Bogen CHB-20A Rebuild

[physics]

Hi Guys

Fuse calculations are given in TUT3 and elsewhere in the series, and also here on this forum:
Fusing for Bias Supply? Fusing for the OT?
post-2

In my amps, I fuse every supply so there is proper protection for each winding of the PT, and that includes the bias supply.

You can always look at every component as a potential breaking point which  maybe cause a cascade of failures. On the other hand, you have to put your faith in something. So, say you fuse the bias supply and that fuse opens - it could be a nuisance failure or because of a legitimate fault, does not matter. Have you also fused the OT? If not, then the OT may be in danger. If you have fused the OT as well, then that fuse will open second and protect the $$ items.

You could devise all kinds of interlock approaches between the bias supply and the plate supply, but then you are relying on these extra components. Are they more reliable than the core circuit components? They shouldn't be.

The reliability rates for most components is pretty fantastic. I choose components for reliability and circuits for tone, therefore I do not follow a specific prototype circuit to attain a particular tone.

The bias winding in a PT is more likely to fail because it is of such light construction compared to the other higher-current windings than for any other reason. Vibration during normal operation can break the wire and then the bias winding no longer functions. I've had this happen on larger PTs and on subsequent runs spec'd the bias winding to be much heavier itself to have a margin against a repeat failure. Really, it was a fluke of manufacturing that lead to that failure.

The tensioning of the wire during manufacture is called "scaning". I had a small audio transformer fail because of a scaning issue - very tiny wire and many many turns. Talking to the manufacturer, I determined that I could use the rest of the batch by adding one resistor to the circuit which did not effect the basic performance but protected the transformer. Another manufacturing issue but also another transformer!

Since transformers are so expensive, i have no hesitation to add tons of fusing to protect them. You have to abandon the larger fuse sizes and go with 5x20mm or even smaller. A friend of mine uses pico fuses that solder to the PCB. I can't say i like that method but for what he builds he has never had a fuse failure and really nothing he builds is going up in smoke anyway

Have fun

Thanks for the info Kevin! 

I dug through TUT3 and all I found explicitly addressing fusing values (besides stuff like "an 8A heater fuse lets us use different valves") is the appendix on the universal fusing method for power transformers. Did I get an older revision of TUT3 and is there a newer revision with more fuse details or am I just blind? That other post you mentioned said that TUT3 called out specific fuse holders, but I didn't see that when I went through TUT3 last night so I wonder if mine is missing some stuff. Granted, I was tired and it could have been a call-out on a schematic that I missed. I'll have to dig through the rest of the series to see what stuff I read and forgot about fuses.

That post you mentioned was really helpful, not sure why I didn't find it early. Guess I need to look harder. Using the 1.25*1.25 fudge factor mentioned, my heater fuses are 0.3A too small, so I'll have to swap those out. Bias supply and HT winding fuses are fine by virtue of not being able to get smaller ones than 250mA at the shop, which is 2x my non-fudged HT calculation and oodles more than the bias draw. 250mA is definitely more than what my bias transformer can handle though... Maybe I should find a smaller fuse. Looks like Digikey has some small current value ones that should probably do it.

I see your point on picking something to trust. For some reason I had it in mind that an interlock of some sort would be better since I can program a specific current value compared to my impression of fuses being a bit cruder. Though thinking about it some more, I'd think a fuse is the simpler, less-moving-parts protection method. Ideal might be an interlock in addition to fusing, for redundancy. Also, like you pointed out, the output transformer should be fused anyway since there are other ways to kill it besides the bias failing. Time to get more fuse holders...

And yeah, 5x20 seems like the way to go. I was wrestling with getting some 6x30's to fit on my board since I had some scrap clips for those, but my life got so much easier when I switched to 5x20 holders I just picked up. Way more space. I can't say I envisioned there being quite this much mechanical design and problem solving involved when I started this project... Not that I'm complaining.

Thanks for the help!