Polyfuse/ polyswitch to prevent tubes from redplating.

[beasleybodyshop]

Anyone ever try to use these?

http://en.wikipedia.org/wiki/Resettable_fuse

my thought was to use them on power tube cathodes, in the event of massive current draw they would essentially disconnect the tubes cathode from ground, preventing problems.

Forgive me if this is well trodden ground, but does anyone use these?

[JMFahey]

They may work to prevent larger damage in a short, like any fuse, but they are not precise/sensitive enough to, say, "let the amp work normally but just block redplating".

Probably will either cut your amp in the middle of a hot solo or not react to redplating.

Big problem is that they are not adjustable, because in that case yes, you could set them to trigger on just barely redplating.

[R.G.]

Back when I first started messing with power tubes, I came up with what I called the tube-saver.

This was a circuit that watched the voltage across a small resistor in the cathodes of a power amp and suitably time-filtered them to not trip on short transients. This signal was then run into a comparator that fired some CMOS logic to turn a power MOSFET on/off in series with the cathode(s). This could be set up to latch, to time out and un-latch, or to stay latched until power was reset, and also to light an LED to tell you which tube had just been saved.

Trip current and time were adjustable, of course.

It's pretty simple to do, and for the life of me I can't figure out why there isn't a commercial product like this. Shoot, I'd do a commercial product to do it if I were not so opposed to me being a shop keeper.

With the 15 or so years since I did that, I'd probably case up all of the CMOS into a single $1.50 microcontroller today, and have a setup with the sense resistors, a MOSFET, a few R/C jellybeans and the controller.

Come to think of it, why *don't* I go make these to sell??

[JMFahey]

Really, you should.

And I'm speaking seriously.

No need for microcontrollers or much Logic either, I just thought on how to make something equivalent with a MosFet and a couple TO92 devices.

Well, I'm cheating in a way, I already use a similar concept, constantly watching power dissipated in (as opposed to delivered out of) power transistors, coupled to a relay which cuts power off.

Designed for customers which have less clue than normal, go figure, so it turns amp off, if they fail to react to danger signs.

[xk49w]

Come to think of it, why *don't* I go make these to sell??

PIC12F1571 for example, 8 pins, four 10-bit ADCs, 1.75 kbytes pgm, 128 bytes RAM, 128 bytes EE, internal oscillator, $0.66 in singles.

[beasleybodyshop]

I can imagine that maybe those types of safety systems arent used due to the fact that its much cheaper to install a 250ma fast blow fuse between the cathode and ground

I was asking about the polyfuse because i just discovered a few of them are used in a live sound console i have been working on - they are between the phantom power circuit and XLR outputs/bipolar supplies for the mic pre op amps. I thought it was a great idea when i finally discovered what they were!

[R.G.]

Yeah, I love to work with PICs.

Actually, I went off and rediscovered some earlier work I did on this. I did a combined "traffic light" bias system with the overcurrent trip in a many-pin PIC.

I used a red-light/green-light bias indicator system in the Workhorse amps, and it was very well received (well, by those that bought Workhorse amps, anyway ) so I just extended that to use a red/green/blue LED for too-hot/just-right/too-cold indication for bias.

That system depends on a cathode resistor for sensing cathode current in each tube, so it's easy to process that same signal for sensing overcurrent. The PIC makes it flexible in application.

The biggest issues are (of course!) powering the add-on circuits inside a tube amp and the mechanical mounting and such to actually be useful as an add-on. I'm actually looking at that a bit now. It might be possible to get this whole mess into a Hammond 1590B box and power it from the 6.3V heater supply. Maybe.

[Leo_Gnardo]

I was asking about the polyfuse because i just discovered a few of them are used in a live sound console i have been working on - they are between the phantom power circuit and XLR outputs/bipolar supplies for the mic pre op amps. I thought it was a great idea when i finally discovered what they were!

What console? Somebody had a good idea!

[beasleybodyshop]

I was asking about the polyfuse because i just discovered a few of them are used in a live sound console i have been working on - they are between the phantom power circuit and XLR outputs/bipolar supplies for the mic pre op amps. I thought it was a great idea when i finally discovered what they were!

What console? Somebody had a good idea!

Leo, for what it's worth, it was a nice Soundcraft GB8 series live console. 40 channels (4 stereo channels) and outputs for left, right, and center. Very well built and designed piece of kit.