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Post subject: Fender Twin Reverb Silverface 1978 humm issue
Posted: Mon Dec 25, 2017 7:45 am
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Good afteroon guys!

Happy holidays for all!

I would like to share an issue with a Fender Twin Reverb 1978 i have, it's a silveface one. There is a terrible humm as background sound which i can't take out and i don't know from where it comes.

https://soundcloud.com/pablo-andres-mel ... -twin-1978

As you can hear in this little audio test there is an Hummmm constantly behind. I had been suggested it would be caps filters. 6L6 has been changed 4 months ago. The 12ax7 i really don't know if they been changed for somebody.

Can you give me an idea or opinon of what this would might be?

Thank you very much!


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Post subject: Re: Fender Twin Reverb Silverface 1978 humm issue
Posted: Mon Dec 25, 2017 8:11 am
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Your playing is beautiful. Loved the clean tone. Didn't dig the distortion - assuming that was a pedal, your playing deserves a better pedal.

I didn't hear any hum. I heard buzz. Completely different thing with different causes.

Buzz is usually from emf/rf interference. Your rig is picking up noise from fluorescent/neon/halogen lights, transformers, dimmer switches, TV's, or other noise sources.

If you've ruled out guitar/cables/pedals, check the amp's lead dress (layout of the wires) and make sure the cab screening is intact (metal foil or metal mesh stapled or glued to the inside top of the cab, above the open part of the chassis).

I could hear voices in the background. If you were playing quietly I'm not even sure that was an excessive amount of buzz.


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Post subject: Re: Fender Twin Reverb Silverface 1978 humm issue
Posted: Mon Dec 25, 2017 8:18 am
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strayedstrater wrote:
Your playing is beautiful. Loved the clean tone. Didn't dig the distortion - assuming that was a pedal, your playing deserves a better pedal.

I didn't hear any hum. I heard buzz. Completely different thing with different causes.

Buzz is usually from emf/rf interference. Your rig is picking up noise from fluorescent/neon/halogen lights, transformers, dimmer switches, TV's, or other noise sources.

If you've ruled out guitar/cables/pedals, check the amp's lead dress (layout of the wires) and make sure the cab screening is intact (metal foil or metal mesh stapled or glued to the inside top of the cab, above the open part of the chassis).

I could hear voices in the background. If you were playing quietly I'm not even sure that was an excessive amount of buzz.


Hi bro!

Thanks! The distorsion also comes from the amp, i leaved the Master at 1 1.5 something like that and used the volumen at maximum.

The buzz starts 2 seconds after i switch the Stand by button and it's quite annoying really :(

I tested everything i would:

Cables KO
Ground cable KO
Another amps KO

Thanks!


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Post subject: Re: Fender Twin Reverb Silverface 1978 humm issue
Posted: Mon Dec 25, 2017 8:41 am
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Yeah, the master volume controls on '70s silverfaces have a bad reputation for that very reason. The preamp is low gain and unless you crank the amp loud enough to get the PI tube and power tubes distorting it'll have that raspy harsh distortion with the master low and the channel volume high.

Converting the master to a post phase invertor master volume (PPIMV) sounds a bit better. But using those amps as clean platforms with good pedals is the most common approach.


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Post subject: Re: Fender Twin Reverb Silverface 1978 humm issue
Posted: Mon Dec 25, 2017 8:50 am
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A bad tube can cause buzz. Usually one of the preamp tubes, but could be any of the tubes.

Electrolytic filter caps that go bad usually cause hum. But if they go bad due to rising ESR they can cause buzz.

Electrolytics have a limited lifespan. Sometimes they can last for 50 years or more, but generally much shorter than that. Most people suggest replacing them every 10~15 years before they fail. Sometimes they can cause catastrophic damage to the transformers and tubes when they fail.

If it still has the 40 year old original filter caps it would be a very good idea to replace them, even if they aren't the cause of the buzz.


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Post subject: Re: Fender Twin Reverb Silverface 1978 humm issue
Posted: Mon Dec 25, 2017 8:56 am
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On the bottom of the chassis (underneath, outside, where the tubes are) there's a metal cover/can/box. Generally called the "doghouse".

The electrolytic filter caps are inside that. There's also a smaller electrolytic inside the chassis as part of the bias circuit. All those should be replaced.


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Post subject: Re: Fender Twin Reverb Silverface 1978 humm issue
Posted: Mon Dec 25, 2017 9:03 am
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Best is to hear the amp alone with no guitar plug.

