Hum in Fender tube amp! Please help!!!!

Tele · From: Andy W. - Wolfenbuettel, Germany

I have a vintage Tweed Fender that I use witha stepdown converter (110V- 220V).
There's a bit of hum ( 50Hz ??) that can be heard even if the amps volume is turned down.
What can I do about it? What to look for? I tried another converter but same thing...

Andy

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Usually, that hum is from the supply line. The 60-cycle (or in your case 50-cycle) hum is a characteristic of AC voltage. Normally, when you get a lot of hum in an amp, it's caused by bad filter capacitors. These are large round components (about 1"x3") usually found near the power transformer. Fender covers most of these on the outside of the chassis with a rectangular steel cover (kinda like a small baking tin). But, in a very old amp such as yours, they may be mounted inside the chassis. There are usually 2 or 3 of them. Replace them all. They are not hard to replace, anyone familiar with electronics can handle the job. They are sometimes hard to find, though. Most supply houses carry very few of these high voltage filter caps (also called electrolytic capacitors or "condensers"). Try to get the values close to the originals, and don't use anything under the capacity and voltage rating printed on the cap! It's OK to go higher, but if you use a lower voltage, they might fail prematurely, and if you use a lower capacitance value, they won't filter as well, causing some hum, and sometimes lowered output.

It's a pain...but not a big deal. You should be able to get it fixed quickly, and for a reasonable amount of money.

[This message was edited by Donny Hinson on 10 November 2000 at 05:32 PM.]

Blake Hawkins · From: Florida

Tele, Check the converters you are using. If they are solid state then the hum will be difficult to eliminate. This is because of the waveforms generated. Most amps will have a hum when used with these type of converters.

The better way to do it is to use a step down transformer to go from 220 to 115 volts. These are usually more expensive but a transformer should work without the hum problem.
The 50 cycle power is a bit harder to filter than the 60 cycle power for which the amp was designed, but the filters should be able to handle it.
Power supply hum will be at 100 cycles. The hum caused by the solid state converters will have some higher frequency components and may sound more like a buzz.
Blake

Vern Kendrick · From: Earth

The Fender Syndrome,Power supply Caps

Tele · From: Andy W. - Wolfenbuettel, Germany

Hm, I think I should go for the caps. The step-down transformers that I used with this amp are usually very quiet when used with my Tweed Twin.
Or is it possible that this particular amp could have that problem with the transformer while others are working fine?
Will check the caps first!
Thanks so far
Andy

Blake Hawkins · From: Florida

Since you have a known good transformer, I agree with the other guys, go for the caps.
Which model Fender amp has the problem?
I have a 1953 Fender Deluxe that I use every day. It has been recapped, resistors measured and tweaked.
If you have trouble finding the caps, let me know. There are several good suppliers, including Antique Electronic Supply (www.tubesandmore.com).
Blake

Blake Hawkins · From: Florida

An additional thought. Most old Fenders have a "Ground Switch" which works with a line bypass capacitor. There was a thread a while back about this. In the original configuration which was ment to eleminate hum by grounding the chassis to the mains neutral, this can cause an electrical shock.
Modern amps, with a grounding plug don't have this problem.
Is your amp grounded to the mains? This cap and switch arrangement can cause hum under certain conditions.
Blake

Tele · From: Andy W. - Wolfenbuettel, Germany

It's a 1958 Pro Amp.

Jay Ganz · From: Out Behind The Barn

Does it HUM as soon as you turn it on, or does it take a few minutes? If it's right
away...then it's one of those 16 mfd caps.
Are they still the old ASTRONS in there?
If it takes a minute or two to start humming,
then check to make sure one of the 6L6
(or 5881) output tubes isn't glowing red
in the center. A short in the output section
will pull way too much current and you'll
hear that HUMMMMMMMMMM....but then it'll
usually pop the fuse, so I doubt that's your
problem.

Tele · From: Andy W. - Wolfenbuettel, Germany

It does hum right away...this amp was rebuilt by a well-known amp-guru here in Europe so I doubt that any of the components are bad. I checked the "ground-switch" idea and I found a solder joint that is connected to...well just nothing..one is connected to the AC-switch, one with the power plug which is grounded.
Do I need to connect this solder point with ground?? Seems obvious to me but ....

Dave Brophy · From: Miami FL

Leave the ground switch disconnected.Hopefully the AC cord was changed to a 3-prong type at some point,making the ground switch (aka the killer circuit) obsolete.If it's not a 3-prong cord,have it changed right away.
I go along with the filter caps diagnosis on the hum.
Btw,Fender reissue amps have the ground switch on them...connected to nothing.

Tele · From: Andy W. - Wolfenbuettel, Germany

yes, 3 prong type...
so back to the caps, but I'd better leave that up to my amp tech
thanks for all the help so far!
Andy

C Dixon · From: Duluth, GA USA

Tele,

I would doubt very seriously if these filter caps are causing your problem. I have seen two many electronic devices share this problem using 50 cycle current through a converter.

Changing these caps and/or having to go through that expense is a bit much only to have the exact same hum after you are through. I would try to explore other avenues first. I serviced Fender and other Amps for many years. I rarely had to change these caps. But many was the time I suspected them, only to find after changing them, it was other things that caused the hum.

CAUTION: please be very careful in removing and/or dissabling grounds. Your life is worth more than all the hum reduction in the world.

God bless you in your attempts to find the true cause of this malady,

carl

Kenny Dail · From: Kinston, N.C. R.I.P.

Carl has given you some good advice. I would recommend before you do anything else, assuming you haven't already done so, lift the ground and see if this eliminates the hum. If not, adjust you "bias" control, if this is out of adjustment, it could eliminate a lot, if not all the hum. Also take the amp to a friends house and see if you get the hum. Could be you may have a problem in the wiring of the AC circuit you are using at home. If the hum still exists, seek the advice or service of a professional repair service.

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kd...and the beat goes on...

Jack Stoner · From: Kansas City, MO

I would not adjust the bias control for hum. Many think this is what that control is for but it is not. It is for bias adjustment on the power amp tubes. If you adjust it for minimum hum you have unbalanced the power output tubes and could have distortion and even damage the tubes.

Use the bias adjustment only for what it was intended.

Kenny Dail · From: Kinston, N.C. R.I.P.

Jack, Thanks for enlightning us on the use of the Bias adjustment. Sorry about making a bad suggestion. I just did not know any better.

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kd...and the beat goes on...

Jack Stoner · From: Kansas City, MO

Kenny, no problem. The bias/hum balance myth has been around for many years. I first ran across it when I was doing amp repair at Little Roy's music store in the early 70's.

Vernon Hester · From: Cayce,SC USA, R.I.P.

Ck the screen resistors on the 6L6 tubes or Voltage should be the same on both of the 6L's
Vern

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