Guitar Gear Forum

So, I noticed this the other day... was hoping I was misreading something and decided to push forward, but felt the nagging urge to check it again

These are supposed to be 1-ohm, 1% (precision) resistors...



But they're all reading more like 1.5-ohm



At least I can say my bias test points work correctly...





...however, I can't really use the measurement, nor can I be certain what the impact would be of the added resistance to the cathode grounding, so I won't be able to start up the amp until I get this sorted out

Called the supplier (Mouser). They're locating replacements and will send to me today, but it'll be a couple days before they arrive and now I gotta pull those resistors out again

Measure the meter leads without the resistor

fretless wrote:Measure the meter leads without the resistor

Ah... resistance in the leads...

I think you're on to something

resistance / capacitance yadda yadda , but that is probably it . The color code is correct . What does the meter read with just the leads ?

fretless wrote:resistance / capacitance yadda yadda , but that is probably it . The color code is correct . What does the meter read with just the leads ?

Almost an ohm on it's own - on both my meters. I believe you nailed it

Suddenly remembered my ESD component tester, and found it to be MUCH closer to the 1-ohm mark. I'm going with it for now.

sweet , hate to see you tear down that for nothing .

Been struggling to reconcile the wiring diagram to the schematic for the mains power connection, and I just realized there's an error (or at least a discrepancy) on Mojotone's schematic versus what's shown on the wiring diagram...

Mojotone's wiring diagram shows the wiring going from input (load) to the fuse and from there to the switch:



...whereas Mojotone's schematic shows load going to the switch first, and then the fuse. I believe this is bad practice, as the switch is subjected to mains power without fuse protection.



Marshall appears to agree with this assessment:



So, that's the route I'm going. Gonna shoot Mojotone a note letting them know about the discrepancy and see if they update their drawing.

...and, power wiring now done. Just need to add the standby supply, once the board is in place.

Got a lot done tonight...

Input jacks:





Small mod to mount the 68K input resistor next to the tube rather than on the main board (which seems like a potential noise source). Also relocated the ground wire for the effects loop board.

Bottom of board

Added wire to capacitor leads so that the cap can be replaced w/o having to pull the board up





First pass at connecting inputs to main board



...and this is where things wound up: front controls mostly done (still need to hook up effects loop between MV and Treble controls), most wiring done at front, still need to add tube wiring (V1-V3)







Not a big fan of these 5.1K resistors in series with the green wires that head to the PI, but I think things are going to work out ok.

Looks great! Are you going to wire the loop to the board after initial power up test, or say "fuck it we're doing it live" and hope you picked the right dropping resistor?

This is all Greek to me but it looks great! I do not have these skills.

Hipster Salad wrote:Looks great! Are you going to wire the loop to the board after initial power up test, or say "fuck it we're doing it live" and hope you picked the right dropping resistor?

Thanks man! Yeah, decided it'd be better to wait to get stock voltages before I start messing with the loop. I'll get the signal path routed through the bypass, though.

...and, just like that, I realize that the yellow wire is connected to the wrong resistor

...so we pull the !#@$^$%&^ thing out



and replace it



and then realize that the left-most red wire isn't connected to the right cap

So, we pull that one, too



and replace it with the updated yellow wire, from Mojotone's latest wiring diagram (it was red before, in a stack of red wires... easy mistake )

Still looking good, I think

Decided to change out the non-shielded green wire going from the preamp pot to the cold clipper stage with a shielded one



then got to the shielded runs for the effects loop



and glued 'em to the MV and Treble pots

Some quick thoughts for next time...

* Adam Savage's "first order retrieval" needs to become my imperative - getting *REALLLLLY* tired of not being able to quickly locate one of the three tools I'm using most.

* I need a wire bin to toss scraps (the stuff that can still be used for shorter runs, etc) into. They're ALL OVER THE BENCH right now, and it's making finding anything that much more difficult.

* Same goes for other supplies (heat shrink, wire ties, etc.). I should be able to reach for anything without even really looking. All too often, I've got parts/wires in my hands held in the right position, and having to let go and re-do after finding and grabbing that tool/wire/solder/etc.

* NOT digging the stranded wire - even the cooler Mojotone pre-tinned stuff. It sucks when I need to re-solder or de-solder a connection and have the stuff fray. Not to mention the inherent flexibility that doesn't exist with the Fender solid-core stuff. When I put a wire <there>, I don't want the fucking thing moving

* Need to think about serviceability a bit more - some of these wire runs and the solid buss wire I used for grounding the pots (from pot to floating bar) will prevent removal of the board without significant re-work. If/when I need to do that, I'll be cutting the solid ground connectors from the pots and replacing them with short wires. Some of the wire loops could stand to be a bit longer, but I'm also worried about lead dress for noise potential.

Also...

* REALLY liking FourT6and2's way of grounding at the top of the pots. Gonna seriously consider that for my next build



* Why aren't we all using DC heaters and buss wire instead of twisting heater wire like animals???!!?!?

It's perfectly fine to run AC heaters like that. That's how they're wired in the slo and they run AC heaters.

Zozobra wrote:It's perfectly fine to run AC heaters like that. That's how they're wired in the slo and they run AC heaters.

Interesting - I thought I'd read that one of the differences between the actual Soldano and JCA stuff was the use of DC heaters (besides the obvious transformers and other premium components).

Last push before I gotta get back to work for now...

Prepped the wires for the smaller tubes



Missed drilling one hole for this green wire, so it's gotta go topside



All soldered to their respective turrets



Hope to get back to this later tonight. If everything goes well, I should be able to wrap up the wiring and start testing the fuck out of everything before lighting it up (tomorrow??? )

Looking good! A light bulb limiter is nice for start up. If there is a short the light bulb will use up the over current. Or use 1/2" amp fuses for the first startup without tubes. The unloaded B+ will be higher.

If you decide to fine tune it after it is running, Crimped and soldered back to back alligator clips work well for clipping in different values. Lots of spots to mess with to dial it in to fit you.

PatF wrote:Looking good! A light bulb limiter is nice for start up. If there is a short the light bulb will use up the over current. Or use 1/2" amp fuses for the first startup without tubes. The unloaded B+ will be higher.

If you decide to fine tune it after it is running, Crimped and soldered back to back alligator clips work well for clipping in different values. Lots of spots to mess with to dial it in to fit you.

Great suggestions - thanks!

Yes, I have a limiter. Also going to break it in a bit slowly with the VARIAC that I picked up