What I see;

I'll test and clean all input jacks's normaly close contact to ground. Often they are bad
12XX7 noisy tubes
Unbalanced 6L6 tubes
Bad wiring dress acting like antenna


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Post subject: Re: Fender Twin Reverb Silverface 1978 humm issue
Posted: Mon Dec 25, 2017 9:37 am
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stratele52 wrote:
Best is to hear the amp alone with no guitar plug.

What I see;

I'll test and clean all input jacks's normaly close contact to ground. Often they are bad
12XX7 noisy tubes
Unbalanced 6L6 tubes
Bad wiring dress acting like antenna


Hi!

Thank you very much for the advices! i will try to test or directly send to a tech professional to change tubes and caps and check something else.

By the way, is it possible to take out 2 of the 6L6 to reduce the power of the amp? to be like 65 watts for example?

Thanks!


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Post subject: Re: Fender Twin Reverb Silverface 1978 humm issue
Posted: Mon Dec 25, 2017 10:01 am
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Change tubes and caps ? Checked them , replace if needed.
Caps sound good to me.
If it is as second hand amp , check fuse to make sure it is the right one.

Remove two tubes ?
https://www.premierguitar.com/articles/ ... duce-power


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Post subject: Re: Fender Twin Reverb Silverface 1978 humm issue
Posted: Tue Dec 26, 2017 9:49 am
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Don't expect 2 tubes to be half the volume of 4. It's only about a 3dB reduction.

As the linked article mentions, the ideal impedance changes with 2 tubes removed. You can achieve a perfect match by disconnecting one speaker. That will also knock down the volume a tiny bit (but sound slightly less "full").


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Post subject: Re: Fender Twin Reverb Silverface 1978 humm issue
Posted: Tue Dec 26, 2017 9:59 am
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'78 135watt Twin Reverbs have the "Ultralinear" transformer wiring. The power amp is much cleaner than the 85watt blackfaces and earlier silverfaces.

Converting Ultralinear to blackface specs is more complex than the earlier silverfaces.

Much better to just get the amp fully up to specs. Healthy tubes and healthy caps. And then use it as a great clean amp with good pedals for dirt.

If the filter caps are the original 39 year old ones it would be a good idea to replace them even if they're still currently working properly.


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Post subject: Re: Fender Twin Reverb Silverface 1978 humm issue
Posted: Tue Dec 26, 2017 10:34 am
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strayedstrater wrote:
'78 135watt Twin Reverbs have the "Ultralinear" transformer wiring. The power amp is much cleaner than the 85watt blackfaces and earlier silverfaces.

Converting Ultralinear to blackface specs is more complex than the earlier silverfaces.

Much better to just get the amp fully up to specs. Healthy tubes and healthy caps. And then use it as a great clean amp with good pedals for dirt.

If the filter caps are the original 39 year old ones it would be a good idea to replace them even if they're still currently working properly.


HI!,

Thank for all the advices, finally i will change the caps as soon as i can because i already tried the tubes and the problem is still there.

As soon as i do it i will re post with video :)

Thanks as always to all of you!!!


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Post subject: Re: Fender Twin Reverb Silverface 1978 humm issue
Posted: Tue Dec 26, 2017 10:47 am
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Filter caps did not make this kind of noise.

Did you change all tubes ?
Don't forget other tests I wrote yesterday ;


I'll test and clean all input jacks's normaly close contact to ground. Often they are bad
12XX7 noisy tubes
Unbalanced 6L6 tubes
Bad wiring dress acting like antenna
____________________________


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Post subject: Re: Fender Twin Reverb Silverface 1978 humm issue
Posted: Tue Dec 26, 2017 12:30 pm
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[youtube][/youtube]
stratele52 wrote:
Filter caps did not make this kind of noise.

Did you change all tubes ?
Don't forget other tests I wrote yesterday ;


I'll test and clean all input jacks's normaly close contact to ground. Often they are bad
12XX7 noisy tubes
Unbalanced 6L6 tubes
Bad wiring dress acting like antenna
____________________________


Hi bro!

Thanks for this, i don't know how to test the rest, the best it will be to take it to a professional guy, it's very funny because in the recordings it doesn't appear the noise so much as here look:



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Post subject: Re: Fender Twin Reverb Silverface 1978 humm issue
Posted: Tue Dec 26, 2017 2:02 pm
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I've never personally experienced this, but I've read that if the ESR of the filter caps has drifted upward, they can let power line buzz enter the signal path.

We both suggested lead dress and other things as being more likely than the filter caps.

Stratele52, you've previously posted that you consider 30 years to be the safe lifespan of electrolytics.

I keep harping on the caps simply because if he's going to take it to a tech anyway he should make sure they're not the original caps. If they're ancient, might as well have it done while it's in the shop rather than bring it back again later.


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