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Thread: Using Jfets for switching in tube amps - high voltage concerns

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    Using Jfets for switching in tube amps - high voltage concerns

    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.Click image for larger version. 

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    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.

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    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?

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    Why not relocate the FET to the grid?

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    Quote Originally Posted by Mick Bailey View Post
    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?

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    Quote Originally Posted by Mick Bailey View Post
    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.

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    Quote Originally Posted by nickb View Post
    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.
    Quote Originally Posted by Enzo View Post
    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 Iíve limited them earlier in the preamp down to reasonable levels.

    Was just concerned about the the DC reaching the fet during the charging time. Iím 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!

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    Quote Originally Posted by Gaz View Post
    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.

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    Quote Originally Posted by uneumann View Post
    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.)
    Click image for larger version. 

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    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...

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    Quote Originally Posted by Gaz View Post
    ... 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.

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    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.

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    Quote Originally Posted by uneumann View Post
    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.

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    Quote Originally Posted by uneumann View Post
    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!

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    Quote Originally Posted by Gregg View Post
    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.

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    Quote Originally Posted by nickb View Post
    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

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    Quote Originally Posted by uneumann View Post

    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.
    In a preamp stage, as we are talking about here, there will no such (switch on ) full voltage B+ transient. The rise time will be very slow due to the filtering action of the preamp supply. A 10K/22uf is typical. Thus the current will be something like 100uA but more importantly the power delivered to the JFET will be quite low. ESD's zap semis because the transient is fast so the power is high but of short duration. JFETs being bipolar are quite tough.


    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.
    A MOSFET is not without merit but remember the gate is very easily destroyed on these so care needs to be taken in handling and circuit design. They are also more expensive, and yes, the low cost of a JEFT is an big attraction.

    If you run a JFET without zener protection I don't see any harm coming to JFET due to the low current. The drain / gate junction can be forward biased on an overload and this needs to be considered in the gate drive. This will also lead to the the coupling capacitor charging and that could have unintended consequences. In the other direction lies the breakdown region but with low currents and so reversible

    SSR are surely most the most robust but cost 20x to 40x that of a JEFT on quick look.

    It seem to me that provided the largest voltage swing is a volt or two less than the used gate off voltage, JFETs are a great solution, zeners a matter of preference I think. If the swing is greater go for a MOSFET but protect the gate. For easy to use, go SSR.

    EDIT: On reflection a single MOSFET won't work due to the intrinsic body source-drain diode. They are only suitable for unipolar applications.

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    Quote Originally Posted by Gaz View Post
    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
    I didn't mean to hit send on that one as it was half baked but I see that I did!

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    SSR are surely most the most robust but cost 20x to 40x that of a JEFT on quick look.
    It's more like 3x to 4x which is not much at all. Here are some high voltage DIP4 SSR prices compared to a typical J175 mute JFET.

    https://www.mouser.com/Electromechan...e&Ns=Pricing|0

    https://www.mouser.com/_/?Keyword=J175&Ns=Pricing|0

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    Cost isn't a big factor to me, I mean the whole amp probably cost $800 in parts or more, but I do understand of course why the big companies do it.

    What I'm wondering is if I can use this same Peavey/Mesa "trigger" circuit with a higher voltage device for muting, or will this only work with a P-channel FET? Although Nickb has pointed our the DC transient is probably not a big concern, there is so much anecdotal evidence of these fets failing I'd like to find a less delicate mute circuit. Plus, being able to handle voltage peaks above the gate voltage would be great. Then this could be used in "vintage" style amps with switchable elements where the signal voltage stays high all the way to the PI.

    Could I use the same triac trigger to close an SSR for example? Having never used one, I'm assuming the SSR's are faster than mechanical relays and click-less.

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    What I'm wondering is if I can use this same Peavey/Mesa "trigger" circuit with a higher voltage device for muting...
    Yes, you can use it. Connect SSR's LED cathode where JFET gate is connected. Run a 1k5-2k2 resistor from SSR's LED anode to 12V supply and then connect the switching part in parallel with the sound path. Add back to back zeners for extra protection (optional). Done.
    Check out the datasheets for the switching part capacitance but it's usually very low like 5-10pF which is inaudible.

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    Quote Originally Posted by Gregg View Post
    Yes, you can use it. Connect SSR's LED cathode where JFET gate is connected. Run a 1k5-2k2 resistor from SSR's LED anode to 12V supply and then connect the switching part in parallel with the sound path. Add back to back zeners for extra protection (optional). Done.
    Check out the datasheets for the switching part capacitance but it's usually very low like 5-10pF which is inaudible.
    Gregg, thanks for the reply, and I didn't expect that! Yeah, no sense in worrying about the low voltage Jfets then! I'm a little perplexed why bigger companies don't use an SSR triggered by a TRIAC. It would seem to give a lot more freedom. Expense I suppose?

    And with a 400V part, zeners do seem unnecessary right? I'm confused about value zener would even protect a part rated so high (sorry for my ignorance if I'm missing a potential failure point here).

    Thanks again.

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    Quote Originally Posted by Gregg View Post
    It's more like 3x to 4x which is not much at all. Here are some high voltage DIP4 SSR prices compared to a typical J175 mute JFET.

    https://www.mouser.com/Electromechan...e&Ns=Pricing|0

    https://www.mouser.com/_/?Keyword=J175&Ns=Pricing|0
    Lucky you! Here in the UK cheapest SSR was about $2.00 yet I can get JFETS for $0.05.

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    yet I can get JFETS for $0.05.
    Where do you buy JFets?

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    $0.05 is very low for anything.

