Amp builders / modders - need some help

[Tortuga]

Having a lot of trouble getting my 2204 build up and running. Without going into all the stuff I've fixed so far (that's what my build thread is for), here's the current state:

I can get a very low guitar output by plugging into the Hi input and cranking the master and gain controls. At that setting, I'm hearing a level of hiss that makes me believe the output section is working ok. I don't currently have a loop installed, so testing using a loop return isn't an option right now.

I've tested all the tubes in a known working amp to make sure they're all working.

I've inspected, poked, and measured everything I can think of (especially around V2 (cathode follower) and V3 (phase inverter), and haven't found anything amiss.

I'm getting low voltages across the board, especially in V2 and V3 (biggest concerns highlighted in yellow).

Here's the voltages I measured vs expected:



This is a schematic showing the main B+ notes:



I'm open to any thoughts / suggestions as to what to try next. I'm going to probably start pulling wiring components and measuring them / resoldering all the connections next, and that is going to absolutely fucking suck...

[Zozobra]

The easiest way to visualize what might be wrong is to add the measured and expected voltages onto the schematic and then post it here. Power amp voltages look close enough. Something is fucky in the pi.

Photos of the pi would help too.

[Zozobra]

And don't forget the preamp grid voltages too!

[Tortuga]

I’ll have it up shortly

[mortatone]

Inputs can be easy to make a mistake on.

Coincidentally, just yesterday I finished up a Princeton build that had low output. The culprit ended up being a bad volume pot. It tested good but when I swapped it out everything went to normal.

Normally though on fresh builds the gremlins are errors, not bad parts.

Another thought is to swap the primaries on the OT. There's always a 50/50 shot at having the polarity backwards. But usually when it's backwards you'll get loud BBBBBBRRRRRRMMMMMPPPPPP, but sometimes not.

Do the 6.3v leads have a CT?

[Tortuga]

Just tried wiring up a new temporary set of inputs using completely different jacks and resistors, and jumpered it to the amp (CAREFULLY!) using clipped leads. Same exact symptoms (low sound only heard from the Hi input)

Gonna check the pots.

Had the swapped secondaries on the output transformer problem last year when I fired it up for the first time (just before that PT crapped out on me and Mojotone replaced it) - Mojotone's wiring diagram I was using had those backwards. Someone here helped me rectify that immediately.

Will check the heaters, but pretty sure they don't have a CT

[mortatone]

If no CT on the heaters look up virtual CT. It's 2 100r 1w or greater resistors from each led to ground. Not sure that it would cause your symptoms though but still..

[Tortuga]

This is a 2204 (Marshall) build, using Marshall paddle-type switches with the integrated LED. Are you talking about the fender-type front panel indicator?

[Tortuga]

Talking a break tonight. Will get more info up tomorrow afternoon…

[Zozobra]

The brown wire on the power supply schem above is a heater CT.

[Tortuga]

The brown wire on the power supply schem above is a heater CT.

Because, of course it is



Told you I was tired last night

[trey85stang2]

Is this a kit build? Or did you source all that parts I think you have some wrong value resistors. The pi and power amp circuit is where you will want to double check every value resistor and measure voltage on each side to see where it might be wrong. Test on both amps so you can visually see the difference.

My very first build I had 2 resistors that my colorblindness led me astray on and it killed the output. Took a couple hours to track down.

[Zozobra]

Looking back at your table, ~200V on the LTPI cathodes is waaaaaay out of whack and I should have spotted that sooner. It should be in the range of 40-50V typically if you look at any annotated schematic.

See here:
https://mhuss.com/MyJCM/JCM800_2204.gif

All your measured voltages are in the right ballpark when looking at a measured 2204. Where did you get the expected numbers in your table from?

[Tortuga]

Looking back at your table, ~200V on the LTPI cathodes is waaaaaay out of whack and I should have spotted that sooner. It should be in the range of 40-50V typically if you look at any annotated schematic.

See here:
https://mhuss.com/MyJCM/JCM800_2204.gif

All your measured voltages are in the right ballpark when looking at a measured 2204. Where did you get the expected numbers in your table from?

Well, shit... those numbers I had came from Mojotone's schematic...

Going to re-check according to that schematic - will report back soon. Thanks!

[Zozobra]

It looks like mojotone have bozoed the information on their schematics. Some of the marshall kits have a sensible enough looking voltages, but most of them are wrong somewhere, which is pretty shitty and furthermore easy for them to fix. I'd be surprised if they didn't know too.

Sensible:
https://www.mojotone.com/Amp_Kits/Briti ... ss_SCH.pdf

[Tortuga]

HOLEY FUCK, I think I got it

Rebuilt the jacks (again again again)...

Re-situated a couple wires on the PI and resoldered every muggafugging connection to the PI, tube base, components, and then went over the whole fucking board with the iron (again again again)...

Ran through the power-up procedures (no tubes, check for basic power, shorts, etc with the current limiter connected, then reinstall tubes and roll through basic startup, power checking etc)...

Amp started making volume...

...and then it didn't

inspected everything I could think of, poked around, cussed a bit

brought it back up and started checking voltages against that diagram that Zozobra posted (thanks man!!!!!!!), and determined that things are a hell of a lot closer to "right" than I thought

grabbed a dowel and started "chopsticking" the fuck out of everything

nothing

cussed some more

chopsticking slowly - realized I hadn't really checked the wiring to the pots... just happened to hit the wire that goes from preamp gain to V1B and the volume responded... went out... poked it and it came back up

MOTHERFUCKER is working solid for the moment

I'm pretty sure I know exactly which wire needs to be changed out (it's a small shielded cable that's kind of a bitch to prep & install), and will do as soon as I can later tonight.

If all goes well, I may even get the loop board back in

[GuitarBilly]

Good news!!! Hopefully that's the last hurdle!

[Tortuga]

Sneak preview

[Tortuga]

SERIOUSLY considering having my son do a custom front panel logo (for the future JMP cabinet) on his 3D printer

[Zozobra]

My first build was pretty ambitious; bandmaster clean with a slo style lead and loop. Needless to say it didn't work first, second or tenth time. The problem was small length of shielded cable had a very small kink in it - this shorted the conductor to the shield, which of course is grounded. That was a bitch for an inexperienced builder to troubleshoot.

I have the soldano font if you want to get wacky with the panel badge white on black with a filament swap for the text would look great.

[Tortuga]

Really have to thank you (Zozobra) and Mortatone for helping point me in the right direction and give me just what I needed to get this figured out. I was convinced I had some kind of component issue that was causing the crazy voltage stuff, but when I used the correct schematic, everything proved out to be ok. Mort pointed me to the pots/control wiring, and if I hadn't started poking around with that, I may have been in absolute "fuck it" mode by now.

Thanks to you guys and everyone else for the awesome help!

[Zozobra]

Np man. This shit is much easier with an oscilloscope and signal generator that's for sure!

[Tortuga]

That’s the next thing I need to learn to use. I’ve got a simple handheld unit that I hope can do the job. Just wasn’t clear to me how to use it for this application, because the majority of my experience with scopes comes from my days taking electronics classes in the mid-80s.

[GuitarBilly]

Not too crazy about the MuggaFugga logo

I'd use a regular Marshall logo or maybe your last name in old marshall/friedman font.


MuggaFugga sounds like Boomer talk might as well call it Rebel Rouser or some other outdated phrase like Hang Loose or Groovy Vibes

Sorry man.

[Tortuga]

Lol