Marshall JTM60 preamp filaments

[rnbboy@comcast.net]

I have a Marshall JTM60 (3-10"combo) that the owner claimed would lose power while playing. I verified the problem as the amp would sound fine for a bit and then would peter out. The problem was traced to no filament voltage on the preamp tubes. The four preamp tubes are run on DC voltage-how much?-and after a LOT of work getting the circuit board out the bridge rectifier for the filaments checks out O.K. I remember working on another Marshall with a blown bridge rectifer a few years ago. Does anyone have any insight on this problem. I'm now thinking that it could be due to cold solder joints on the PCB as I can tell that the BR runs pretty warm. Thanks for any help with this.

[BrotherJ]

I think its 6.3volts to the heaters in the JTM60

[Enzo]

You found the bridge was OK, but did you determine that the solder that was holding it to the board was making a good connection to its legs? I find a common problem is that the solder never wetted the leg well from the start. Pull the bridge, polish up the legs with abrasive, and reinstall it, making a GOOD solder conection. or just replace it. If those tubes are dark, follow the voltage. If the AC gets to the AC terminals of the bridge, then whatever voltage is on the DC terminals will be right - assuming the bridge is OK. Then follow the traces carrying that DC over to the sockets. it will lose continuity somewhere. I'd bet on the bridge solder myself though.

[PerroGrande]

I agree -- a poor solder connection is very likely to be the cause here. As the tubes heat and things expand, the connection goes bad. I've worked on tube-based amps and televisions (geez, I feel old) with this exact problem...

I would follow Enzo's advice, and also inspect the traces to make sure that one of them isn't damaged, lifted, etc. If you notice any carbon from arcing, etc, you'll want to clean this off as well (rather unlikely at lower voltages, but worth taking a quick look for just in case).

[Amp Kat]

I had this problem with this amp several times and it is indeed the heater voltage dropping out. Sometimes retouching it with solder works but sometimes have to change out the whole tube socket. you can get the sockets at Antique Electronic Supply. It's usually just one socket. You can take the board out pretty easy but remember to take the preamp tubes out before you pull the board out are you'll bend the pins up pretty good.

[Mars Amp Repair]

well,
I have also experienced all the above plus bridge rects that go intermittent when they get super hot..as these do. I'd also replace the bridge & stand it off of the board about 1/4" to help dissipate some of the heat. That seems to work rather well for the longevity of the repair.

Also, there are many print through connections on some of these amps that have double sided pcbs. The print-throughs also can go bad. That's a real fun one to find.

Replacing the bridge with a size 1amp higher also helps to ensure future operation....g

[Amp Kat]

That bridge is a big pain and we have discussed it before here. One of the signs is you'll see dark brown area all around it and if you do yes I would change it to a higher rated bridge. I believe the one in there is a WO8 round British style. When powered up look at the back and monitor the preamp tubes. If you see one drop out that is the one with the problem but I would also take these guys advise on that bridge.

[rnbboy@comcast.net]

Well, first off thanks for all the replys to my post. After reheating the bridge solder connections on the board because it does look brown on the PCB under the bridge and repowering-no sound-and some strange readings on the pins of V1. I'm getting 1.2Vdc on pins 4,5,&9, but am also seeing 482.7Vdc on the plates of the preamp tubes. The EL-34's lite up fine with around 6.0 Vac (I know it's summer) on pins 2&7 and -45 Vdc for the bias. I also am getting no bias voltage on V1 or V2 on pins 2&7. Oh and by the way, this amp is layed out wrong and has 0 serviceability. Thank you all and Peace...

[Amp Kat]

You may have both problems but no doubt sounds like that rectifier is toast and I'd replace it no matter what. You are checking DC on 4,5/9 right ? Power tubes AC preamp tubes DC. And you negative bias voltage with respect to ground should be on pin 5 of the power tubes not 2 & 8. Your heaters are on 2 & 7. But 495 is to high on preamp tubes which makes it seem that BRD is causing problems and I've seen it before and changing the BRD solved the problems. If you change it and it still drops out it could then be the sockets but all of your voltages should be right.