Does anyone have a schematic, (or sell a PCB), for a circuit that can monitor the voltage drop across a 1 ohm sense resistor and output a latched high output if the voltage goes over the pre determined set point? Whereby if the voltage rises over a set point it will output a latched high that can then be used to energize a relay that cuts mains power via its NC contacts. For example your amplifier loses its bias for some reason, your four 300B tubes worth $1,000 start to melt down and you want the whole amp to simply turn itself off. I see that the INA300 chip does exactly what I need, but I don't want to start from scratch on that.
I'd use an arduino to that, powered by it's own psu and equipped with some display. The arduino may
be used to control the power and after sensing overcurrent should display the reason for power down in the
display, that's why it should have it's own psu.
Just breaking power without any more disgnostic outputs won't tell why the amp just shut down.
be used to control the power and after sensing overcurrent should display the reason for power down in the
display, that's why it should have it's own psu.
Just breaking power without any more disgnostic outputs won't tell why the amp just shut down.
If that's solely the only function of the circuit, I would imagine it would be obvious why the amp shut down. That's just my .02.
You can buy simple comparator PCB kits online, no problem. But a circuit that turns off the power when the current is too high sounds a lot like a fuse... ahem
You could consider fusing the cathodes of the tubes, and maybe the anodes. I think that if the 1 ohm resistor is sized (in terms of watts) properly, that might work as well, eg as a fuse. This is assuming that you've got that resistor in between the cathode and ground.
Of course, that's not an easy reset 🙂 But I would imagine that losing bias would not be a common thing to have happen, and if it did you'd probably need to open up the amp anyways.
Edit: but yeah, an inexpensive SBC that has enough oomph to drive a mains relay could do that with very few parts.
Of course, that's not an easy reset 🙂 But I would imagine that losing bias would not be a common thing to have happen, and if it did you'd probably need to open up the amp anyways.
Edit: but yeah, an inexpensive SBC that has enough oomph to drive a mains relay could do that with very few parts.
Few years ago (during the Covid) I played with fixed bias protection problem.
Simulated this schematic, but I've never tried it in practice.
BTW I used fix biased SE amps for decades ... never needed anything like this before.
Yes, arduino IS an inexpensive SBC that can control a relay and has analog input(s)
If you want to protect in the event of ''lost bias'' you could just power a relay off the bias voltage that opens the heater/filament circuit and you'd hear the loss of the music. You want enough bias filtering capacity to make that loss a slow and smooth process, but drop the heater quickly.
Yup. So is a Teensy, lol and there are others, including very inexpensive older Raspberry Pi models.
Anyways, any of these that include an ADC and a single digital output pin can be used for this purpose. It should be a trivial exercise to use any of these SBC to perform threshold detection in code that will drive the logic for a relay.
So simple, just disconnect the $250 tube! Thanks Merlinb! (excerpt below circled red):
Patrick Turner (RIP) had circuit schematics for this very purpose on his website. The complete mirror site is here https://atrad-audio.co.nz/turneraudio/www.turneraudio.com.au/index.html there are several examples under various projects.