Question on 70's Fender Pro Reverb

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mcontour, Aug 22, 2005.

  1. mcontour

    mcontour Supporting Member

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    Hello All,

    I'd appreciate any help I can get here.

    This weekend I picked up a silver face Pro reverb. It has a master volume with push/pull. What is the function of the push/pull? I can't really tell any difference.

    Since the tubes are of unknown age/condition to me, I'm thinking of changing them out with some fresh ones. The back of the amp has some adjustments for tube bias and hum balance. What is the procedure to adjust these? I have a quality multimeter to use.

    Is there a place to get a manual (online preferred) for this amp?

    Many thanks.

    Michael
     
  2. Swarty

    Swarty Member

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    The hum balance balances the filaments and has nothing to do with the bias. The bias control is a bias balance circuit.
     
  3. John Phillips

    John Phillips Member

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    Is this a 70W model (it will say under the speaker jacks) ?

    If so, you really need to use NOS power tubes - they have a tendency to fry most modern types. 70W at the plate voltages these amps use (over 500V) is extremely hard on a pair of 6L6s. If you do want to use modern-production types, you may need it rebiased (an internal job, you need a resistor changed) and possibly the screen resistor values increasing, or other alterations - I've dropped the plate voltages on a few of these amps with large resistors in the power supply to make them less destructive on the tubes.

    Personally I'd fit NOS tubes anyway for tone reasons alone. Actually I wouldn't rush to change the tubes at all unless you're sure they're on the way out - if it has its original GE and Sylvania tubes (especially in the preamp) they may well be in perfect working order and better than modern replacements, even now. I've come across many Fenders from this period exactly like that.

    Assuming a reasonably well-matched set of tubes, you don't need any tools at all to adjust those controls on the back.

    First, turn the MV down to zero, and let the amp warm up for a few minutes. Turn the Output Tubes Matching control until you get the minimum hum from the speakers - it will (hopefully) be somewhere in the middle of the range, if it's right at one end the tubes are not well matched and you may not be able to reach the correct point.

    Then, turn all the knobs up full and adjust the Hum Balance until you get the least hum.

    That's it.


    The pull knob on the MV turns on a crude distortion circuit fed from the reverb drive. If it does absolutely nothing, check that the reverb drive tube hasn't failed (you'll get no reverb either) - these amps are also quite hard on this tube which is run very hot.

    You probably won't want to have it working though - it sounds horrible. To get much distortion you need to have the vibrato channel volume (and usually the midrange) most or all of the way up, and then it sounds harsh and scratchy.

    IMO it's much better to use these amps for what they do extremely well - a very loud, clear, bell-like clean tone.


    Welcome to the Gear Page! :)
     
  4. drbob1

    drbob1 Silver Supporting Member

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    IIRC this will be one of the "ultralinear" Fender models. That means that it becomes extremely complicated and pretty expensive to "blackface" the amp. It excells at a clean tone, like a slightly smaller Twin (not much, BF Twins ran at 90w) and takes pedals well. Let it be what it is and you'll be very happy.
     
  5. mcontour

    mcontour Supporting Member

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    OK, I took a look in the back.

    It does say 70W below the speaker jack. The tube chart inside the cabinet shows four 6L6's, but the amp only has two. I guess that means this chassis wasn't born in this cabinet.

    The two power tubes were different. One was a Sovtek. The other was a Ruby. They both had corroded leads. The only other tubes I had were mesa tubes, so I stuck those in and made the hum and balance adjustments described in an earlier post. The "best" settings were in the middle of the range.

    The amp is a whole lot brighter with the Mesa tubes. When I have time this weekend, I'll see what's hiding under the other metal tube covers.

    Thanks for the help, guys.

    Michael
     
  6. DoobieK

    DoobieK Member

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    Nice score. I had the same amp about 14 years ago and regardless of what anybody says, it sounded very nice. Never used the pull switch for distortion. THAT sounded horrible.

    For output tubes, you could use a pair of KT66 or 7581 tubes. They are beefed up, for lack of a better term, versions of 6L6 and would handle the higher voltages easy.

    The chassis may have originally been in that cab as Fender used the same tube layout for a lot of amps. The only difference is 4 vs. 2 output tubes. What is the model number stamped on it?

    These amps LOVE pedals too.
     
  7. mcontour

    mcontour Supporting Member

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    I'm not sure about the model number. On the back of the chassis it looks like a serial number engraged....A992607. Below that is an ink stamp of 9743.

    I checked out the rest of the tubes: four 7025/12AX7 Sovtek and two 12AT7 Jan Phillips ECG.

    In response to a previous entry in this thread....the reverb does work....almost too deep. Between 2 and 2-1/4, it goes from nothing to a very long echo. The tremolo works too, but nothing to write home about.

    As best as I can tell, the push/pull on the master volume makes no difference. Maybe something inside has been modified?

    Michael
     

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