Good to see this ended well (sort of ). No idea if you can do this with Switchcraft jacks, but I always wire my Cliff jacks to short when nothing is plugged in. Thus nothing plugged in means the ot secondaries are always grounded. No idea if that would have helped here, because it is unclear to me if the fault happened due to an open load.
Fusing the ht right after the the first reservoir caps but before the ot centre tap. This way you limit inrush currents through the fuse.
Better still would be before the rectifiers, so the fuse also trips when a diode goes bad. Never had that happen though.
Rootz, the usual setup with Switchcraft jacks uses a shorting type for the main and one non-shorting type for the extension (second) speaker. Lately I've seen people using multiple jacks in lieu of an impedance selector, which usually eliminates the safety shorting feature. In any case that feature is disabled if the speaker swap is done at the cabinet end of the speaker cable. A quicker way to safely compare speakers would be to use an A/B box with a shorting switch.
In the case of an open secondary, I'm not sure fusing the OT primaries would help since 1) the large voltage is generated due to a lack of current flow, and 2) when the current does start flowing, it's after the arc and the resulting damage have already occurred. Going on standby is the safest way, but there is a window of some seconds after the standby is switched where the stored energy in the capacitors could still be a problem. It's clear that the HT voltage went to ground through the filament circuit, but that can happen via a couple of paths.
I agree! Education is the single best cure and any component can fail. I've never had to put any flyback diodes in my amps because I know better. I always tell my customers to listen for the thump after you hit the Stby to tell you the circuit is complete. Most players who don't know better will keep playing and reach for the vol knob thinking the sound will come out soon. It never can and that energy eventually finds gnd Poof! goodbye socket, tubes or worse arcs out the OPT.
Tony
" The psychics on my bench is the same as Dumble'"
Agreed, knowing what you are doing helps best, period. It’s just that many people haven’t got a clue, or you yourself make a seldom mistake. Now I’ve never blown up a pt or ot because of that, like you Tony. I’ve been around tube amps and building them since I was 16 or 17 years old. Some experience and education with and about I’ve amps since. Every new amp owner deserves a builder like you Tony, one who schools them on how not to blow up a ot or arc a tube (socket). That’s pretty scarce service though!
Anyway, here’s what I usually do for the output jack wiring. As far as I understand the secondaries are always grounded with nothing plugged in the amp. No matter where you set the impedance selector. It offers zero protection when the cord is pulled out of the cab instead of the amp! You’re right about that Martin.
But yeah, ignorance or lack of knowledge is the ultimate enemy of an amp. You can’t cover all the possible things that can go wrong.
Any fans of fly back diodes across the ot here?
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This page has a nice description of wiring output jacks "Hiwatt style". It allows you to plug into either or both output jacks and also have a safety short.
I did this wiring with the amp that I built.
I believe that this wiring would have prevented the problem that we are discussing.
Marcus, re: speaker swapping, our amps are far more robust than you credit them. Over the past decade I have done every stupid thing to my builds--playing with no load, output plugged into the input of another amp, mismatched loads, etc. And the ONLY time I ever damaged anything was an accidental use of a 16-ohm Marshall cab, powered by my 50w, 2-ohm Bassman OT, turned up to maximum. Still, it took 10 or 20 seconds of playing before the tube's plate shorted thru the heater balancing resistors, same as yours.
I build and repair tube amps. http://amps.monkeymatic.com
Interesting Aaron. So some of the energy made it across the speaker load but the rest headed upstream through the output transformer primaries and then tubes. Did your transformer survive this?
CW
Charlie Wilson wrote: ↑Fri Jan 24, 2020 12:28 am
Interesting Aaron. So some of the energy made it across the speaker load but the rest headed upstream through the output transformer primaries and then tubes. Did your transformer survive this?
CW
Yes, the iron was saved by the heater balancing resistors.
I build and repair tube amps. http://amps.monkeymatic.com
Charlie Wilson wrote: ↑Fri Jan 24, 2020 12:28 am
Interesting Aaron. So some of the energy made it across the speaker load but the rest headed upstream through the output transformer primaries and then tubes. Did your transformer survive this?
CW
Yes, the iron was saved by the heater balancing resistors.
That is interesting. Can we assume the heater balancing resistors protected the transformers more than using the PT's heater centre tap if it had one?
Guy77 wrote: ↑Fri Jan 24, 2020 2:27 amCan we assume the heater balancing resistors protected the transformers more than using the PT's heater centre tap if it had one?
This assumption has been confirmed by several others. Same folks who recommend sticking with low-wattage resistors, in screen resistor position also, to provide fault fusing as a benefit.
I build and repair tube amps. http://amps.monkeymatic.com
That is interesting. Can we assume the heater balancing resistors protected the transformers more than using the PT's heater centre tap if it had one?
Yes. That is if you use 1/2W resistors. They will usually pop before the OT windings melt. So don't use any high wattage resistors. The filament winding and center tap are much heavier gauge wire than the OT primary winding wires so they are not likely to burn up before the OT does.
Thanks Aaron and thanks Sluckey for the quick updates. Thanks for the valuable information.
I had a heater centre tap melt on me once about 8 years and it was not a pretty site . There was smoke and a bad stink!. The insulation had melted the centre tap wire all the way down to were the wire entered the transformer. I tried to salvage the transformer by cutting down the worst melted part of the wire but when I cut the wire the bare wire looked like it was heavily corroded and the wire was brittle and would break easily if you bent it. It was not an expensive unit since it was made in China for Weber.
Cant remember exactly what happened but it may have been a short at the pilot light or a loose screw on the heater ground wire.
Very interesting read and learning. For my last amp, which uses the 40-18042 classic tone PT I didn't use the balancing resistors on the heaters because the PT already has a central ground. Would it be advisable to use the 100R (1/2W) regardless of the PT having that central ground? I think I will add it to my amp...
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Not at all Guy, this is what threads like this are about, all the combined experience and knowledge of members coming together.
It seems I was lucky as the resistors I used were Dale metal film 1w . The replacements are 1/2 W and it's good to know a lower wattage resistor will act as a 'fuse' so good info for other builders, thanks @SteveL
I suppose this is a random/rare thing to happen, no? if not, would having an internal fuse on the heater CT be a good thing? as it's not something I've seen I presume not.
Bombacaototal wrote: ↑Fri Jan 24, 2020 9:32 am
Very interesting read and learning. For my last amp, which uses the 40-18042 classic tone PT I didn't use the balancing resistors on the heaters because the PT already has a central ground. Would it be advisable to use the 100R (1/2W) regardless of the PT having that central ground? I think I will add it to my amp...
that's a good question and I'd be interested in people views on this too. I've always used the CT if it's there.
). No idea if you can do this with Switchcraft jacks, but I always wire my Cliff jacks to short when nothing is plugged in. Thus nothing plugged in means the ot secondaries are always grounded. No idea if that would have helped here, because it is unclear to me if the fault happened due to an open load.
