Bluesbraker problem

[chungaleta]

Hello mates.
Lying flat on my bench is a Marshall 50w JTM Model 1962 Bluesbreaker reissue made in ´94

It has a rattling sound when you play low notes, specially the 6th string. It sounds as if it was a broken or torn speaker, but speakers are fine (they´ve been tested, and so the amp with other speakers)
Funny thing is the muddy noise is only noticeable on the lower notes, the lower the noisier.

I´ve measured-checked ceramic caps, AC filter caps, and ALL resistors. the F.E.T beta is what it should be.

Cleaned tube sockets, all jacks, the board and soldering points, looking for cold joints... none.
Tubes are brand new, I swapped them around and the problem remains unchanged.
Gosh, it´s such a simple amp to be giving me so much trouble.

What else is left to check??
Any Hints???

[Stanz]

I had a similar issue on an ODS I built. In this case it was a fizzy sound triggered by the low E. The low A was almost too high to trigger it. It turned out to be the NFB. I had it coming from the 8 ohm tap when the circuit was set for it to come from the 4 ohm tap. Assuming this is the same issue, I have seen others describe this issue as a "farty bass" or low end. That may aid in looking for what others have done. The other way to have dealt with it would have been to keep using the 8 0hm tap and use differrent cap/resistor to set cut off frequency correctly.

You may very well have checked this, but is the speaker tight against the cabinet, or some other object rattling. It is remotely possible the JTM plays differently through the speaker, so simply playing another amp through these speakers may not reveal something that is loose. I say this only because sometimes it is hard to get what the problem sound is without actually hearing it.

[Zippy]

Have you checked the mechanics of the cabinet? Loose baffle mounts, etc.?

[chungaleta]

Thanks mates.
Actually I´ve played the amp through my own 4x12 and 2x12 cabs, the problem remains the same regardless of the cab it is played through. Honestly I didn´t check the housing of the amp, only the speakers, cause it gives me the exact same problem with other cabs that I´m 100% sure they are in fine working order.
I´m off right now to buy some other parts I need for other amps, but as soon as I come back, I´ll check on that NFB rout once a gain, I´ll re-check its connections and its sources, It came from the 8ohm tap of the OT if I´m not mistaken, but I´ll have a look at both the amp and the schematic to make sure.
And of course I´ll post my findings here.
Thanks a million.

[chungaleta]

Thanks mates.
Actually I´ve played the amp through my own 4x12 and 2x12 cabs, the problem remains the same regardless of the cab it is played through. Honestly I didn´t check the housing of the amp, only the speakers, cause it gives me the exact same problem with other cabs that I´m 100% sure they are in fine working order.
I´m off right now to buy some other parts I need for other amps, but as soon as I come back, I´ll check on that NFB rout once a gain, I´ll re-check its connections and its sources, It came from the 8ohm tap of the OT if I´m not mistaken, but I´ll have a look at both the amp and the schematic to make sure.
And of course I´ll post my findings here.
Thanks a million.

[fishy]

I've had a transformer end bell bug me before. I had to have the amp on the cabinet to get it and the lower frequencies set it off. By that point I had checked all over and secured every bolt I could find in the ampand the speaker cabinet.
I put a big elastic band around the PT and it went away.

[M Fowler]

Filter caps can cause off sounding notes, and can add a warble sound as well. This is especially heard on Fenders. I suspect that the tone caps are to low a value causing low frequency problems. Verify the caps with the correct schematic and if they are correct change some to a higher value for experimentation. In the NFB I use anything between 68K to 100K and no problems that you describe.

Mark

[Firestorm]

The filter caps in the reissue are larger than in the original (should be 32-32 and 16-16) in the reissue they're all 50s. Might be something there, but it sounds like a parasitic to me. Good luck.

[selloutrr]

I had the same problem on my bench last week from a Peavey Classic 30. It sounded sweet (well as sweet as a peavey can) as long as I didn't play a G note or Lower on the Low E string then it would break up and fritz. I swapped all tubes in the amplifier for known good ones (Not just new!) and the problem was solved!!! Then I went back and replaced the tubes one by one with the originals until I isolated the problem to the output tubes. Ordered a new set tested them, mod'd it to adjustable fixed bias and it's as good as new. I don't think you'll find the problem in the NFB if the amp has been working fine since '94, that is a common mistake made and fixed at the time of building but not really years down the road. It could turn out to be in the transformer, though. I'd go back and check the tubes again, and then the filter caps, and transformer. check your shunt resistors and make sure the legs of the transformer are metering in MegOhms to ground.

[chungaleta]

Done! at last.
After having done anything possible, including new tubes, I decided to try out old tubes from a friend´s amp just to see what happened. With awe and disbelief, I noticed that as soon as I replaced the Rectifier tube the problem stopped.
I put the amp´s old tube back in, and then the new one and both gave me the same farty-bass response, however, a 3rd tube, known to be good solved the problem.
I guess I went in too naive assuming new tubes are to
be considered "good" just for the sake of being new.
And even when I knew they don´t always honour their status of New-in-the-box, I thought the problem was somewhere else and continued to shoot in the dark for days.
Damm I feel embarrassed!!!! Way too green!!!!

Thank you much for all the help.

[Structo]

So it was the rectifier tube?

How odd!

[chungaleta]

Or so it seems, the amp owner just bought another new tube, put it in and no noise has occured.

Damm if it starts bothering again I´ll tell that guy to take the amp to a Shaman.

[drz400]

Hello mates.
Lying flat on my bench is a Marshall 50w JTM Model 1962 Bluesbreaker reissue made in ´94

It has a rattling sound when you play low notes, specially the 6th string. It sounds as if it was a broken or torn speaker, but speakers are fine (they´ve been tested, and so the amp with other speakers)
Funny thing is the muddy noise is only noticeable on the lower notes, the lower the noisier.

I´ve measured-checked ceramic caps, AC filter caps, and ALL resistors. the F.E.T beta is what it should be.

Cleaned tube sockets, all jacks, the board and soldering points, looking for cold joints... none.
Tubes are brand new, I swapped them around and the problem remains unchanged.
Gosh, it´s such a simple amp to be giving me so much trouble.

What else is left to check??
Any Hints???

I have also seen this problem having to do with parasitic oscillation and lead dress
Usually from the input jacks to the circuit or from the primaries on the OT too close to the secondaries around the power tube sockets.

Good you found it though, it is the simple things that get you.

[drz400]

delete