12000 is cursed

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Probably Loud
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12000 is cursed

Post by Probably Loud » Fri Jul 24, 2020 10:04 pm

Hey all,

I have built many amps and at least 3 Marshall amps thus far in the past 5 years. I am familiar as to how to build them and typically when built they work immediately and have no issues. I decided to build a 12000 series amp as it was one Marshall I wanted to do but haven't built yet. I got it working and took it to practice and it was producing a smell. It seemed that the PT was getting super hot where I couldn't touch it without feeling like I was going to set my finger a flame.

I decided to move the ground for the heaters to way out under the preamp board and give it a super good connection. I also decided to rebuild the preamp board as my board shipped with the vintage style turrets (even though I didn't order those). I put the modern style turrets and redid the board. I also changed the grid resistors to the 4 5.6k ones attached by wire thinking that the singular 5.6k ones would not be enough. Since I have done that I figured it would work great. All I get is very weak output similar to a dying or dead fuzz pedal. I know it is not the tubes as I have swapped them all out for a known working set from my JTM100. Today I rebuilt the power board replacing the vintage Erie caps with known working F&T 100uf ones. I still get the same result!!! I have been pulling out my hair trying to figure this out because it was working before.

One concern I had was that when I installed the M***** PT that I had to crack some of the goop at the bottom in order to fit it into the chassis as it was a bit big. I realized today the smell I was smelling at my practice a couple of weeks ago was the varnish inside the PT as I saw it on the bias wire. The PT had gotten hot enough to melt some or the varnish maybe?

Also this amp was supposed to have Piher resistors in it but they never showed up due to the mail being absolutely terrible.

I have attached some photos of the amp as it is currently and what it looked like prior with the big blue Eries.

Thanks in need as I feel like throwing this amp in the garbage!!


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Re: 12000 is cursed

Post by danman » Sun Aug 02, 2020 7:05 pm

Disconnect all of the PT secondaries (heater and HT) from the circuit and measure each pair. See if you get 6.3vac when measuring across the two filament wires. On the HT wires, measure each one to the centertap and see if you get the expected voltages. Also check and see if the PT warms up at all when all of the secondaries are disconnected. It should stay cold and you should get correct voltages on all secondary wires if it's still good. If the PT checks out good, I would hook up each secondary one at a time and power the amp up and see if the PT get's hot again. This will help you to see if the fault lies in the power supply or the heater circuit.

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