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Old 10th January 2011, 08:15 AM   #11
Arnulf is offline Arnulf  Europe
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Originally Posted by tubelab.com View Post
There is no way to protect against this with a fuse. The usual protection method is to place a 100 ohm 10 watt or so resistor across the speaker terminals. This will steal a couple of watts of output power, and is not a guarantee either. Gas tube spark arresters like used in telephone equipment and big zener diodes across the speaker terminals is another possibility.
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Old 10th January 2011, 03:27 PM   #12
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Originally Posted by sampleaccurate View Post
PS: Why would you run your amp with no load???
Nobody here would. But if the dog wanders behind the gear and rips out the speaker leads, most of our spouses will happily listen to one channel of background music until hubby returns to fix the other. (...or until the smoke alarm triggers.)

...Todd
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Old 10th January 2011, 03:30 PM   #13
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Exactly. So an alternative approach would be to leave out all protection devices for the tubes, and simply eliminate dog & spouse from the equation, resulting in even better sonic properties (no barking wife & screaming dog in the signal path).
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Old 10th January 2011, 03:36 PM   #14
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Exactly. So an alternative approach would be to leave out all protection devices for the tubes, and simply eliminate dog & spouse from the equation, resulting in even better sonic properties (no barking wife & screaming dog in the signal path).
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Old 10th January 2011, 11:39 PM   #15
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Thanks for all the replies. I will definatley look into using MOVs in order to prevent high voltages from damaging the output transformer.

For the short circuit protection, could I just use a current sense resistor (~ 0.1 Ohm) on the HT and then turn off the HT when the current gets too high? I will also use a fuse but I want this amp to be as robust as possible to any abuse. I have the power supply mostly designed and it includes a 20 sec delay and soft start for the HT voltage.
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Old 11th January 2011, 01:50 AM   #16
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A couple of my Conrad Johnson's (MV-50 and MV-55) fuse the B+ to the center tap of the OPT. On the MV-50 they use 2 - 270K, 2W resistors in parallel then an LED in series & this string is in parallel with a 1 amp, 600v fuse (BBS-1 or BLS-1). If a fuse blows the LED lights. Seems to work. I've blown a couple fuses when tubes went bad.
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Old 11th January 2011, 05:56 AM   #17
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Originally Posted by desolationx View Post
Thanks for all the replies. I will definatley look into using MOVs in order to prevent high voltages from damaging the output transformer.

For the short circuit protection, could I just use a current sense resistor (~ 0.1 Ohm) on the HT and then turn off the HT when the current gets too high? I will also use a fuse but I want this amp to be as robust as possible to any abuse. I have the power supply mostly designed and it includes a 20 sec delay and soft start for the HT voltage.
Your initial question was about how to protect against no load and shorts, I assume a shorted output.

The MOVs should protect the transformer from no load conditions (BTW, I was being sarcastic about why anyone would run an amp with no load having done it myself more than once ).

Using fuses to protect the tubes at the possible expense of fidelity is not a wise move IMHO, but again, that's one person's opinion. I would try to keep all fusing on the mains side of the power tranny if possible, or certainly before any filtering on the secondary side of the power tranny. If you use good quality KT88s and you don't abuse the amp I think you're a little overly concerned, but it's your amp and your application. I think all the advice in this thread taken together should be a good starting point.
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Old 11th January 2011, 08:11 AM   #18
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Fusing the B+ only to pentode PP output stage (eg. KT88) has the disadvantage that if the fuse goes then the tube still has screen supply. Fuse may go through age, or for an event that doesn't damage the tubes, but likely that tubes will be damaged with just screen supply energised. Hence a common preference for fusing the cathodes. Not sure how well the parallel resistor-LED alleviates the stress on the tube - but sounds reasonable for a triode PP.

Some prefer to use a small cathode sense resistor that will 'burn up' on overload, as a poor mans fuse - but that has a rather ill-defined I2t characteristic, and provides an ignition source, and doesn't protect the heater-cathode insulation if the tube is not at fault. Fuses are pretty quality consistent with respect to fusing I-t characteristic, and can be fairly well matched to a PP loadline.

The thermal mass of the fuse element is likely to attenuate the voltage drop parasitic signal that adds to the fixed bias level in a fixed bias PP stage. Given that a hot 2A fuse is circa 0.11V steady state, I would anticipate any thermally induced parasitic signal from resistance change would be lucky to get in to the 10's of mV, and that would only be likely at low frequencies. The parasitic effect is likely to be even less in a cathode biased PP.

Ciao, Tim
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Old 11th January 2011, 10:38 AM   #19
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Some protection against overvoltage in the OT (due to over-light loading) can be made by installing antiparallel diodes across the output pentodes in PP. The diode-anode goes to ground , diode cathode to pentode anode (get this right or blow your B+). Series-connected 1N4007s x2 or x3 will do.

This is somewhat frequently used in guitar amps. The Laney LC15 is notorious for blowing OTs like this, and adding the diodes fixed a bad example, IME.

The idea is that when the negative-going end of the PP gets to 0V, the diodes conduct. This helps to limit the positive-going end to B+ x2.

It's helpful, cheap, and doesn't get into the way of normal operation. Whether it can protect a fully unloaded PP against overvoltage is doubtful, though.
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Old 11th January 2011, 04:26 PM   #20
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Originally Posted by trobbins View Post
Fusing the B+ only to pentode PP output stage (eg. KT88) has the disadvantage that if the fuse goes then the tube still has screen supply.
???

Who suggested doing that?

Nevermind, I see it now. Agreed, not a good idea unless the screens happen to be tied in UL.
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