12xxx Completed.

[Vegard]

Nice work!! Would love to hear sound clips of the amp!

[wrea398]

Thanks! I still have some things to work out but I will definately post some when I'm ready. I have a light squeal to tend to and a possible bad presence pot/ground. Still tweeking though. It can only get better from here.

[wrea398]

UPDATE...

Presence pot replaced and works fine now. Added Larry grounding. Squeal is gone. Sounds pretty darn awesome. Just hold a string or a chord and those sustaining harmonics just start to sing naturally like Marshalls are known for. Couldn't be much more pleased! Great vintage sound with a bit more gain and drive. Just what I wanted. I may decide to 100k/8ohm the NFB for a bit more gain. I just signed up for another 68 chassis from George. Damn... this is addicting!!

[wrea398]

Well that was quick...Rocking out with the amp dimed for an hour today and ZZZZZT!! Started blowing fuses. First the HT, , replaced it ZZZZt again when cranked to 2-3, then the mains, a pop thru the speaker cab when the SB switch is switched on, etc. After draining the caps for an hour I put a HT fuse back in and the speaker made a ZZZZZT noise when inserting the fuse with all power off. Weird....Checke everything yesterday. Bias set at 36mV, voltages were all OK. Weird... Mabey a shorted tube. Guess I will see....

[VintageCharlie]

Congrats on the build! Looks great! Hope you find the cause for the blowing fuses soon!

Best,

VintageCharlie

[Strat78]

Have you made any progress on this yet? I think I had the same problem, but it was because I had pushed the front end too heard while burning in the amp. Somehow the power tubes were compromised and after replacing fuses and persisting to keep trying to play the amp, finally the V7 socket arced quite spectacularly. Once the socket arcs, it's best to just replace it because they say after arcing, a pathway is formed through the socket which will continue to cause problems and ruined power tubes. vhjunkie can explain it better than me.

[ValveStorm]

My two sense on all of this. Let the amp warm up for twenty minutes. Monitor the bias voltage pull (36mvdc or so, depending on B+) as the amp warms up at idle. This will give you an idea of which tubes may be a little weird. Sometimes one tube (or a pair on one side) will go off the reservation during an idle warm up, but then come back to normal. Once you have an idea of which tubes might be odd. Start to play the amp louder and louder, watching the power tube the whole time. See if one or more tubes redplate during the process. If you see redplating, go to standby immediately. Swap the suspect tube to the other side of the quad, and see if the problem follows the tube. Like strat78 said if you see arcing on a socket, it must be replaced.

[wrea398]

I guess I failed to mention that the amp was built and played for the last 6-7 months but not alot. It has mabey 60 hours on it. The fuse pops with the power tubes out. The day before this happened I wanted to re-checked the voltages and the bias. It had been sitting unplayed for a month. It was reading 34.5mV and I reset it at 36mV and I had 449v on the plates (460v tranny). All voltages were still good. No redplating, no bad tubes. I warmed it up for 10 minutes in standby,fired it up and played it for over an hour. I was recording some preliminary sound clips. After about an hour of playing it DIMED, I lost power and saw that the HT fuse popped. After replacing the HT, both fuses popped. I replaced both fuses with slo blos and the HT popped again. I then drained the caps and started over. I pulled the power tubes and started checking voltages. All voltages were OK in standby. 120v at the ON/OFF switch, 165v at the SB switch, 95v at the feed resistor, 3.20v at the heaters and
-41v neg bias voltage. I flip the SB switch and HT fuse blows. No shorts, no chafed wires, etc. (as I can see)

Jon Wilder of Wilder Amplification on the Marshall forums told me the OT tranny is shot with an internal short. He said that if I have any continuity or resistance between the individulal wires and the core, the OT is shot. The resistance should be zero and I have 8 ohms on the BRN, 19 ohms on the WHT and 33 ohms on the RED. Also also get a solid beep on my continuity tester from all three wires to the core between the endbells. The resistance from wire to wire is right on (RED to WHT 17.4, BRN to WHT 15.8 and RED to BRN 32.9) but not from the wires to core, which again should be zero. I would like to confirm this info further but I know JON is a pro. I have a call into MM. If this is the case, it's going back to Mercury Magnetics for warranty.

Even though I trust Jon Wilders opinion, if anyone has another take on this problem, please feel free to let me know.