Preventing Failure on Stage (bad switch!) | 100w Vintage Marshall JMP Restoration Pt. 3

[@YeatzeeGuitar]

Bright cap comparison test coming up next! Really fascinating that one...

[@bambule5268]

That will be great!

[@lordgraga]

One recommendation: you can jumper the channel inputs over the first tube in order to avoid using a jumper cable on the front. It’s not period correct but it’s a lovely feature. Furthermore, those speaker impedance switches are so sketchy. I got a rotary from TAD and a baseplate that worked well but regular 2-way switches are probably even more reliable.

[@Lu_Woods]

I recall seeing a Dumble modded marshall where he used a multi gang impedance switch(gangs wired in parallel) for redundancy.

[@YeatzeeGuitar]

I've been playing with jumpering, mostly off camera, and it's been fascinating. I always thought, out of ignorance, that it didn't matter which two you jumpered or went into with the guitar cable for the 3rd, the end result was always the same tonally speaking. How wrong was I! Every combo makes a huge change to the sound, including using a Y cable vs a jumper and a third cable for the guitar. The former, running one guitar signal into both hotter inputs is quite the tone! For now, I made myself a Y adapter cable, as that was my favorite so far.

[@YeatzeeGuitar]

Regarding the impedance, it's solid now so I'm not concerned. Contacts cleaned, can't pull it out without intentionally doing so.

[@tonya4157]

Coming along nicely! Sounds great. Can't wait to hear the cab(s).

[@YeatzeeGuitar]

Thanks! Some fun stuff coming up, including a big bright cap comparison!

[@stevendehaan1078]

Good stuff, great progress.

[@robertlyon1360]

Re those cable ties - If you want a slightly more factory look, rotate the 'ratchet' clip thingy to point downwards. I always do as it looks neater IMO.

[@soapboxearth2]

You can loosen the internal nut on the switch that rests against the inside of the chassis, so you never hurt the face plate

[@YeatzeeGuitar]

Yep exactly what I did!

[@MichaelSmith-rn1qw]

For those knobs with the voids on the inside, you could fill the voids with epoxy to beef them up a bit. I assume the knobs are readily available, so maybe just buy a few spares.

[@YeatzeeGuitar]

They're incredibly rare

[@mtbmadman011]

coming along nicely! Is that bass pot rated a bit? top view inside presence and bass pots look slightly rotated to left in vid. the jumper to treble looks a bit stretched as a result.

[@YeatzeeGuitar]

They are, but it's factory. There are guide "pins" off the pots that fit into slots in the chassis, so they're oriented how Marshall intended evidently 🤷‍♂

[@RustyTaylorGuitar]

Shout out to whatever mad genius had the idea of using mouse pads...

[@YeatzeeGuitar]

🤣 Thanks Rusty. Your shoutout got lost in the 13000x fast forward of that portion of the work lol

[@NINEWALKING]

I wouldn't go for those powerful screen resistors. Schematic asks for less power rating, and that's important because you do want them to burn when the tube develops short to a screen. 5 or 7 Watt's resistors with such a powerful supply and high voltage is dangerous for your power transformer. Regarding the Standby switch, I would have just swapped it for the phase switch that is disabled anyway. Keep the amplifiers originality as well.

[@YeatzeeGuitar]

I've heard conflicting recommendations for those, 7w is what I was reffered to get. I've heard some recommend even 10w for these 100w El34 amps. All good, is what it is at this point I'm not swapping them again any time soon 😆 Polarity is 3 pole double throw vs 2 on the standby. I wanted that intact between them, but yes that could have been an option

[@NINEWALKING]

@YeatzeeGuitar in normal operation, those resistors see 1Watt power dissipation. 5 Watt rated resistors are fine. All you want is for them to burn out before the power transformer does if, for example, in the transport tube develops screen to ground short. Smaller power rating resistors will die sooner, making it less dangerous for the power transformer. And die they will doo, in a short to ground scenario. They will see up to 500V. With 1K Ohm, that's in theory 500mA or 250Watt. Power transformer HT is capable of 300mA or 150 Watt maximum. You just want resistor to die and save the power transformer. That's how that works. So screen resistors have two functions. Dropping voltage and being additional fuses 😀