    I checked Farnell's prices and they look kind of same as Mouser's:

    https://uk.farnell.com/panasonic-ele...503695?st=aqy2

    https://uk.farnell.com/search/prl/re...d&sort=P_PRICE

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    Quote Originally Posted by Gregg View Post
    $0.05 is very low for anything.

    I checked Farnell's prices and they look kind of same as Mouser's:

    https://uk.farnell.com/panasonic-ele...503695?st=aqy2

    https://uk.farnell.com/search/prl/re...d&sort=P_PRICE
    Which equates to USD2.00 as taxes are 20%

    I checked my inventory for JFETs and that was the price I paid. Mind you, I see it was a reel of 3000 from a few years back. In current prices $0.15-$0.20 might be closer for small qtys. That's still at least 10x.

    Anyway, the OP doesn't care about the price so it's of no consequence.

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    Quote Originally Posted by Gregg View Post
    Yes, you can use it. Connect SSR's LED cathode where JFET gate is connected. Run a 1k5-2k2 resistor from SSR's LED anode to 12V supply and then connect the switching part in parallel with the sound path. Add back to back zeners for extra protection (optional). Done.
    Check out the datasheets for the switching part capacitance but it's usually very low like 5-10pF which is inaudible.
    Gregg, got the Panasonic SSR and hooked it up like this, however it's not working quite right. When the panel switch is thrown that swithes the mechanical relays (and triggers the TRIAC), the sound gets muted, but stays muted, rather than just muting the "pop." If I wait several seconds I can flip the switch again to switches channels, but then it gets stuck in mute. Any ideas?

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    Firstable lease check if everything is connected properly. Stays muted for how long? Longer time constant?
    Disconnect SSR from the triac and check manually if the if it's working properly (repeatedly short SSR's LED negative end to gnd). If it doesnt get stuch then follow back down the circuit.

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    Thanks. Ok, so everything seems to be hooked up correctly with the SSR. I can disconnect from the triac and mute the signal as I ground LED. But when I hook up the triac it doesn't mute fast enough to stop the relay click and it gets stuck, muted. If I cycle through the channel switch a few times it will unmute, but the mute again.

    I also noticed if I removed the 12V supply from the Triac the same behavior continues. Did I destroy the triac perhaps?

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    Probably not. If you can break the circuit to release it, then it activates the next time you chanel switch, then it works. The triac won't release until the current drops to zero. Perhaps your SSR is a current path, the LED inside perhaps?

    That resistor isn't there for teh triac, which will conduct just fine without it. The resistor is there to provide gate voltage to hold the JFET OFF.

    You do realize, I hope, that the triac grounds the gate voltage of the JFET to allow it to turn ON.

    What does the data sheet say about the rise time at turn on?

    I'm a little perplexed why bigger companies don't use an SSR triggered by a TRIAC.
    Why would they? The part costs a lot more, and the JFET works just fine and is super simple.

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    The 1M/100nF combo has a 100msec time constant which means that after it shorts the gate (LED in your case) to ground it should release shortly after that.
    With disconnected SSR measure the voltage at the 1M/100nF junction. Then try a regular LED with resistor to 12V and cahtode connected as I suggested earlier and let us know.

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    Quote Originally Posted by Gregg View Post
    The 1M/100nF combo has a 100msec time constant which means that after it shorts the gate (LED in your case) to ground it should release shortly after that.
    With disconnected SSR measure the voltage at the 1M/100nF junction. Then try a regular LED with resistor to 12V and cahtode connected as I suggested earlier and let us know.
    Thanks, I did the LED test and it was still behaving in the same way (LED getting stuck on for several seconds). I checked the connections to the MAC97 again, and all was good so I replaced it with another I had just in case I had overheated it.

    The new one behaves differently now, but still doesn't work correctly. Now the LED lights up when the relays are turned OFF, but stays on. When relays are switched back on the LED does not light at all. The effect with SSR connected is it mutes one channel and not the other (and the pop comes through on the unmuted channel as well). Hope that makes sense.

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    Well, at least now we know that the problem is somewhere on the driving circuit side and the SSR is not the problem.

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    Yes, true. I also hooked up J175 again, and circuit works as expected, so I think perhaps there need to be some modification for it work with an SSR, but it's beyond my ken to reason why it works with the p-channel fet and not the LED. I attached a drawing of exactly how I have it hooked up. Maybe I'm missing something.

    Click image for larger version. 

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    Quote Originally Posted by Gaz View Post
    Yes, true. I also hooked up J175 again, and circuit works as expected, so I think perhaps there need to be some modification for it work with an SSR, but it's beyond my ken to reason why it works with the p-channel fet and not the LED. I attached a drawing of exactly how I have it hooked up. Maybe I'm missing something.

    Click image for larger version. 

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    With a 2.2K resistor the SSR LED current is about 5mA. The holding current for a MAC97 triac is typically 1mA and may be much less than that. Therefore the triac turns on and stays on. Enzo mentioned this above. A small triac just isn't suitable to use with an SSR. I suggest you use a transistor instead. The time constant with 0.1uf and 2.2k is going to be too short. You'd need more like 50uF but the current would need to be limited to avoid damaging the transistor, perhaps a 120 ohm in series with the transistor. I think it would be better to move the timing to the base circuit rather than the collector. Try 10K instead of 47 ohms and 4.7uf instead of 0.1uF. You can omit C2 in your schematic. Just about any small switching NPN transistor will do.

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    Well sorta. The reason they used a triac in the first place - in my opinion - was because it will fire in the quadrants. In other words a trigger pulse can go either direction and trigger the triac. A transistor would only be triggered by a positive pulse (or negative depends upon NPN or PNP)

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