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Cathode bypass caps in Bassman 135

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  • Cathode bypass caps in Bassman 135

    Hi all,

    After a little hiatus to learn the names of the strings on my guitar and occasionally playing them in the right order, I'm back on the iron.

    A friend of mine has asked me to fix the buzzing and popping on her Bassman 135. On opening it up, the first thing I notice is that the Mallory caps all have original 1979 date codes. So I have replacements on order.

    But something puzzles me. The cathode bypass caps are rated at 25v or 50v (on diagram and as built) , yet the voltage indicator (on diagram) at each of the cathodes is 1.9v. Knowing Leo's propensity for all things inexpensive (and that gain stage bias is rarely more than 2 or 3v) , why do these caps have such a high voltage rating?

    Many thanks

    Jeff
    It's not microphonic - it's undocumented reverb.

  • #2
    25v or 50v parts could be used in many locations and they were cheap in large quantities.
    "Stand back, I'm holding a calculator." - chinrest

    "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

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    • #3
      I'll just use the 16v ones I have in my parts box then. many thanks.
      It's not microphonic - it's undocumented reverb.

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      • #4
        Another reason that Leo used the 25V rated caps was likely due to availability. I scanned through some old parts catalogs from the 1950s and found that the 25 uF caps for sale then tended to start at around the 25 V rating. A few companies offered lower voltage ratings but the price was almost the same as the 25V line. For a quantity buy the 25uF / 25V part was probably a sweet spot for need, availability and price.
        Cheers,
        Tom

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