Metropoulos Forum

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!!

https://imgur.com/a/9NM3Xg9

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.

Just saw this tucked at bottom of board.

Presume that you have made a voltage chart and everything is in spec.

If that is the case, your bias is correct, the selectors are clean and the jacks are grounded and the common directly hooked to the ground then you start to suspect the OT.

If you are still here can you do that? Or maybe problem solved?

I had indeed made a voltage chart and most hings were in spec, but some were way the hell off! Pin 1 of v1 and v2 should have been 156 and 168 respectively, but were 70.6 and 79.7!

Pin 3 of v1 and v2 was supposed to be 2 volts and its come up as .43!pin 8 of the same tubes was super low as well.

We need to see a picture of your under preamp board links to see where you are losing the volts to V1.

Have you measured the voltage as you follow the B+ line from the corner turret (choke/OT CT/Voltage from HT fuse)?

Likely there is a dry joint between lugs (one of the problems with people building amps with the bus wire and connections hidden underneath.

The other thing that I can see is that you have the 820R cathode resistor (that should be under the 0.68uF cap) is going to the tone stack. I am guessing that the 100k that should be feeding the tone stack is on V2a cathode - not good.

The turret of the first preamp cap that has the 10k across the board and the 10k hanging off the board looks empty of solder, I would look there after correcting V2 cathode arrangements.

Note the swapped resistors

Maybe solve your problems!

I just noticed this! I'm an idiot! This is rookie shit hahahaha

I have practice tonight and tomorrow I will swap the two and I bet that the biggest problem. Thanks Neikeel!

Hopefully this solves the issues but I will not hold my breathe yet...

She roars again!!!!!!!

I still have a hum issue which may be from a lead dress issue and I need to put back in the freshly reformed the Erie caps in the power section. I had tossed in old F&T caps that have sat for about 5 years from an old build so hopefully that will fix that issue.

I will also say that pulling out the bright cap made a huge difference in the sound at low volumes. It was way too bright before and its now perfect without them! I also need to check the bias again on the tubes but for the few minutes I was able to play it today it sounds damn amazing!

That's a relief.

Amp blew up during recording.

I got another PT and redid the grounding scheme to The Larry style star ground. Amp is dead quiet when I stop playing! I couldn't be happier. The PT didn't even get warm to the touch after 5 hours of cap reforming! All seemed well!

However....

The new PT got hot as hell again after about 2 hours of playing at practice. Even after a 2 minute drive home and lugging it don to the basement, the PT was still hot! What the fuck is going on here? I thought I did everything right this time as I basically redid the whole amp and tried to make it perfect. It has shaken my confidence and I don't know what to do.

What should I do? I don't want to use it again because I don't want to ruin the new PT which took many months to be built.

Hope that can help you:

https://robrobinette.com/Tube_Guitar_Am ... itar_Audio