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Topic: Vibro-champ volume drop |
Hiro Keitora
From:
New York, New York
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Posted 14 Jun 2008 3:57 pm |
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Folks, I have a question here. I have this Black Face Fender Vibro champ, and everything works except, when I turn up the "intensity" knob for the tremolo, Over all volume level drops.
Is this the nature of the beast of this circuit?
Also, tremolo is not as deep as, say, Princeton Reverb. I didn't notice that much of volume drop on princeton Reverb when I turn up the intensity knob.
Is there any way to improve this?
Or it is what it is??
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Hiro Keitora
From:
New York, New York
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Posted 17 Jun 2008 4:34 pm
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Aw, C'mon, no one cares about Vibro-Champ!?
Help me on this, please!! 
<H> |
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Loren Claypool
From:
Mequon, WI
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Posted 18 Jun 2008 7:50 am
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I have a silverface VC. I don't experience the volume drop when I turn up the intensity of the tremolo. I don't understand what's happening in the circuit though so I can't comment on possible cause.
Hope this helps.
_________________
Loren Claypool
genre-indifferent instrumental guitar music
www.lorenclaypool.com
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Hiro Keitora
From:
New York, New York
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Posted 18 Jun 2008 1:57 pm
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Hey, thanks! Well, that's good to know.
That means, maybe, something's not quite right with this BF Vibro-champ.
I can't seem to locate any "tips" of how to take care of this problem...Perhaps, some faulty parts like faulty caps??
Can anyone who knows chime in?? Pleae?? |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 18 Jun 2008 6:42 pm
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Just saw this - been pretty busy here. It is not normal for the volume to significantly drop when the tremelo intensity pot is turned up on a Vibro Champ. It may well seem quieter because the volume oscillates, but it should not overall be way lower, IMO.
My caveat is that you should give this to a qualified technician familiar with high-voltage tube amp electronics unless you are thusly qualified. You can hurt or kill yourself and wreck the amp doing things improperly here. If you understand this kind of stuff and look at the very simple Vibro-Champ circuit diagram, you'll see a few obvious places where there can be problems with the tremelo in this circuit. You can get both a schematic and layout diagram here: http://www.ampwares.com/ffg/schematics.html If you don't understand this circuit diagram, you should give the amp to someone who does.
This tremelo circuit is connected to the cathode bias circuit of the second half of the main 12AX7 preamp tube, and is quite different than the tremelo in the BF/SF Fender Deluxe and larger, and even different than the Princeton Reverb tremelo, which modulates the fixed bias controlling the output tubes.
First, I assume you have subbed known, good 12AX7 preamp tubes. Do the easy and obvious things first. Next, I would look between the 68K resistor before the intensity knob and the 47 and 1500 ohm resistors and 10 uF cap. Leaky caps or seriously out-of-spec resistors could cause problems. Then, there could also be some kind of problem in the tremelo circuit itself - again, leaky caps or out-of-spec resistors could cause problems. There are not a lot of parts here. But you need to know how to assess these things - if you don't, again, get it to someone who does.
There are more subtle potential problems. You could, somehow, have a parasitic oscillation, which can cause a loss of volume. I have also seen a few amps with the dreaded conductive-fiberboard problem. This is not common, but when it happens, can drive you crazy. If you're not an experienced tech, you probably won't be able to figure this kind of stuff out.
Again - unless you're familiar with tube amp servicing procedures, you should leave this to someone who is. Just going in and replacing parts blindly is not a good idea, and will not necessarily fix the problem. |
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Hiro Keitora
From:
New York, New York
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Posted 19 Jun 2008 2:30 pm
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Well, Dave, thanks you so much for your imput.
I did check all the voltage and specs of indivisual resistors,and they are not too drifted. When I restored Blond Tremolux, for some reason, .03 cap before the intensity pot was causing the tremolo to work, but weak.
So you might be right on money about the doubt of that 10 uf. I'll find out.
You are absolutely right about replacing parts "blindly". Like old days, when we used to have to do "tune up" for a car, we used to change spark plugs, wires, Rotor cap, points, and the capacitor on the distributor in one shot. And if the engine didn't idle smoothly, then you don't know where to look! ( but it was usually silly mistake of misplacing wires in wrong firing order, V-8 was fun 
To be honest, I should just change all those caps anyway, but surprisingly, this little amp isn't humming at all, so I was hesitated, but what the heck, I can always save those 40 year old caps in Ziplock bag, but I don't think I'll put them back...
