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Topic: 75 fender twin mv |
Mike Bowles
From:
Princeton, West Virginia, USA
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Posted 22 Dec 2014 3:35 pm |
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my 75 silverface twin master volume started humming pretty loud when i turn the mv up past 3 or 4 when i turn the master volume off it stops it has 2 jbls in it amp is in great shape other than that. i put new ruby tubes in it a little while back any help would be appreciated.
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Mike Bowles |
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Lane Gray
From:
Topeka, KS
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Posted 22 Dec 2014 8:05 pm
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1) does it matter if anything is plugged in?
2) Does it matter which way the amp faces?
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2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Tony Prior
From:
Charlotte NC
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Posted 23 Dec 2014 2:21 am
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it's not actually the MV thats humming, it's the power stage or power supply, you just don't hear it at lower volumes. Be sure it is NOT the AC power source ( where it is plugged in) first. If it still as hums it's more than likely time for service , seeing it is an amp from 1975 ! I suspect main power supply. You have heard this before " cap job" This is what electrolytics do when they fail. And no, do not attempt to repair it yourself, there is 465 V running around inside that kool amp and it will find you !
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Emmons L-II , Fender Telecasters, B-Benders , Eastman Mandolin ,
Pro Tools 12 on WIN 7 !
jobless- but not homeless- now retired 9 years
CURRENT MUSIC TRACKS AT > https://tprior2241.wixsite.com/website |
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Mike Bowles
From:
Princeton, West Virginia, USA
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Posted 23 Dec 2014 7:21 am twin
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thanks guys yes lane it hums with nothing plugged in tried it in different places tony i kinda figured it needed some tlc i wont try i know the hazards thanks
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Mike Bowles |
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Richard Keller
From:
Deer Creek, Illinois, USA
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Posted 23 Dec 2014 7:28 am
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| You could try adjusting the hum balance pot. That fixed the hum on my 1974 Bassman. |
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Mike Bowles
From:
Princeton, West Virginia, USA
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Posted 23 Dec 2014 1:34 pm twin
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where is this richard? thanks
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Mike Bowles |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 23 Dec 2014 7:32 pm
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If your Fender amp hums with nothing plugged in then something's wrong... Fenders have a reputation as being dead quiet when all is working right. A good technique is to start pulling preamp tubes from the front end, headed for the PI, and see where the hum stops. This tells you a lot about what to do about the hum. Since your amp doesn't hum with the master down, the hum balance control won't help, since it is meant to cure hum in the final amp (which is past the MV control). It's either a preamp tube or filter caps.
The hum balance control on that era of Fenders changes the bias of one side of the final amp. This is meant to balance the final section... the problem is, you don't get to set the bias properly. Fender bought lots of tubes... rather than match them, or include an extra pot for bias and match, they just chose resistors to set one side's bias for the tubes they were getting (Philips/Sylvania STR6L6GC, a really great tube) and changed the other side's bias (using the hum balance control) to match the idle at the point where hum was minimized.
Other amps have hum balance that actually is a pot across the filament supply, with the wiper grounded... you get to bias the chassis with filament hum, and attempt to cancel all sources of hum at once.
So the best thing to have is two pots... one for bias, and one for balance. Or you can have two pots, one for the right side, and one for the left... both ways work. Tubes made today don't idle where the STR's did... for optimal bias you need to be able to adjust it properly. See your local tube amp tech for more on this.
Your amp can hum for other reasons than bias... bad filter caps can cause hum in the preamp section, which then gets passed on to the final amp. Hum in the power to the final section is normally cancelled, since the two sides are out of phase... a balance control tunes this, but if the preamp is humming you won't tune out all the hum with final balance. Tubes can have filament hum leak into the signal due to gassy condition or filaments shorting or bent... good preamps use DC for the filaments to try to stop this. You can try different preamp tubes in slot 1 (the 'gain hole', nearest the input) and see the difference in signal, noise, and hum dramatically.
