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

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    Quote Originally Posted by nickb View Post
    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.
    Thanks, Nick! I actually missed Enzo's reply (which I think I would have missed the point of with the triac anyway). Before I try to hack this up, I redrew the schematic. Is this what you meant?

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    Quote Originally Posted by Enzo View Post
    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)
    Oops, missed this before my last post. I think you're correct because the relay clicks with high gain can be heard turning the relays on or off. It's seems there's more than one reason they don't use SSRs. I understand the tried and true method is cheap and simple as you said, but it would be nice to have an alternative muting device that could handle higher signal voltages, especially when trying to use relay switching in vintage style designs where there's not much opportunity to bring the signal down to safe levels.

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  3. #38
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    Quote Originally Posted by Enzo View Post
    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)

    Yes indeed, thx. I missed that point. I had wondered why they chose a triac and what you suggests make complete sense. So, no, a transistor will solve the hold issue but won't get the dual edge switching unless you add quite a bit more complication.

    The original Peavey design is starting to look close to optimal.

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  4. #39
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    Quote Originally Posted by Gaz View Post
    Thanks, Nick! I actually missed Enzo's reply (which I think I would have missed the point of with the triac anyway). Before I try to hack this up, I redrew the schematic. Is this what you meant?

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    Well, not so fast with the praise. Yes you understood

    But in order to do both edges you need yet another transistor. [Schematic deleted as it was wrong]

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    Last edited by nickb; 03-04-2020 at 08:00 AM.
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    Nick, it works! However, when the relays are switched ON the mute is shorter. I can hear the click come through and visibly see the LED flash for a shorter period when I have it hooked up in place of the SSR.

    Any reason you can think of for this? Right now (if I understand correctly), the mute should be about 47ms right with the 4.7uF/10K? I think it's right when the relays shut off, but not on.

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    Quote Originally Posted by Gaz View Post
    Nick, it works! However, when the relays are switched ON the mute is shorter. I can hear the click come through and visibly see the LED flash for a shorter period when I have it hooked up in place of the SSR.

    Any reason you can think of for this? Right now (if I understand correctly), the mute should be about 47ms right with the 4.7uF/10K? I think it's right when the relays shut off, but not on.
    Sorry. It was late last night. With fresh eyes this pnp/npn thing will never work right. The led will never get fully turned off.

    Honestly, I cannot think of a simple way to make this work with an SSR. The best I've got is to go a variant of the original JFET design but instead of the JFET doing the muting, it drives the SSR. Something like this:


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    This begs the question, can't you just use the JFET on it's own?

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    Last edited by nickb; 03-04-2020 at 07:39 PM.
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    There's another way to pick up the relay spike in both directions using an opto element with back to back diodes like the one on the schematic below.
    The other ways is to use Mesa's mute circuit but in order for it to work properly you need at least one relay switching on and at least one off.

    Click image for larger version. 

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    Quote Originally Posted by nickb View Post
    Sorry. It was late last night. With fresh eyes this pnp/npn thing will never work right. The led will never get fully turned off.

    Honestly, I cannot think of a simple way to make this work with an SSR. The best I've got is to go a variant of the original JFET design but instead of the JFET doing the muting, it drives the SSR. Something like this:


    Click image for larger version. 

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    This begs the question, can't you just use the JFET on it's own?
    Hey, yeah, I did notice (not at first) that the signal was reduced even with the LED "off."

    That's cool, another idea to try, and really the original circuit is fine, but using an SSR means I could stick the mute anywhere in the circuit even with high signal levels. I am out of town for a few weeks so can't try anything until I'm home :/

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    Quote Originally Posted by Gregg View Post
    There's another way to pick up the relay spike in both directions using an opto element with back to back diodes like the one on the schematic below.
    The other ways is to use Mesa's mute circuit but in order for it to work properly you need at least one relay switching on and at least one off.

    Click image for larger version. 

