With the Cary 300SE Monobloc amp in the previous post, turned out the intermittent popping sound was one of the expensive 300B valves!
Customer had already considered new valves, so with nothing to lose thought lets just check the connections and after extracting some solder from the pins, found this! One of the wires loose, in this case the grid, this explained everything!
Question is, how can we get in to fix? The base was well cemented in place, unlike some others that loosen with heat (not a good thing for 300Bs that can reach almost 200 degrees C)!
Tried various solvents, heat, superglue softener, but nothing seemed to touch the glue that holds the base on – which was needed to be removed to see if we can reattach the wire easily. Frustrating as what could be a simple fix but impossible to get to! There’s plenty of info about reattaching loose bases but not much about getting stuck bases off!
I had one more trick up sleeve – Proton 703 which is used for dissolving silicone, failing which, probably have to cut it off with a dremel and fit a new base. Valve sat in an aerosol cap full of 703 for a couple of days.
I think it worked as the base pretty well fell off after this soaking! The wires inside had all corroded (not the Proton 703), perhaps acid from flux during manufacture, with heat as a catalyst, and all 4 wires separated.
4 of the wires broke at a solder joint where the sleeving met, but the one we need (at far right above/below) broke right by where it enters the glass 🙁
Still, we give it a good go, cleaning up what we can and hopefully we can solder a tiny loop of wire to what’s left:
Had to grind a little bit of the glass around the stub, and also around the wires as the green stuff (sulphate?) wasn’t accepting any solder, but persistence pays off – formed loops around some other wire and soldered in place:
Testing with the uTracer v6 before and after gluing cap in place (all seemed to be respectable readings):
Just prior to re-soldering and trimming the excess wire:
Testing – all good! The Cary 300SE’s ready to go back into service!