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Hiro Keitora
From:
New York, New York
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Posted 21 Jun 2008 9:04 am
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Yup, Thanks for the tip,Dave! it was 10ufcap that was causing the trouble. It now has much deeper tremolo too.
I must say Vibro-champ's tremolo is different from other bias modulating tremolo, it's more like, "Leslie" kind of effect I hear from it. Very cool.
Now, I isolated the problem, I can go ahead and re-cap all those old ones. I might as well do it now, so I don't have to deal with it for next 10 years.
Thanks everyone! |
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Loren Claypool
From:
Mequon, WI
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 21 Jun 2008 4:10 pm
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Glad it worked out. I love mine, which is a '68 silverface with the drip-panel trim around the grillcloth.
These are great amps, and I agree that the tremelo is good sounding. I use it for guitar mostly, but occasionally it is nice when playing steel in a strictly acoustic situation. It can even get above an acoustic b@njo without drowning everybody out. |
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Hiro Keitora
From:
New York, New York
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Posted 22 Jun 2008 8:41 am
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Yeah, yeah, so I'm now tinkering an idea, that I can probably squeeze 2 more tubes and a driver tranny in to hook up a short 3 spring pan to have a reverb!
Torres offers the kit, but I think he's dis engaging tremolo to have reverb. That's not cool... |
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Jim Sliff
From:
Lawndale California, USA
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Posted 22 Jun 2008 5:53 pm
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| Quote: |
| Yeah, yeah, so I'm now tinkering an idea, that I can probably squeeze 2 more tubes and a driver tranny in to hook up a short 3 spring pan to have a reverb! |
Well, it's *your* amp - but that's what those of us in the vintage amp tech/collector circuit call a "hack mod" - an irreversible "destructive" modification of a vintage amp.
Mods that simply change parts values or add/remove features *without* cutting, drilling or other non-reversible work are generally thought of as OK, but the second you hack an amp you destroy its vintage value and take one more working vintage collector's piece out of circulation, just as easily as tossing it in a dumpster. Torres' is widely known...and widely discussed....in the tech world (check the alt.guitar.amps newsgroup) as one, along with Fuchs, who not only isn't bothered by but encourages the practice.
Punching tube socket holes and bolting a reverb tranny to a VC is the same as routing the top of a '57 Strat to mount a humbucker...or bolting a light fixture to the top of your '39 Martin D-28. Not as significant financially, but exactly the same effect.
There ARE instruments/amps that aren't now and likely never will reach the collectible category and have not increased significantly in value over the years that irreversible mods sometimes simply improve the utility of, and most steels (excluding Bigsbys, Emmons Push-Pulls and very few others) fit that category...but hack mods on vintage tube amps - especially classic Marshall, Fender, Vox, Ampeg, etc. models...are considered by most a bad practice.
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 22 Jun 2008 7:35 pm
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I agree with Jim on doing irreversible mods on that nice old Vibro Champ. Honestly, it wouldn't be so tough or expensive to wire up a hot-rod Vibro Champ chassis. The real money would be in the chassis and transformers, and I don't think they're so expensive. Then you could have everything just the way you want it. The guys associated with my old guitar store are doing some amps like this and mounting them in a cab with a single 12" speaker - these things are just amazing.
Just another possibility. |
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Clyde Mattocks
From:
Kinston, North Carolina, USA
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Posted 22 Jun 2008 10:34 pm
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Dave brings up the subject of a conductive fiberboard. I started a thread on this a few months ago, prompted by an article by Craig Weber in Guitar
Player Magazine. While it does seem to be rare, it
is not unheard of and can manifest in some strange
symptoms.
_________________
LeGrande II, Nash. 112, Fender Twin Tone Master, Session 400, Harlow Dobro, R.Q.Jones Dobro |
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Hiro Keitora
From:
New York, New York
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Posted 23 Jun 2008 4:01 am
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Yeah, I heard of that conductive fiber board situation. There were some argument about Rails VS Board. Some says if you directly mount those tone caps on pots, you save a few feet of capacitive wire.
So the argument goes on.
Kendrick years ago was offering "add-on" spring reverb unit, I wonder whatever happen to that. |
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