I know this has been discussed a 1000 times.

[dude]

I have a Marshall 18 watt TMB that I built years ago, over time it has developed a loud hum (many hours without a problem till lately, hum came slowly). Still plays well but the hum interferes with recording and annoying.


I know there is a procedure to find the culprit, like with pulling tubes, does it do it... etc?


Can someone direct me to a link maybe a pass post about this, can't seem to find anything on a search here. 


al

[dpm309]

Here is a link to the Tube Amp Debugging Page that has a section on hum. http://www.geofex.com/ampdbug/ampdebug.htm
Since the hum came on gradually, I would suspect your power tubes or filter caps.

[dude]

Probably the el84's but they seem to not lack any power, amp still sounds good. The tubes are old Phillips made in Holland and hard to find. They are indeed old and have many hours on them, perhaps they have drifted too far apart...?


Thanks, I'll check that link

[Willabe]

.......perhaps they have drifted too far apart...?

Check them for current draw, they very well may have drifted apart over the long hours of being played.

Or just pop in a new matched pair and see if the hum is less or gone.

[SnickSound]

Swap the cathode follower preamp tube (don't remember if it's the 2nd or 3rd one on that design) with a known good tube. It's often the first to go due to high heater to cathode voltage difference (cathode hangs at 120-130V or so).

Just to be clear, I'm not saying this is definitely the problem, I'm saying it's quite possible it is and swapping a preamp tube is an easy test.

[sluckey]

Hum...

The first thing I'd do is change the output tubes, mainly because that's easy. Then I'd use a 20µF test cap with gator clip leads to bridge across each filter cap one at a time.