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    Thanks Gregg, this is a cool schem too. Did you come up with this? What are the resistors for? I will have to try that when I get home in a few weeks too.

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    It's not my idea. It comes from the link below (Pops, clicks section but there's other interesting info there as well) and it's all explained. I just added the SSR. It's tested and works.

    http://guitaramplifiers.prv.pl/misc.htm#Pops

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    Quote Originally Posted by Gregg View Post
    It's not my idea. It comes from the link below (Pops, clicks section but there's other interesting info there as well) and it's all explained. I just added the SSR. It's tested and works.

    http://guitaramplifiers.prv.pl/misc.htm#Pops
    It's still good idea. I didnt know optocouplers with back to back LEDS existed. It made me wonder if anyone makes an SSR with back to back LEDS. If they did then you could feed the SSR directly and that would be the simplest possible scheme.

    Anyway, it did give me the inspiration for this, using diodes instad of the bi-directional opto-coupler:

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    Can't wait to try these ideas. I will report back!

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    Quote Originally Posted by nickb View Post
    It's still good idea. I didnt know optocouplers with back to back LEDS existed. It made me wonder if anyone makes an SSR with back to back LEDS. If they did then you could feed the SSR directly and that would be the simplest possible scheme.

    Anyway, it did give me the inspiration for this, using diodes instead of the bi-directional opto-coupler:

    Click image for larger version. 

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    Back home early from my trip for obvious reasons, and have more time to play with this to distract myself.

    I tried this setup, but the LED stays on in one direction. I'm guessing I may have it hooked up wrong. Are there any other parts necessary in this schem besides a LED limiting resistor (I am using 2.2K)?

    I have the relay ground going to the 470R resistor for the trigger, and the 12V power source through the 2.2K resistor to the anode of the LED. Is that correct?

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    Quote Originally Posted by Gaz View Post
    Back home early from my trip for obvious reasons, and have more time to play with this to distract myself.

    I tried this setup, but the LED stays on in one direction. I'm guessing I may have it hooked up wrong. Are there any other parts necessary in this schem besides a LED limiting resistor (I am using 2.2K)?

    I have the relay ground going to the 470R resistor for the trigger, and the 12V power source through the 2.2K resistor to the anode of the LED. Is that correct?
    Nick, you crazy son of a bitch. You did it I realized the schem you drew needed no extra parts. It works great, and it seems as though you've come up with a robust mute circuit using a resistor, cap, ssr, and 4 diodes that can be used anywhere in the signal path!

    The only thing I had to adjust, and maybe you can explain this is the resistor cap combo. At first the mute was quite long, and I got a short time (but long enough to silence the pops) using a 220-470R resistor and 22uF cap. Can you explain why this might be so?

    Thanks again!

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    The resistor needs to be small enough to ensure the LED lights up, but not so small so at to break it. The cap needs to be big enough to just mute the pop. The original 220uF was just a conservative guess. If 22uf does it then great!

    So happy you got it to work in practice rather than on paper.

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    Quote Originally Posted by nickb View Post
    The resistor needs to be small enough to ensure the LED lights up, but not so small so at to break it. The cap needs to be big enough to just mute the pop. The original 220uF was just a conservative guess. If 22uf does it then great!

    So happy you got it to work in practice rather than on paper.
    Thanks, Nick, yeah I'm really happy with the performance, and came across something interesting. I noticed that when I got the limiting resistor down to 220R or less the click transient was silenced completely. I wasn't sure why this was so, but came across an interesting article from Vishay that mentioned faster turn on times using a peaking circuit.

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    With 220R and below the SSR switches so fast it's able to catch the transient completely, and since the duration is so short 50mA through the LED shouldn't be a big deal, right? The article says limit to 300mA, and the absolute peak rating for the SSR I'm using is 1A. I'm thinking

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  17. #52
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    You are correct. Yes, the right thing to do is use the biggest resistor that give you the pop suppression you need but not so small as to nuke the SSR.

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