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Old 15th April 2012, 06:08 AM   #1
nazaroo is offline nazaroo  Canada
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Join Date: Feb 2012
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Default Output Stage Protection Circuit

I was looking at tube amps, and noticed this slick protection circuit in a 1990s Marshall rack unit:

Click the image to open in full size.

I've put it in an example self-bias Push-Pull stage.

Here's how it works:

(1) If one tube shorts out, and stays that way, the 300 mA Fuse blows, which is far better than a 1 ohm resistor which could start a fire or wreak havoc to other parts. (also, strategies like one 1 ohm resistor per tube don't work, because the other half of the push-pull pair still conducts as if everything's ok!)

(2) The High Voltage (HV) appearing at the bottom of the tube (top of cathode resistor) has been cut off from the cathode resistor, as well as the current in the tube on the other side of the push-pull (if they share a common fuse).

(3) The HV now may appear through both cathode resistors at the bottom of the other tube, but since this virtually puts the voltage across the good tube at zero, it remains safe, and no current flows that way either. Remember, the heaters may still be on in both tubes! *

(4) Although there is HV appearing now at one side of the fuse-holder, hopefully the lack of sound and/or indicator warns the user to turn off the amp. The fuse-holder itself can be recessed deep within the amp, while the LED indicator can drop most of the HV on the other side of its feeder resistor, which is also recessed back at the cathode resistor network. No HV needs to appear near the front of case.

(5) There is only one snag. Which tube blew? Well, both need to be pulled (with the light indicating which PAIR has the problem, if there is more than one pair). Possibly inspection can reveal the dangerous tube before even plugging it in a tube-tester.

(6) A nice side-effect is that if the fuse just failed because of a surge or fault, and the tubes are ok, there is no HV at all, either on the cathodes or at the fuse-feed. Its safe to change fuses. Because of this, No LED lights up either, indicating no tube is permanently shorted, creating an unguessed danger.

(7) On the other hand, if the LED fails to light up, or someone doesn't know what it means, there will be danger in unexpected places inside the chassis. Its not foolproof.

(8) One way to improve the design (with some costs) is to have the current also control a relay to the HV at the top of the Output Transformer. This would remove the HV at the plates, and redirect it to indicate the blown fuse (another way to ensure the warning light stays on).

(9) All in all, its an improvement over a simple fuse at the c.t. of the Output transformer, because the HV won't be sitting at the fuse-holder, tempting some idiot to replace the fuse with cigarette foil in a pinch and kill himself.
The fuse at the low end of the HV supply offers some measure of additional safety.

But I think the idea could be improved more, and made even safer, if others want to have a go at this. Its such a good start that it needs a good finish.

* there is an additional problem with common cathode configurations like this, because the HV is now being applied through the bad tube to the cathode of the GOOD tube: This sounds good until you realise that the GRID is still being held at virtual GROUND. It is possible that a reverse-current between Grid and Cathode could take place, and this would be very bad! (arcing and damage?) One tube could still take out both tubes! Additional safety could be added to prevent this, by automatically lifting the cathode and/or grid of the other tube via a relay).

Also, most heaters expect no more than a 100 v difference between the cathode and heater! If the heaters are floating supplies that might help (although noise might increase). But arcing between cathode and grid or cathode and heater could take place in the good tube if it is tied to the bad one.

Obviously there is room for further improvement of the protection circuit!

Last edited by nazaroo; 15th April 2012 at 06:16 AM.
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