Power Transformer advice needed - Marshall 9005 - fixed???

[Jooshtin]

Hi all, I'm looking for some advice about my Marshall 9005 50w+50w power amp. The power transformer hums loudly and seems to be mechanically coupling the hum to the OTs and my preamp. I'm used to Marshalls & transformers humming but this doesn't drop to a usable level off standby.

So far I've replaced all the elec caps, tubes and worn out volume pots. The background mains hum from the power transformer can be heard though the speaker - even in standby with no tubes in. You can feel the transformer vibrating the amp. I've tightened the bolts which made no difference. The PT still vibrates and hums with all the wires apart from the mains AC disconnected.

All the voltages seem good and it does sound great apart from the hummmm....! I'm starting to wonder if it needs a new PT? I'd be really grateful for any ideas and advice, I've attached a schematic and some pics, many thanks
Justin

[Phil_S]

Have you tried putting some rubber washers between the PT and the chassis? This is one way to tame a "mechanical" hum. Look in the plumbing department for rubber washers. If it works, it's a quick 50 cent fix. You don't have to pull the PT. Just withdraw the mounting bolts enough to slip them in.

[Structo]

That is a good idea Phil.

That is de-coupling the transformer from the chassis.

Another thought, I remember reading an old article that said sometimes it has to do with the phasing of the primary winding on the transformer and if you reverse the wires of the primary it can help with the mechanical vibration of the core.
I don't know how or why but it is something to try.
I would try Phil's idea first.

As far as, why does this transformer do this?
Not sure, but it must be something magnetic/ mechanical that is causing the laminations to vibrate.
Did you make sure the screws that pass through the laminations are good and tight?

BTW, is this a factory amp?
Good luck.

[Jooshtin]

Cheers Phil & Tom, I've got some rubber washers so I'll try that this eve, as well as reversing the the wires to the primary. I'll let you know how I get on.

It's a factory made amp - made by Marshall in 1990, model 9005. I've read that these amps have a reputation for running hot and the PT giving up - it does run v warm (a rack unit with no fans) so I've got cooling fans in my rack. I can't imagine it hummed like this when new, so I guess 20 years of heat hasn't helped the PT...

J

[Richie]

Just guessing, it didn't do that when new,and i doubt now anything you do will cure it. It might lessen it some. But in the end you'll probably have to swap out the PT. It will just get worse if you let it go.

And those PTs are junk in my opinion. marshall vox transformers through those years were nothing but problems.
Looks like they would have changed transformer makers if they couldn't get them supplied without all the problems. After the 800 series, the transformers went way downhill. Again, only my opinion.

[David Root]

I have to agree. No interwinding electrostatic shield, no copper flux strip around the lams, not even bell covers! Must have hummed from day one.

I bet Heyboer could make you something better that would fit mechanically and electrically. Would cost you in a one-off unfortunately.

Since you are in the UK, I would look up Sowter on the web and talk to them. They ain't cheap either, but you might find a stock unit that is close enough and it WILL NOT hum!

[Jooshtin]

Yup, I've given in and ordered a new PT and end bell from Tube Amp Doctor, hope that stops the hum...

I was surprised to not even see a cover on on the old PT. I agree, Marshall (and all the other big manufacturers) save every penny they can for max profit, so always use the cheapest methods they can get away with, cheapo PT and "suicide" bias as a good case in point! Overall this amp is well built and sturdy, from the end of the 800 era, apparently they were too expensive to build so weren't around long. +1 on things going downhill after the 800 series, and the newest models I've recorded recently are so poor in build and sound quality, no match for an 800 or older model IMHO

Thanks again for the advice and I'll let you know if the new PT does the trick.

[Richie]

And 'IF" you did have a transformer that had a cover or bell, save it, because usually the new or replacement wouldn't have one,and you could use the one off the old transformer. They are makers that have the 900 series transformers.

[dynaman]

Yup, I've given in and ordered a new PT and end bell from Tube Amp Doctor, hope that stops the hum...

I was surprised to not even see a cover on on the old PT. I agree, Marshall (and all the other big manufacturers) save every penny they can for max profit, so always use the cheapest methods they can get away with, cheapo PT and "suicide" bias as a good case in point! Overall this amp is well built and sturdy, from the end of the 800 era, apparently they were too expensive to build so weren't around long. +1 on things going downhill after the 800 series, and the newest models I've recorded recently are so poor in build and sound quality, no match for an 800 or older model IMHO

Thanks again for the advice and I'll let you know if the new PT does the trick.

What's "suicide bias"?

[Jooshtin]

The bias voltage comes off the wiper of PR2, if the pot fails or gets "scratchy" there will be no bias voltage going to the power tubes! The tubes will then draw maximum current and overheat, and worse, overheat the output transformer.

I've run a 220k resistor from the junction of R22 and PR2 to the wiper, and a pair of 100k resistors from the wiper of PR1 (the AC balance pot) to R9 & R20. Now, should either pot fail it will hopefully default to a higher bias voltage and run colder....

[Jooshtin]

I got a new PT from Tube Amp Doctor and it's now nice and quiet - TAD were very helpful and shipped it fast. Looks like a well made trans with hum band, interstatic shield and end bell. There's now only a tiny bit of normal background hum, barely audible, I've tried raising the heaters etc but it all makes no difference so I'm happy it's as good as I can get it! With new tubes and biased it sounds great. Cheers to everyone for your help & advice

[Structo]

I'm sure you have tried this but what effect does turning the PR1 balance control have on the hum?

[Jooshtin]

It doesn't do much to the hum, at extremes it just ads a bit extra buzzing hum - I have read that it's for AC balance. If anyone knows more I'd like to know.. I've just measured the value either side and set PR1 in the middle. I checked the voltages today and realised the heater is running at 7.09vac with tubes in and both channels working I'll try lowering the heater voltage as explained by Merlin at http://www.freewebs.com/valvewizard/.

The mains voltage here is 240vac and the transformer is 230v - TAD did tell me it was fine for 240v though... The B+ voltages are also up by the 230 to 240vac ratio (345vac output printed on PT, actual 360vac and 365vac). The voltage at X on the schematic is 484vdc.

Cheers

J

[Richie]

Without changing anything, you could get a vaiac for your voltage, then dial it in to match the power for the PT,which would be about 10%.
Then everything should be normal.. But then, that means you have to pack it around with ya..

Kinda of like running the old transfomers here, a 110-115v transformer on some old amps, plugged into our now about 125v and same thing happens,all the voltages in the amp come up. Not real good for the heaters to run above.

[Structo]

It doesn't do much to the hum, at extremes it just ads a bit extra buzzing hum - I have read that it's for AC balance.
J

Yes some of the old Fender had a Hum Balance control but that would be instead of a bias control.
You didn't get both!
The bias was truly fixed by a fixed resistor and you would turn the hum balance control for the lowest audible hum.
But what it was actually doing was balancing the bias voltage between the grids so if one tube was drawing more current you could sort of "even" them out for the lowest hum.

I saw an old Stromberg Carlson PA amp that had a DC balance and a bias pot which I thought was a pretty neat idea.