1973 Fender Vibrosonic, inputs different sound/power level?

Discussion in 'Amp Central Station' started by saintgerty, Oct 4, 2018.

  1. saintgerty

    saintgerty TDPRI Member

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    Hi, ok, I just got myself a Fender Vibrosonic from 1973. All looks well and beautiful. Except for the fact.... that in the normal channel the sound is different, more laid out tonally than the vibrato channel (without the vibrato being switched on, no reverb, etc). I thought the channels were meant to sound identical... but with vibrato you could just add vibrato or reverb as you wished. But they actually sound different. Vibrato is brighter. (... the bright/ normal switch is in the same position of course for both channels)
    Is this a common occurance for vintage silverface amps? Or just mine....

    Also...The second input of the normal channel is lower in volume. I assume this is a mistake?

    And has anyone noticed that the bass is really boomy for these old vibrosonic reverbs? I definately have to get an attentuator, this beast was meant to be played loud... and anything not.... 'LOUD' doesnt really hit the sweet spot to my ears.

    Ok, thanks for reading, any suggestion much appreciated-
     
  2. Old Tele man

    Old Tele man Friend of Leo's

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    Look at the input circuits of the two channels (in schematic).
     
  3. LOSTVENTURE

    LOSTVENTURE Tele-Afflicted

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    First of all, yes the two channels sound different and deliver different volumes. It was designed that way.
    Second, if you have to crank it to get it to the sweet spot, you need to evaluate the performance of those speakers. If they are the originals, regardless of manufacturer, they are probably on their last legs.
     
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  4. Axis29

    Axis29 Poster Extraordinaire Ad Free Member

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    The second inputs on Fenders are normally 6 dB quieter. I would assume the same on the Vibrosonic?

    And yeah, the difference between the two channels, is a gain stage for the reverb recovery.
     
  5. schmee

    schmee Doctor of Teleocity Silver Supporter

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    The channels on Fenders from BF and SF do sound different. Not sure about the VS though. How different depends on the amp. But generally, the normal channel is thicker and smoother.... possibly quieter. The signal path is a lot shorter than the trem/reverb channel. Dead simple in fact. Here's a typical one for DR through ProRev with the reverb/trem removed:
    [​IMG]
     
  6. gridlock

    gridlock Friend of Leo's Silver Supporter

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    I wasn’t very familiar with a Vibrosonic. I guess that it is basically a Twin Reverb with one 15” speaker.

    Cool amp. Congrats.
     
  7. Paul in Colorado

    Paul in Colorado Telefied Ad Free Member

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    There line up then was:
    Twin Reverb (2 x 12)
    Dual Showman Reverb (head, usually with a 2 x 15 cab)
    Quad Reverb (4x12)
    Super Six Reverb (6 x 10)
    Vibrosonic Reverb (1 x 15)

    Same chassis, different speakers.
     
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  8. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

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    The vibrato channel on all vintage Fenders amps has one more gain stage than the Normal channel - that's the difference in basic clean tones. Without getting too technical, the #2 input in each channel has a resistorr that drops the signal to that channel's preamp - so it always has a lower level than the #1 input in the same channel.

    The differences between the channels themselves can be affected by the tubes and general condition, though.

    You need to find a qualified amp tech - asap.

    Looks mean nothing as far as the sound or health of the amp. If you don't know if the amp has had normal service done sometime in the last 10 years or so (with receipts to prove it - and you'd need to have someone explain them) it needs to go to a tech ASAP - no matter HOW good it sounds.

    Roughly 75-90% of the vintage Fender amps I see that have been recently purchased are WAY over due for normal service. A few parts - the electrolytic filter capacitors and bias capacitor in that model - have a service life of about 15 years. They can last longer, but fail without warning. If this happens there's a chance of blowing the power transformer - a $2-300 repair job *besides* the service that still would need to be done. And you lose quite a bit in vintage value.

    I tell everyone shopping for vintage tube amps to budget an extra $150-$400 (the range depends a bit on work needed but also allows for replacement of some tubes, and your amp has 4 power tubes) in addition to the price of the amp. The sad part is the number of players who are not aware of this, or who ignore it, and en up with ;;$250 in repairs plus another $300 in normal service AND lose several hundred dollars in value because the amp no longer has the original power transformer.

    So - if the amp has not been serviced in a while, or has been sitting for several years (VERY bad for electrolytic capacitors) or you just don't know (and no matter what the seller TOLD you or what it sounds like) IMO it should be taken to a qualified amp tech asap - like before playing it any longer - and ask that the "filter and bias cap dates be checked, and that they be replaced if over 10 years old or there are no date codes. Also perform a full checkup, and clean/lube pots and jacks".

    Some techs will only do what you request, so you need to be specific. I'm using 10 years in this case because if you are going to have the amp opened up, will you remember to do it again in 5 years or so if the caps are 10 years old?

    This work is typical for ALL tube amps. It's just like changing the oil or tires on a car.

    Good luck!

    PS - I also suggest the book "Fender Amps - the First Fifty Years" by Teagle & Sprung. It has detailed descriptions os the evolution of every Fender vintage tube amp model, and fairly easy to understand descriptions of how the electronics work.
     
    saintgerty likes this.
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