Silverface Fender Bassman 100

A Bassman 100 on the bench.

Missing PT (burnt, I suppose …)

bassman_100_schem

Thanks to www.wallybill.com for the layout, thanks to Chris A. for pointing out an error


This model of Bassman was made from 1972 to 1976. This one seems to be from 1975 (Schumacher date code 606-5 … on the OT) :


OK, once the new PT is in place (a Hammond 290FEX) and the four 6L6GC changed, time to rebias this thing. On this model, there is a balance bias pot and not a “real” pot to set the bias. Balance bias is easy : the minimum hum, the better !

Unfortunately, this does not allow to set the current bias, and on this Bassman, the current was as low as 18mA (for 440V to the plate). So I replace the 15k resistor by a 1k “security” resistor and a variable resistor (a 50k I had, but a 20k should be even better) :

Before :

After :

Hence, I can set balance AND bias !

I replaced the 40 years old Mallory caps (I keep them, maybe someday I will try to reform them ?)

I noticed a wire going … nowhere ! Soldered at one end, cut at the other end ! A previous mod ? A “factory” mistake ? Anyway, I removed it (and the amp works well after having serviced it)

New caps …

Another small issue : a very nice blue light in a 7025 tube :

Air inside (and the getter is half gone, btw) : little break in the envelope … and one of the triode does not work (bass channel does not make any sound) :

Didn’t have any 7025 left, so I used a JJ 12AX7 “V1″ (low noise)

Anyway, the baby can sing again !

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14 Responses to Silverface Fender Bassman 100

  1. Sander says:

    “I noticed a wire going … nowhere ! Soldered at one end, cut at the other end ! A previous mod ? A “factory” mistake ?”
    Noticed the same wire going nowhere in my ’77 fender PA100, so factory mistake

  2. Oneball says:

    Thanks for the pictures! My 1974 Bassman 100 has no “wire to nowhere.” Just replaced all 6L6GCs with a matched JJ quad and smoked one of the caps you replaced. I’ll be replacing all five.

    • t-geek says:

      Oops … for the cap !
      Let us know if everything is right after having new caps replaced !
      Thanks for the information about the weird wire …

  3. Chris A. says:

    In the schematic, which is very nice btw, a wire on valve 8 goes to pin 3 instead of pin 1. I know it’s from http://wallybill.com/Bassman100.html.

  4. Matt says:

    Hi, noticed on the above layout – the 6L6 screen grid connections on the two outermost sockets aren’t connected to the supply, just to each other (ie pin 6 to pin 6 to nowhere). Otherwise this has been a lifesaver! Thanks

    • t-geek says:

      Thanks Matt. I already corrected the layout (orange “error”), but you are right, his layout is very useful (thank to its author) but not perfect ;-)

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