with placement. I'm just super green when it comes to fet and Si in general, so any advice would be appreciated. I do understand I would need a negative supply for an NPN, but not sure if I would need depletion or enhancement for example. Any help would be appreciated!


I'm copying this Peavey circuit like below (also here is the full JSX schematic it's from, for example.)


They use this circuit in countless amps, but just noticed this one they actually use it after the blocking cap. They do use zeners (I don't know if they are to limit signal voltage or DC start-up voltage like I am worried about). There is 100K resistors in series as well, but I don't know if those are relevant to the jfet mute circuit or not...

[uneumann]

... I'm just super green when it comes to fet and Si in general, so any advice would be appreciated. ... They use this circuit in countless amps, but just noticed this one they actually use it after the blocking cap. They do use zeners (I don't know if they are to limit signal voltage or DC start-up voltage like I am worried about). There is 100K resistors in series as well, but I don't know if those are relevant to the jfet mute circuit or not...

These are tricky circuits to just copy/paste into a new amp - You have to view the FET, the signal levels being muted, the control signal to the FET all together as a system. In particular, the control signal has to be properly created at power up and power down to ensure that the FET is "on" so the power transients don't damage it. It's more complex that a simple "do this" and it will work.

You could consider this an "opportunity" to do some reading and get familiar with solid-state basics. JFETs and mosfets like the LND150 operate in ways that are similar to tubes so the leap is not hard.

That said, for a new design, you may want to look into solid state relays - like the AQY210EH series of parts. They are far more forgiving and simpler to use (IMO) since the control and relay signals are completely independent (they are optically coupled) and power transients are less of a concern when the parts are rated at 350v or higher.

[Gregg]

A 300-600V DIP4 SSR will do the job. It has a DCR of ~50 Ohms when on which is the same like J175. You can also put some back to back zeners for extra insurance.

[nickb]

Sure - zeners would help, but the transient coming through the cap is potentially full B+ voltage.

I don't see this for realistic applications unless you are talking about fault conditions. This application would be in the preamp. The supplies will be filtered by something like a 22uf and 10k resistor making the time constant about 0.5 seconds and the voltage say 400V. The current through the FET will be limited by the 47nf cap to something like i = 47nF * 400/0.5 ~= 40uA. I can't imagine such a small current could cause damage to the JFET.

Not that zener protection isn't a bad idea, but it just might be overkill.

[Gaz]

These are tricky circuits to just copy/paste into a new amp - You have to view the FET, the signal levels being muted, the control signal to the FET all together as a system. In particular, the control signal has to be properly created at power up and power down to ensure that the FET is "on" so the power transients don't damage it. It's more complex that a simple "do this" and it will work.

You could consider this an "opportunity" to do some reading and get familiar with solid-state basics. JFETs and mosfets like the LND150 operate in ways that are similar to tubes so the leap is not hard.

That said, for a new design, you may want to look into solid state relays - like the AQY210EH series of parts. They are far more forgiving and simpler to use (IMO) since the control and relay signals are completely independent (they are optically coupled) and power transients are less of a concern when the parts are rated at 350v or higher.

Thanks, I do understand the complexity of adding this to an existing amp, and my. One of the attractions to the Peavey mute circuit is that for most designs that rely on preamp distortion you can shoe-horn it in . Peavey and Mesa among others use it on almost every amp. I've gotten the circuit to work and the signal levels are low where I'm muting so that the FET does not turn on with audio signal. Just was having trouble seeing how to protect the fet from DC voltages at start-up and if it was necessary. My confusion lies with more of the tube part of circuit!

In particular, the control signal has to be properly created at power up and power down to ensure that the FET is "on" so the power transients don't damage it.

Can you explain this more?

Thanks!

[Gaz]

A 300-600V DIP4 SSR will do the job. It has a DCR of ~50 Ohms when on which is the same like J175. You can also put some back to back zeners for extra insurance.

Thanks Gregg, I'm going to look into these for future projects. They won't work for this specific mute circuit, but I could see using these instead of mechanical relays all around with some considerations. I've read they have high capacitance and worse "offness" than mechanical relays.

[Gaz]

I don't see this for realistic applications unless you are talking about fault conditions. This application would be in the preamp. The supplies will be filtered by something like a 22uf and 10k resistor making the time constant about 0.5 seconds and the voltage say 400V. The current through the FET will be limited by the 47nf cap to something like i = 47nF * 400/0.5 ~= 40uA. I can't imagine such a small current could cause damage to the JFET.

Not that zener protection isn't a bad idea, but it just might be overkill.

Thank you, Nick. Working out this real-world example for me is really helpful. I didn't know how to do the math to find the current. So even though there is a transient of 400v (for example) because it's only at 40 microamps it will likely be okay? I guess I don't really understand the relationship...

Thanks again