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OLDAMPS
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Watkins Westminster Mk 9

Picture
Another faulty amp purchased for fun and my education.  This one was a squealer!  As soon as it warmed up it hummed and then screamed unbearably so you just had to shut it down.  These are a bit awkward to repair as you have to strip out everything including the speaker to remove the chassis.  To get to the solder side of the PCB, you have to strip it down even more.  After checking for obvious signs of trouble and trying different valves, I came to the conclusion the Mullard ECL82s were shot and there were some leaky coupling caps in the powers section.  I was not prepared to change one at a time, then reassemble each time to test it, so I went ahead and replaced five green polyester coupling caps, all rated at a mere 250V.  It seemed a lot happier with 550V equivalents.  However, one of my spare Pinnacle valves was running too hot.  I replaced the Mullards and was surprised to find they now worked, which confirmed the leaky coupling caps.  However one of them was also looking a bit hot.

I know others working on these have upped the 200 Ohm cathode resistor to stop the ECL82s burning up, so I substituted a 250 Ohm, then a 390 Ohm which seemed to keep the valves from melt-down.  I checked the schematic of the Mk 1 Wesminster and noted that it used a 390 Ohm shared cathode resistor.  Why did Wem use almost half the value in the later Mk 9?  Anyway, the Mullards are back in and sounding great now. 
Musicman RP 112 Sixty-five            JMI Vox AC 10
Elpico AC52                                     Carlsbro CS 60
Filmosound Amplifiers                      Hiwatt Lead 50R
Lab Series L2                                   Burns
Lab Series L5                                   Fender Twin Reverb
Selmer Constellation 20                   Fender Musicmaster Bass
Selmer Treble & Bass Mk 2              Impact 60
Schaller KV40                                   Northcourt 50
Fender 5F1 Champ                           RSC Bass Major
Traynor YGL 3 - Mark 3                     Mystery Amp
Watkins Westminster                         Denco 912-Plus
Denco 912-Plus                                 Interesting Waveforms
Wem Dominator Mk 3                        Mullard 5-10
Wem Watkins Copicat Mk 3               Decca Amp
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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
    • Tripletone Amp Conversion
    • VOX >
      • Vox AC 10 (JMI)
      • Vox AC 30 TB Restoration
      • Vox AC 50 Repair and Overhaul
  • Blog
  • For Sale
    • Amp Sales
    • Speaker Sales
  • Other
    • Marconi C10A radio
    • Interesting Waveforms