Safe to put V1a coupling capacitor on a switch for variety?

[Chuck H]

Adorable! And your boy is good looking too

If that's a standard spray paint you'll definitely want to put some coats of clear on the outside after any graphics are applied. The extra film thickness helps the finish "adhere to itself" as it were so it's less prone to chipping with any abrasion. If that's some sort of two component auto paint (with the proper metal prep done prior) or a powder coat then ignore what I just said.

[Mr. Bill]

Thanks Chuck! If only I was as good at making amps as I am making cute kids, haha. It's Rustoleum 2 in 1. Graphics plan is a dry transfer with a clear coat on just the front and rear panels. That's if I ever decide on what each knob and switch does and actually finalize a graphic to begin with. I did a scratch test on the ears and it's really durable at this point. That's about 7 or 8 thin coats.

[Mr. Bill]

Well I hit my first snag. I decided to try the higher bandwidth Classictone 40-18072 OT on this build and thought the mounting orientation was the same since it's advertised as a replacement for a JCM 800. Well, the holes will line up if I orient the OT 90 degrees from the original installation, but the I need to either drill new holes for the wires......or install stand-offs for the transformer to give them a little wiggle room. I think I may go with the stand-offs temporarily in case I don't like the wider bandwidth so I won't have mutilated the chassis. Any thoughts?

Lesson learned on paying super close attention to spec sheets.....

Also, since I'm forced into parallel orientation of the Mains and OT, should I go ahead and wire up the mains and test for coupling with that orientation by wiring up a speaker or headphones to the OT before moving forward?

[Chuck H]

Also, since I'm forced into parallel orientation of the Mains and OT, should I go ahead and wire up the mains and test for coupling with that orientation by wiring up a speaker or headphones to the OT before moving forward?

I would. Though I'm not sure if that test proves out. I don't know if loading the PT makes a difference regarding EMF. So an unloaded test doesn't necessarily mean anything. I think Juan Fahey or Mike Sulzer could answer this one. Maybe they'll see it and post. Or you could private message them.

[Mr. Bill]

I did the test and it's definitely better the way it "should" be. It's not enormous though. It was hard to hear on headphones so I wired the OT 16Ohm tap and common to a guitar cable and plugged it into a DI through the studio speakers. Then turned it up until I could just hear the hum with the OT at 90 degrees. With turned facing the same way the hum increased a good bit, but nothing major. Measured in Volts AC of the guitar cable, the meter couldn't even read the signal with it at 90 degrees, but was .001 VAC when facing the same way.

Now moving it over next to the PT was something else, LOTS of major hum going on there. I think I will just proceed and if it's a problem swap it out later. Unless my logic is incorrect, this slight hum isn't being amplified in any way since it's being induced in the actual output or the circuit. Is that correct?

[Chuck H]

No, that hum won't be amplified. But like I said, I don't know if a load on the PT affects the EMF.

Your chassis has the four filter cans between the transformers. I had a 70's JMP 50 watter that had the iron VERY close together. If both were oriented the same it would have hummed like it didn't know the words.

Well it's a small bummer. I know your brain isn't resting on the matter either. You may yet come up with a way to reorient the OT or PT that doesn't offend your sensibilities.

[Mr. Bill]

Yeah, it really is a bummer. The footprint of that OT is so big it overlaps with the output tube cutout when butted up against the face of the amp. Sideways it does fit. I could move the PT 90 degrees, but that would require more drilling as well. At this point I'm just going to get it together and see how detrimental it is under real world circumstances.

Here's the progress.

[Chuck H]

Ah. I see (a wee thumb and a wee finger as well ). It's probably unfortunate that the lams are oriented parallel rather than linear. I think the EMF pattern off the iron has a "blind spot" at the outside edges of the lams. But it's somewhat inconsistent too IME. I've dealt with it orienting reverb pans it tight places and was sometimes surprised at where the quiet spots were. Still, way more clearance that you get with the typical 50W chassis.

[Mr. Bill]

That is an extremely tight setup. For what it's worth, I moved my OT that close in proximity to my test as well just to see. It was insanely loud with the lamination facing the same way. However, the quiet spot with them almost touching like that was still louder than the loud spot that I'm stuck with, so fingers crossed.

[Mr. Bill]

I would like to bring something up that I addressed earlier regarding the filament wires going across preamp tubes. I patiently practiced getting this neat and maintaining the twist right to the pins, and it looks pretty tidy in some places. My question is when the preamp tube is wired as a cathode follower right over the socket. Would it not make more sense noise wise to actually loop around that tube stage rather than laying the heaters right across the cathode follower resistor? I could easily pull it off, but it seem counter intuitive.

[Chuck H]

That's sort of how Marshall did it. They snugged the filament leads against the back of the chassis in lazy loops and then ran from there to the tube sockets untwisted. I don't think it's going to make a huge difference either way. Personally I usually come up on top, twisted, and give myself whatever lead length I need for ease of operation. Rather than have filament wires laying RIGHT on top of the cathode follower tube crossover pin work.

[Mr. Bill]

It's alive!!!!!! Still have to work some kinks out, but it's sounding pretty great at the moment.

[Chuck H]

Sweet! How's the hum at higher gain settings? I would think that any hum from transformer proximity wouldn't increase with gain, but I'm asking. What "kinks"?

[Mr. Bill]

Well through the "stock" hole with all the switches in their marshall state, it sounds like a stock 2203. The resonance switch pops and makes the amp wayyy louder, so that's going on a pot tomorrow. All I have left in shop is a 50k linear, but it'll work for now.

The extra gain stage hole has way too much gain on tap. Overkill with the gain on .1, haha. I'm going to have to either lower the divider or give it it's own pot which I'll need to drill for.

The hum isn't too bad, but the high gain input squeels with my p90's with the head open like this at anything above maybe 2 on the gain knob.

All voltages are good, and everything is functioning, I guess it's just about tweaking the voices to actually all work well enough.

I will tell you one thing though. I don't care how subjectively better some people think the UF5408's are for rectifiers. I will never try and solder one to a turret board ever again, hahahahaha. What a pain those things were.

[Mr. Bill]

Oh, and the actual transformer hum is a negligible. Without a guitar signal plugged in the amp is silent unless you crank both controls. If I understand correctly that the transformer hum would be present at the same volume no matter the settings, then it's not even noticeable. I'll be chop sticking it tomorrow too to see if I can get the noise any better than it already is.