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Vibrasonic Reverb


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8/10/1998 7:23 PM
Zippy
Vibrasonic Reverb
I have a customers' amp, that checks out in every respect to voltage, power and gain specs shown on the schematic, but has to be run at "10" to be loud. Won't really distort, will go to about 92 of rated 135 watts. The tests showed every measurement to be within 20%, at the low side, but still won't overdrive, even with Master volume down and gain all the way up? One tech I know says this is normal with current tube quality, owner says bull- he just bought the amp off the Net. Help!
 
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8/11/1998 10:21 AM
Mark Hammer
So why not just momentarily swap the tubes with something else and see if it makes a difference?  
 
Alternatively, what is driving it: hot output humbucker pickups, or more restrained lipsticks? Remember, gain equals loud and distorted only if the input signal is sufficient to make use of the available gain.
 
8/11/1998 6:41 PM
Zippy

To Mark: The guitar is the shop guitar, a Kramer with stock humbuckers. It's used to test twenty amps a week.  
I have swapped tubes, and replaced some cathode bypass capacitors, checked plate voltages, measured gain with my Scopemeter, and the amp checks out within the 20% tolerance in power, gain etc.  
The speaker is not stock, but I have subbed a Community PA cabinet and a Twin's 4 10's and the amp does not have the GAIN from guitar I would expect. It WILL go to full power if input and Master vol. are at 10.
 
8/11/1998 7:38 PM
Richie

Zippy: Your resistors may check out ok, but what is used in the driver circut? Some of the newer fenders[70s & 80s] have different resistors in the driver circut,[to get a clean sound] They just won't distort very much,and some have to have the controls turned up,bass,treb,mid,etc.Also look at the input resistors to see what they are using,and check these.If the 1 meg is off on the input it will effect the gain.What I'm saying is this amp may sound this way,you may need to change a few things in different circuts to make this amp come alive! The ones with the master volume usally don't sound very good. [Richie]
 
8/11/1998 8:56 PM
Zippy

Richie, you are correct- the amp is not designed to distort much, and the issue of modification has not come up. The owner simply believes that it should be "louder".  
I think it is performing as designed, as spec'd on the schematic.
 
8/12/1998 10:19 AM
Doc

Most of the Fender schematics have signal voltages marked at various places throughout the circuit, with a specified nominal input signal level. An audio voltmeter or scope is all that's needed to locate any trouble spots with respect to individual stage gains. Does the Vibrasonic schematic have signal voltages shown?
 
8/12/1998 7:08 PM
Zippy

Sure Doc, I have the schematic, and have done the ac voltmeter (I have a Fluke 99B)gain- testing, and the amp is about 20% off.  
I see the overall reduction in gain as a sign of age, but the amp, I believe is operating within it's design characteristics.  
I am more than willing to do an overhaul, but the customer may expect something more(or diferent) from the amp than it will deliver.  
I have received opinions from several techs that support my measurements, so I can proceed with the "customer satisfaction".  
Thanks for all who helped.
 

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