Weird howl--Marshall

[Valvehead]

Picked up a 1980 JMP 2203 , it was disgustingly modded. Ive went through it and its wired back to stock . It does pass signal , but it has a very loud howl . It reminds me of the sound when you hook up your plate wires on the OT backwards. I tried reversing those.

I went back and traced through the amp and I cant find the mistake..... its got to be something simple...re-checked all the filter cap wiring (they are new)

Im trying to attach a video of the sound...anyone heard this distinct noise before ??

No i dont have a scope ...

it gets quieter when i remove the PI tube...but still there. The preamp and MV dont affect it ...tried moving the NFB wire to different taps ..

https://www.dropbox.com/s/l70g908sc6p7xgi/Jmp.mp4?dl=0


i keep thinking its something in the filtering ??

[The Dude]

If caps are new and wired correctly, you could simply have an output tube problem. Try a new set or remove 2 at a time and try pairs to see if you can narrow it down. Did you change the bias supply filters?

[Valvehead]

all caps and tubes new. i can try and swap tubes...voltages and bias look great .

[Valvehead]

tube swap = no change. I think i see something....
i wired the amp to a 1976 scem..2203 JMP . Im not so sure the PT is stock in this 1980....no writing on it... Looking at the scems for a later version , like a jcm 800 2203..i see the different filter caps , and maybe 1 tap (center tap) not used on the 800 . The 800 uses 3 50/50 cans and i have this wired to use the 100s like in the 1976 scem.........i wonder if this PT wants to be wired as the jcm 800 ???


https://www.thetubestore.com/lib/the...-Schematic.pdf




my amp is wired like this --

https://www.thetubestore.com/lib/the...-Schematic.pdf

[The Dude]

Is the voltage across one of your filter caps the wrong polarity?
Does your PT have a CT and is it connected to anything?

Edit: CAUTION: If one of your caps has reverse polarity voltage across it, it may well explode!

[Valvehead]

yes ct connected--like the 1976 scem above.

ill check the polarity

[g1]

That JCM800 schem you posted is for a re-issue. This (bottom) one should be right:

[Valvehead]

ok, i traced wiring again and cant find a mistake....yet...


look at the above scem...posted by G1 ...see the filter cap right below " A "...the 100uf/500v . Right there i have a 50/50 cap with the positives wired together. Is that a problem?

also...i always thought the choke could be wired either way ..as in its not dc ??? maybe my 2 choke wires are reversed causing the howl??

[g1]

If the voltage rating is sufficient, then the 50/50 there wired parallel makes a 100, that's fine.
Is the center tap connected between the two 100uF caps?
Can you post some pics?
Also, how did you determine the correct phasing of the OT, is the howling less one way than the other?

[The Dude]

^^^^^^ That. Plus, the caps I was wondering about polarity wise are the two sets of series caps. The totem pole right next to the cap you replaced with 2) 50UF and the two caps just left of the bridge rectifier. Note that one cap's negative lead goes to the other caps positive lead in both spots.

[Valvehead]

Ct is connected to the 100ufs from power trans. Ill try to listen to the noise when flipping the plate wires---

yes the pos / neg are correct on the 100u caps.

[Pedro Vecino]

The noise is totally uniform. It could come from a short circuit or leakage between heater and cathode in some preamp tube or in its socket. Inspect them carefully.

[Valvehead]

The noise is totally uniform. It could come from a short circuit or leakage between heater and cathode in some preamp tube or in its socket. Inspect them carefully.

does it with all pre tubes pulled...but at lower volume

[Chuck H]

The noise in the opening clip sure sounds like 120Hz clipping. And with all the preamp tubes out it's still there, but quieter (I wonder if that sounds more like 120Hz hum?). I'm thinking you must have a ground plane that isn't grounded AND has ripple on it. There's a mistake, a fault or a short in there somewhere having to do with the grounds and the power supply. Possibly both.

[Valvehead]

update------i found a speaker wire that was where it was supposed to go...(to ground) but someone forgot to solder it............(i am an idiot).

the LOUD hum is "mostly" gone now....still has some noise..so im going to try and move some wires around tomorrow with the ol chopstick . I just blasted this this jmp 100 ---yes it was loud--yes it sounded like Angus. Sounded like a cranked up old Marshall should sound at 1am.