JMI Vox AC30 Repair Pt.3 - FRAGILE ...

[@vintagetubeamplifiers]

Personally I replace all out of spec resistors if they're way off like those were. I've bought up several lots of vintage carbon comps on eBay so when I do have to replace some they blend in visually and are barely noticeable.

[@goodun2974]

At 2:00 It occurred to me that If the red wire you're reattaching is a plate B+ feed, that's a pretty long length of wire carrying high voltage between the resistor and the terminal of the tube socket, and it passes right by another tube socket and underneath other parts of the adjacent circuitry on the board. That sort of layout makes it more likely that you would pick up hum and noise, and ideally the plate resistor(s) should be as close, physically and electrically) to the tube socket terminal as possible. Vox could have learned a few things from studying how Leo Fender laid out his amps!

[@YeatzeeGuitar]

They definitely could have, this thing is laid out in such a weird way 😵‍💫

[@matthewf1979]

If the amp was a museum piece, I would leave it alone. It's not though. Replace those resistors!

[@goodun2974]

With that much solder to remove from the individual terminals on the phenolic strip, i'd be using a solder sucker of some kind (I have access to a vacuum desoldering station, but a piston-type, spring-loaded desoldering device would work). Desoldering braid has its uses but it's slow to wick up the solder, especially old solder (though an added dab of flux would help), and forces you to linger a bit too long on the terminal being desoldered.

[@YeatzeeGuitar]

Yeah it's a balance. I've heard from more than one person a spring loaded solder sucker can easily break the tabs from the recoil so I've opted to rely on braid. If there's no force bending the tabs while they're hot I feel good about it. Worked so far at least!

[@goodun2974]

@@YeatzeeGuitar , There are also rubber bulb solder suckered, rather like a giant medicine dropper but with a big round rubber clown nose for the suction; those do not have any recoil. Ideally, anybody who does this type of work as more than the very occasional hobbiest should seriously consider buying a vacuum operated desoldering station.

[@ampyamps]

Try this. Clip a second resistor in parallel across the drifted out of spec carbon comps to bring the resistance in line with what’s on the schematic, then play the amp. If having proper resistance in that position is an improvement, swap the drifted resistor out. If not, leave the drifted resistor in. You can use a parallel resistance calculator to find the values you need.

[@YeatzeeGuitar]

That's actually a great idea, butttt I already swapped them out (shown on latest video). Good thing to remember for next time though

[@goodun2974]

A switchable resistor substitution box would make this exoeriment very easy. And if you don't have one of those, why the hell not? Every amp obvious or technician should have a resistor substitution box and perhaps a capacitor substitution box as well.

[@YeatzeeGuitar]

@@goodun2974 do have one, but never used it!

[@goodun2974]

@@YeatzeeGuitar , At work I have an old Eico resistor substitution box where I changed some of the resistors (carbon comp, originally) to higher wattage values and therefore I can use them in power supply applications as well. I mostly work on solid state hifi gear at work (and tube-type stuff at home), and when it comes time to upgrade incandescent backlighting in old stereo receivers to LED's, the resistor switch box is invaluable in determining what value of series dropping resistor to add to the lighting circuit so that the LED''s aren't garishly, hideously over-bright nor being run too hot with too much current, shortening their lifespan. Without that switchbox i'd go nuts if I had to substitute in various resistors one at a time with alligator clips....