Dumbass tht I am.....

[Randall]

Put an OT in this Marshall JTM 30 that I have had multiple issues with. Studied it, asked questions, got answers, was confident I had ID'd the terminals, and I STILL put it in backwards! I was thinking backwards as to which wires were the primary and which were the secondary. And yes, I ran it at full power for a little bit, just long enough to take measurements that made NO sense. That's when I discovered my error. Now I have MORE problems with this. It has a hum that goes away with the Master Vol, an oscillation squeal when either Clean or Boost is turned up to about half, and some strange ripple on the OT. One plate has 9 vac, the CT has 4.2 vac, and the other plate has 2.8 vac.

I tried hooking the original back up, and the noises are still there, so I went back to the new one. The volume is much louder than before, so at least that part is confirmed. I tried new tubes, no change. My voltages on the 6L6s look good, and both tubes are pulling about the same current, 62 - 64mA at 438v, which I know is high. I have -38v of bias.

What do you think happened here?

[olddawg]

IMHE.. when I got into a weird situation like that the only real short cut is to have a working example of the same amp to compare it to. That’s also the advantage of a big busy shop.

[g1]

Try reversing your 2 outside connections of OT primary.

[Randall]

"Try reversing your 2 outside connections of OT primary."

I thought of that, but didn't try it because the wires are cable tied and will only reach in the order they originally were. Worth a try I guess.

Well Darned if it didn't correct the squeal, does that mean the two are wired reverse from each other? So much for drop in replacement. Still have the 120 Hz hum that goes away with the Master Volume, and is effected by the bass control and to a lesser extent the Mid control. The ripple followed the plate wires, so now the 9 vac is on the left side instead of the right. I'm not seeing how that is possible?

One thing for sure, the amp is roaring loud now, so at least that problem is solved.

[The Dude]

It would mean that the wires are "reverse from each other" and you had positive instead of negative feedback basically creating an oscillator. I find this often on replacement transformers- often enough that I've learned not to trim the leads until I know everything is wired correctly and working.

If you look at the bias circuit, there are 3 zeners there. Two are 15V and one is 9V, so it makes sense that you have about 38-39V bias voltage. You may have to modify the bias circuit to get proper bias. Maybe different zeners, add a zener or bias pot?

[Dave H]

There's no output stage negative feedback on the JTM30 that I can see (it's hard to follow the POS schematic) so it shouldn't have had positive feedback. It must be the layout causing positive feedback when the OT is reversed.

I'm getting a sense of deja vu. Didn't we already discuss the bias problem in this thread?
https://music-electronics-forum.com/...ighlight=jtm30
The 9.1V zener was changed to 15V

[nickb]

To measure the ACV on the transformer take them from the center tap to the plates. Otherwise I think you can get confused by the different phases on the three nodes.

[Randall]

The bias situation has already been dealt with. Just on background, it was the bias circuit that caused the other problems, cooked the AC inlet board, and possibly the OT.

The final issue here is the 120 Hz hum that follows the Master volume control that was not there until I put the new OT in backwards. Help me understand why/how the two are related? I assumed the ripple I see on the OT is related.

[nickb]

If there is no hum and it gets louder as you turn up the MV I would look before the MV. If it gets quieter then hum prior to the MV is cancelling hum introduced later that is in the opposite phase i.e hum bucking. How much hum are are talking about anyway (measure at the speaker)?

[Randall]

I see 2 vp-p (0.7v rms) with master on full and tone controls on 5, and the hum is very audible. It drops to 0.2v p-p (0.07 vrms)with master on 5, and is barely audible. `Since I doubt this amp will be run on full volume, perhaps I am chasing a ghost?

[nickb]

I'd call that a lot of hum. As a general rule I'd expect the hiss and hum to be in the same ballpark with the gains up. Time to track down the cause, I think. how do the other controls affect it? Channel? Reverb? Pulling tubes. You know the drill.

I had a fun one today where apower tube had a cathode to heater short so defeating the center tap balance network.

[Randall]

Pulling the PI tube stops it, pulling V1 and V2 have no effect, neither do channel vols, or reverb. FX in/out jacks have no effect. So it seems to be somewhere between V2 and V3.

[g1]

You've tried other preamp tubes, including PI tube?

[Randall]

Yes, I tried all the tubes. It does it with the two preamp tubes out.

OK, I am getting there! I traced the hum back from the Master vol and found it was coming from the second plate of V2 with no tube installed. I pulled it's filter cap C105 and bingo! It has splooge on the board (radial) and measures 2 nF. While I was at it, I checked them all and found the one upstream C107 to be bad as well, the rest measure good. I am going to go ahead and assume my reverse wiring stunt either caused this, or made matters worse, and not charge him for it.

So now it is back to reasonable hum levels, and now everything looks and sounds good. Or does it? While putting the amp thru the paces I notice a little squeal when I touch the input plug. I find the jack ground tip is sprung. No way this guy is going to want to hear this, and I'm not going to pull that board and put in a free jack, so I do my best to retension, burnish and spray it. But after that is *ahem* fixed, I notice that with a guitar pluged in and the amp at moderate to high volume, it squeals when I touch the positve speaker terminal. And when really turned up, it sqeals when I touch the speaker basket, which shows no continuity with either terminal, What the heck is that? Could this have something to do with the OT polarity?

Seriously, I really want this amp to go away.

[nickb]

Sorry. 5 hour time diff meant I didn't get the chance to say this: While the board it out check the bridge rectifier as they run very hot and the solder joints can fail.