Nice mess in a 78 JMP Superlead...need advice!

[Firestorm]

Martin may be right, but bias pots in Marshalls are crap. Maybe that.

[martin manning]

I think there is likely more than one problem. A flaky bias pot would explain the drop. At least it fails in the "safe" direction.

[donzoid]

I will power up the amp and re-check my bias. It held steady for an hour and stayed there...I will check it again. Martin, should I remove the tubes for the time being or leave them in at idle?

[martin manning]

As I noted above if the wiper loses contact with the element it fails "cold," which is a good thing. You could try to clean the pot with some spray lube.

If there is an ultrasonic oscillation problem there is the possibility that it could break into it even at idle. I think put the tubes in, connect a speaker or load and check the bias. If you don't have a scope you can measure AC voltage at the speaker output. Turn up the tone controls and presence, and then the MV. If the voltage shoots up at some point that would indicate an oscillation.

[donzoid]

Fired it up with the probe in place (and the tubes) and it started at 30.x mA!

It's been on for 5 minutes and is (very slowly) climbing now at 34.5. I will leave it on until it stabilizes (IF it does).

[donzoid]

Ah...didn't see your last response. You are referring to the MV (dual-ganged) pot for cleaning? No alas, I don't yet have a scope but oddly enough, have been looking at them for one of my next acquisitions.

I will let this thing go for a while to see if it stabilizes at "some" number. Then power it down and clean that pot. Thanks!

[donzoid]

Oh...I bet you mean clean the Bias adjustment pot, don't you? Its probably the only thing that might cause a drift like that.

[martin manning]

Yes... I was just typing exactly that.

[donzoid]

OK .. I cleaned the trim pot (wow that was fun getting the straw on my can o' Caig pointed in the right direction) and yes Firestorm you are right these are crappy little pots...but I digress.

Amp on, everything down...reset the bias to 38 and after 20 minutes it had risen slightly but not much (40) and stayed put. Backed it down to 38 and pulled the bias probe and did as you said (tone knobs and presence first, pre all the way down and slowwwwly brought up the master to halfway. Then I just opened the pre enough to get some volume at first.

Ended up with master at 3/4, and preamp at half with no apparent redplates. But (no surprise) no increase in volume either. Then it occurred to me that last time I checked/tested it (before I butchered my finger) I had it set up to a 4 ohm load. Well in the interest of keeping the test consistent, I reset the ohm plug and since this cab has one of those select-able ohms inputs, put it into the 4 ohm side. Tomorrow before I go back at it, I'm going to clean the MV pots and re-inspect them since they came out for the bias fix along with the pcb. I don't really expect the volume to jump up way louder...but its worth a try BECAUSE I see that the neg. feedback is tied to that tap. At that point, I'll check my AC at the output to see if it "runs away" when I jack up the preamp so I will know if its oscillating out of control.

[donzoid]

Oy vay! I tried this thing on 4 ohms and its redplating. Granted, I had it up to 3/4 on the master and halfway on the pre. Oh and its even less volume on 4 ohms at the same settings, which to me makes NO sense a'tall.

I'm letting it cool now so I can pull the bias probe off (I checked it again...at least THAT is seemingly steady now). I dunno...maybe these tubes are just shot and I need to go over and have the guy down the street (with a MUCH better tester) check them out. I'm stumped! Anyway i will measure the AC at the speaker out shortly. What kind of voltage am I going to see? Hmmm....Ohm's law guesstimates somewhere around 10 to 15 volts?

[martin manning]

I guess if the idle current measured at the socket is holding steady that rules out leaking coupling caps. You had some issue like this before when you were measuring voltages right? I'm still suspicious of the MV wiring. If it were mine I think I'd convert it to a KF type 2 (aka Lar-Mar). In your pics there are two big 10k resistors that I think used to be the 220k grid leaks, and there is also a 100k hanging below the MV pots. Is that 100k part of the MV ground?

Yes I think 10-20VAC might be expected if an oscillation starts. That would accompany the red plating.

[donzoid]

Well Martin...I have "all kinds of issues" now don't I? Heheh! The original post started with the popped fuses trying to measure voltages...progressed to a measurement out of whack, the corrective actions with your clever diode placement, annnnnd now this.

Now, you are correct there is a resistor on the ground leg of the MV pot. And it "might" be possible that its something I did on the back side of the pcb but I have an "ok" iron, new tips and was very careful not to burn traces etc. In all this is why I titled this thread a "nice mess" as everywhere I look I find more...uh...gunk.

Being the obsessive on details kinda guy...I remeasured the plate voltage too, using my new "better quality" meter. Its actually at 512 with this meter. And that puts correct bias at about 35 or 36. OK I will measure my AC now and report what is there.

[donzoid]

Ok I think I have reached the end of my ability here. No significant jump on the AC reading when pushed into the red. One final question on this one...I selected DC volts at the end and it read a small (mV) amount. Normal?

I'm going to run the tubes down the street for a "sure" test.

[martin manning]

Whoa! 512V is really high. I'm afraid we're back to the power transformer again. A quick look at a replacement (same Magnetic Components unit we were talking about earlier) shows the secondary at 350VAC, which would translate into about 470VDC, and I believe there is the expectation that there will be quite a bit of sag to keep the tubes from over-dissipation when playing at high volume. This is consistent with what you are seeing in that it works ok at low MV settings, but not when you turn it up. I don't know what to think about the low volume you are experiencing except it could be a symptom of oscillation, but with the plate voltage that high I think there will be trouble even if you set the idle current low.

Yes a bit of DC on the speaker is normal. It comes from the NFB connection to the PI tail.

[donzoid]

Just as an FYI... ye olde Hickock says that while these tubes don't test "new", they have plenty of life left in them so thats a good thing! Still well matched and 85%-90% (not bad I think after 25 years in the amp). Of course, now that I "get to" rethink the whole thing I think perhaps some time, a turret board, and a spec PT are the ticket for this beast. And as you said Martin, a re-vamp of the master volume to a LarMar. Of course if I'm gonna do all that, I doubt those el34's will ever see this amp again! I still quite like the idea of a quad of 6v6 in here.

Thanks all for the time and input. In spite of it not working for now, I have learned a LOT. Oh and Martin I went out on a couple of other threads and looked at some of the amps you've built. The term "surgical precision" came to mind. Very impressive!