Holy smokes…it was used at a rehearsal today and after about 40 minutes it…stopped working. The fuse hadn’t blown and the rectifier was still “lighting up” and producing DC voltage, but the filaments on the 6.3V tubes were all dark. No sound, obviously.
I started to calculate a doom hypothesis - it being an NOS power transformer most likely from the 1950s or early ‘60s at the latest, we’re looking at a 60 - 70 year old transformer. And although it was in the original box and looked very clean God knows what kind of conditions it was stored in regarding temperature fluctuations, etc. Indeed, during the build while I was removing housing with a razor blade I found the stranded wires a little too easy to cut through. So I started to wonder if the possibility of these theories having some bearing on the transformer’s general health combined with the seemingly very hot bias that maybe the secondary windings had fused themselves together or shorted or just melted or whatever.
Smaller amps like this there usually do not have a fuse on the HV center tap like you’d typically find on larger amps (50/100 watt Marshall, etc) I have built maybe 8 or 9 amps using EL84s (sometimes 6GK6s) and 6V6s and have never put a fuse on the HV center tap. But I started to regret not installing one and wonder if I had simply been getting away with a careless build method and was now paying the price for it. To top it off, the PM-8405 like lots of old Stancor transformers has a pretty unique size and that nice, hard to come by low voltage HV secondary which would be basically impossible to replace. Damn Sam.
Not being able to accept reality I took the back cover off in traffic on my way home (ok, I was in a parking lot) and looked very closely at all the soldering, etc. I luckily saw that one of the wires from the 6.3V winding going to the pilot light was not properly soldered. A ‘cold’ solder joint as they are called. My domestic situation does not allow for prompt crankage immediately upon return to my house, but I was able to plug the amp in my kitchen and see, to my great satisfaction and pleasure, the filaments of the 6F6s and 12AX7s lit up again and that faint, glorious hiss emitted from the speaker. It would appear my doom hypothesis may have been incorrect, thank Gawd.
However, I wonder….my loud tests at home are limited to 15 mins or so. All had been well. But is it possible that, like an old carbon comp resistor once it warms up it could allow a ‘gap’ or ‘short’ to develop which ‘heals’ as the amp cools and only re-emerges after getting real hot again? OR does it not work that way with transformers: like, once they’re dead they’re DEAD.
Additionally, I had a member message me about the local grounds as opposed to a star or buss grounding scheme. I forget where I saw it recommended, but at some point I remember someone suggesting placing the electrolytics near the tubes they are supplying B+ to and grounding each of them at those points. For the simple circuits I like to make this seems to work pretty good…but, could this have some detrimental effect? Or do you guys think it was just a cold solder joint that once addressed, with all else being ok, I’ll be back in Flavor Country
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