The Steel Guitar Forum

Question for the tube amp whizzes

I own two mid 70's Fender Vibrosonics. Both are the 135 watt bi-linier models with orange frame JBL's. Both have been gone through in the past and work and sound great overall.

The amp in question has new caps, plate resistors and Wing C power amp tubes. This work was performed by a very reliable shop just outside of Nashville before I moved to Florida two years ago. The amp has around 75 hours of use since this work was performed.

The question concerning this amp is that it has developed some "hum" when first being turned on. The hum will diminish in a few minutes to somewhat acceptable limits. Flipping the ground switch to the other position does not make a difference.

Any thoughts on what can make this ? I had talked to Ken fox about bringing the amp in to his shop for a check over but his house fire has since put this on hold.

I really do not fool around much with the technical work on these all tube amps. Is the hum adjustment control on the back pannel something that I can try and adjust at home to see if this might help eliminate it ?

I do plan on taking it to Ken in the future if he has time to work on it. If not, I need to find a good tube amp shop near the Tampa FL area.

Im just trying to get some idea of what can possibly cause this problem.

Thanks
Ray DeVoe

I've got the same hum in my twin when I first turn it on, for about 5-10 seconds. My amp tech says it's the old transformer(s). I don't worry about it since it goes away quickly.

The hum balance is user adjustable and won't hurt anything. You basically just turn it each way until you find the spot with the least hum. Don't expect miracles from it.

the hum balance adjusts the "false" center tap of the heaters, and will not make much difference in your situation. I would not touch it until the amp is warmed up and idling.

most tube amps do a little funny business when they first turn on - think about it: you've gone from a cold nothing to about 400+ volts with heat in under a minute. Any corrosion, dust, moisture etc will take a second or two to settle

Also, as Brett suggests it could be the laminate of the transformers vibrating for a moment as they get used to the voltage/heat

I wouldn't worry about it. If the hum returns during normal warmed up operation, I'd take it to a tech.

I believe that Brett and Tim have this one figured out, you are hearing the sound of the power transformer laminates vibrating, as they warm up and expand the noise should stop. It will do no damage to the amp or the tone of your guitar but if the noise becomes a constant problem you might want to consider having it replaced.

Hey Guys.

Thanks for the information. My main concern was to try and find out why it has the hum when its first started. It has a heavier hum for about a minute, then it lightens up. The amp soungs great and works fine.

I really should sell one or both of my Vibrosonics as I have quite a few amps. I play out very lightly these days and a heavy tube amp is the last one to go out the door. I just do not use them enough to justify keeping them around.

Thanks again
Ray DeVoe