PTC voltage ratings - fusing B+?

[Enzo]

And sometimes we have a solution looking for a problem. AFter all, the guitar amp industry has been using these basic fuses in their high voltage circuits for over 50 years, and as a professional repair tech, I cannot think of a single instance of such a fuse opening then arcing over from lack of voltage rating.

[NateS]

Guys, I know that we all use 250V rated glass fuses in our amps. The sarcasm was directed only at the "shut up and don't ask questions that rock the boat" comments. In this area, lots of things get used way outside their design parameters, or at least their stated specifications. Ever check the screen voltage limits on the garden variety audio power pentodes? Or heater-cathode voltage in your favorite DC coupled cathode follower? Or the voltage rating of resistors? Or DC switching rating of your standby switch? Or for that matter running 450VDC to a user accessible front panel control from a safety standpoint?

[Steve Conner]

Yes, I have addressed all of these things in my home built amps. I learnt a lot about what issues really matter from years of reading the Troubleshooting & Repair section of MEF. Some examples:

HT fuse goes in the transformer secondary so it breaks AC. Standby switch likewise, unless inrush is a problem. In that case I've put it in series with the filter choke, using two sets of contacts in series and a flywheel diode.

I only use pots and switches with metal shafts, and they're bolted to a grounded front panel.

I also run lower screen voltages (or bigger screen resistors) than the classic amps, and try to avoid DC coupled CFs. I'd have trouble building a Plexi clone.

Finally, I've always avoided complicated protection circuits in spite of RG's exhortations. I experimented with them when starting out, but they mainly seemed to be good for shutting the amp down in mid-solo.

[R.G.]

The sarcasm was directed only at the "shut up and don't ask questions that rock the boat" comments.

I went back and read through the stuff. I didn't find "shut up and don't ask..." anywhere in there. Did I just miss it?

In this area, lots of things get used way outside their design parameters, or at least their stated specifications.

Yes, they do. This falls entirely in the area I mentioned to you. I'd say exactly the same thing to the guy who put those things into the tube amps:

I fully believe in everyone's inalienable right to do whatever they want, given only that they get a chance to get the info to make an informed decision. You have the ultimate responsibility there.

Pushing the envelope is something all engineers do. It's just that you're betting your job, not your bedroom amplifier, that you'll guess right, so the pressure is a bit bigger. Test labs and incoming component inspection, paying for out-of-spec selection, or just relying on plain old luck are techniques that get used.

In the stock market, there's a concept called survivor bias. All existing [whatevers] can point to their stellar performance. This is because the survivors write the prospectuses (prospecti? prospectices?) and reports.

A tube amp company that guessed wrong about what parts they could get working in the delicate balance of actual performance and claimed advertising benefit for sales didn't make many amplifiers. It amounts to what engineers would today call a genetic sampling algorithm - selecting a random mix of out-of-bounds stuff and using only the ones that worked.

Near as I can tell, the comments you received were appropriate, and were not snide comments about you or your thinking. Mother Nature doesn't care what we think or feel. She just enforces the Rules. What's written on datasheets is not the Rules - it's humans telling other humans what they think will work. Sometimes they're conservative, sometimes not. Whether conservatism is appropriate in your particular application is entirely up to you if you have enough facts - including history - to make an informed decision.

Ever check the screen voltage limits on the garden variety audio power pentodes? Or heater-cathode voltage in your favorite DC coupled cathode follower? Or the voltage rating of resistors? Or DC switching rating of your standby switch? Or for that matter running 450VDC to a user accessible front panel control from a safety standpoint?

Let alone the safety aspects of running tubes with a 200C surface temperature where a child could reach around the back of the amp and get second degree burns on touch, or electrocuted. Yep, there are a lot of things done in the Golden Age that aren't necessarily good ideas - just what they got away with and made money on then. Times - like mileage - may vary.

[trobbins]

I think the highest available red-spot fuses are 460VDC rated (2A min)? That 808 style are still looking pretty damn good for a DC fuse.

Bloody hell - a red-spot fuse holder of that size would be at home in a rack mount industrial transmitter of 6+ RU ;-)