No good deed goes unpunished, helping a friend on a build

[Madjackhatter]

OK, so a friend bought a 100w JCM800 clone Weber kit put together and has had some succesful builds but he could not tame the hum and eventually made a mistake that poofed his PT. So I look at it and instead of chasing the demons out oc his build, I went into full Mulligan mode and rebuilt it. I am about to fire up and run the test voltages. But he had a cool idea about making it 2 full channels and he drilled the chassis for another tone stack. I really hate the tubes being in 2 rows FWIW but I did all the best practices on ground loop elimination and ran a 2nd cathode follower and tone stack values to make a Plexi channel and 800. Switches aren't in until individual channel tests. So before and after, swapped the eyelet board out, Hammond PT, more robust in series caps, and bridge rectified, and got rid of several feet of wire. I also am running heaters for the preamp topside and cutting a aluminum cover plate. Wish me luck...

[Madjackhatter]

Before

[dude]

Suggestion: Always get a new build working before you start modding and do them one at at time.

[Madjackhatter]

Oh dear, don't I know. This thing was a rat's nest. But here's the after...don't mind ghe long OT leads. We are pre squeal check

[Madjackhatter]

Same pic, sorry.. the real after

[Madjackhatter]

Also I went the route of 2 CF's to make sure the only 2 switch points are after the first gain stage and at the PI feed so the unused channel can switch to ground and keep out the interaction. Also I was able to feed different values in the stack and bypass the 820R in the Plexi channel. I used tried and true values on the modded parts of the circuit so I am really hoping the 2 channels play nice with eachother. With the 2nd CF being unbypassed and hotter attenuation into channel 1, I hope goes a long way to help the overall volume balance even though the extra gain stage is going to be louder no doubt.

[tdvt]

That is a striking difference.

Fingers crossed for you.

[shaun]

Nice work! Clean enough to eat your dinner off.

I notice the 5w wirewound resistors on the screens. I just read an article by blueglow stating that having such a highly rated res in this position prevents it from blowing if there is a short in the tube, the idea being that you WANT them to blow in order to save the tranny. He points to Fenders use of 1w, as well as the use of fusable resistors. I'm no expert on this, but it seemed worth noting.

[Madjackhatter]

I have read that same thought as well about the screen resistor size. I have also contemplated adding another fuse in the mix and leaving the wire wounds in, I mean so many Marshalls have them and there's not a lot of iron getting smoked that way. 🤔
Anyway, I found a few minor mistakes and corrected them and fired it up less tubes through the trusty light bulb limiter and had 520v at the resevoir and roughly what I expected down stream. So I guess I'm giving it the acid test tonight one channel at a time before putzing around with the relays.