Big hum can be a catastrophic symptom... when one of your final tubes shorts out, you will hear a great deal of hum until the fuse blows. If a final loses bias, the amp will hum loudly (and the tube will 'cherry up' and turn an angry red before it melts). If there's a 20A fuse in there, you'll hear lots of hum until the smoke makes you forget the hum. Caps can short too, but normally the fuse blows (or the smoke happens) much more quickly. Shorted or blown rectifiers can cause hum.
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Richard Keller
From:
Deer Creek, Illinois, USA
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Posted 24 Dec 2014 11:59 am
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I still think it could be the hum balance pot, just turn your master volume up until you hear the hum and then reach in there and put a screwdriver into the hole for the hum balance and turn it back and forth until The hum goes away. See the picture for the hum balance pot. It looks like an input jack.
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Mike Bowles
From:
Princeton, West Virginia, USA
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Posted 24 Dec 2014 1:09 pm twin
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thanks guys i better find a tech this is way over my head
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Mike Bowles |
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Mike Bowles
From:
Princeton, West Virginia, USA
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Posted 1 Jan 2015 10:39 am twin
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teven i pulled the 1st small tube looking from the back of the amp and the humming stopped put it back in and it started again any suggestions? thanks
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Mike Bowles |
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Mike Bowles
From:
Princeton, West Virginia, USA
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Posted 1 Jan 2015 10:49 am twin
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also the big tubes dont take very long to get real hot they will burn you.
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Mike Bowles |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 1 Jan 2015 11:54 am Re: twin
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| Mike Bowles wrote: |
| [S]teven i pulled the 1st small tube looking from the back of the amp and the humming stopped put it back in and it started again any suggestions? thanks |
Normally that tube (right-most, from the rear) works for the Normal channel... you should be able to play without it in place... this also gives your amp a little more punch and clean headroom, since the preamp supply voltage goes up a little.
Now if you pulled the *left-most* small tube, you pulled the phase inverter, V6A/B... this will completely kill the amp's sound, you would only hear hum if you had a really bad problem in the final tubes' circuit (V7,8,9,10).
Here's the link to the schematic:
http://schematicheaven.net/fenderamps/twin_reverb_sf_100_schem.pdf
You can see that V1A/B do the work for the Normal (left-most) channel... if you don't use this channel you can leave the tube out.
You can swap the two channels' tubes and see if the problem follows the tube or not. After this it gets deep and you'll need the help of a tech. The tubes are numbered from the right to the left, looking in the back. You start pulling from V1 to isolate problems... you can leave the power on, just go to Standby when replacing tubes (use a rag, they're pretty warm). Wiggle tubes in a rotary (swirling) way to loosen them when pulling... carefully align the 'gap' when replacing.
The final tubes are supposed to get hot... up to a point, the hotter the better. If you see the metal case inside the tube get red (the plate) then you are in danger territory... this is the kind of thing you can have happen by twiddling the 'hum balance' control, which is actually jacking with the bias. I just noticed that this amp goes back to a bias tremolo... very interesting... looks like the wiper and tap on that pot are reversed IMO, more research needed here. Bias tremolo with a roach!
PS the 'roach' is the ellipse thing with the CdS photocell and the neon lamp.
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Mike Bowles
From:
Princeton, West Virginia, USA
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Posted 1 Jan 2015 12:25 pm twin
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thanks steven way over my head tech bound
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Mike Bowles |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 1 Jan 2015 8:30 pm Re: twin
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| Mike Bowles wrote: |
| thanks steven way over my head tech bound |
Can you at least tell us which tube you pulled?
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New FB Page: Lap Steel Licks And Stuff: https://www.facebook.com/groups/195394851800329 |
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Mike Bowles
From:
Princeton, West Virginia, USA
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Posted 2 Jan 2015 11:10 am twin
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steven i pulled the first small tube looking in the back of the amp its a 12at7 i pulled the cover over the caps the red wire connected to one of the caps has been crushed have not had time to check it out yet i will let everyone know if i find anything the caps didnt look bad but i dont know
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Mike Bowles |
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