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Vox AC 50 Repair & Overhaul

Picture
This lovely Jennings Vox AC 50 from around 1967 came in with red plating power valves.  Several other faults were found, a previous modification being the main cause of trouble.  There was also a capacitor soldered to the wrong terminal! 

After researching the various different circuits it is back to it's original specification.  This model uses a mixture of fixed and cathode bias for the power valves.  However, someone had replaced the 47 Ohm cathode resistors with 1 Ohm ones, presumably to make it easier to set the fixed bias.  Unfortunately, this removed a good safety feature, so that if the bias is adjusted too high the valves can overheat when played at higher volume.

Most of the electrolytics were replaced, including the ones in the bias section, to ensure reliability.  The amp was eventually adjusted to around 50% of maximum plate dissipation, which gives a good safety margin without affecting the tone.

These are seriously loud amps!   Whilst monitoring this one flat out for 15 minutes, with no sign of red plating, it measured 86 Watts into an 8 Ohm dummy load with a 1kHz signal!  That's from a pair of EL34 in mixed bias!

Another thing worth noting, is that the grounding system used here is much better than the rather random AC 30 mess.  It all makes for a much quieter amp in the background, which makes it great for recording.

A good year later and this very good regular customer says he really loves the amp and it still works flawlessly.
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  • Repairs & Servicing
    • Guitar Amp Service & Repair
    • Business Card
  • Filmosound
    • Filmosound Guitar Amps
    • Filmo New Style
    • Filmonator Prototype
    • Testimonials
  • Contact
  • Amps & Stuff
    • WEM / WATKINS >
      • Watkins Clubman
      • Watkins Westminster
      • Watkins Westminster Mk 9
      • WEM Control ER15 No. 1
      • WEM Control HR 30
      • WEM Dominator Mk3
      • Watkins Copicat Mark 2
      • Wem Watkins Copicat Mk 3
      • Wem ER100
      • Wem PA100
    • Baldwin Burns Amplifier
    • Carlsbro CS 60 PA
    • Elpico AC52
    • Leo Fender >
      • Fender Concert Repair
      • Fender Twin Reverb 1977 Silverface
      • Fender Twin Reverb 1972 Silverface
      • Fender Blues Deluxe
      • Fender Musicmaster Bass Amp
      • Musicman RP 112 Sixty-five
    • Gibson >
      • Gibson Mercury
      • Gibson Falcon GA19RVT
      • Lab Series >
        • Lab Series L2 Service and Repair
        • Lab Series L5 100W Combo (Gibson/Norlin)
    • Hiwatt Lead 50R Combo
    • Marshall >
      • Marshall Artiste
      • Marshall Superbass Mk2 100W Head and Cab Service Repair
    • Miscellaneous Amps >
      • Harmony H307x
      • Hotpup 5F1 Champ
      • Impact 60
      • Mystery Amp
      • RSC Bass Major
    • Orange >
      • Orange OR120 1970s Service and Repair
      • Orange TH 30 repair
    • Schaller KV 40
    • Selmer >
      • Selmer Constellation 20
      • Selmer Treble & Bass Mk 2
      • Selmer T&B 50R SV
    • Traynor YGL 3 Mark 3
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    • VOX >
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      • Vox AC 30 TB Restoration
      • Vox AC 50 Repair and Overhaul
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