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Questions on a fender bassman

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  • #1

    Questions on a fender bassman

    This amp seems to be around 1970. The transformers code at 69, but the serial is in the middle of what is thought to be 70. Anyway;

    It mostly matches to AB165. Some things seem like AA270. But these following 2 things are not on either.

    1. From pin 8 (6l6) to pin 8 (other 6l6) there is a non polar electrolytic 5uf 25v. It looks the same as the 25uf 25v Mallory cathode caps. (except is non polar).

    2. There are 2 wire wound 1.5k 7w resistors. From pin 8 (6l6) to ground. one on each 6l6 tubes.

    Original?
    The caps look original. The resistors look kinda old, but maybe not original. ??

    Thanks
    Tags: None
  • #2
    Look for an AC568 schematic. I bet that will answer your questions.

    Yes, it is indeed original. And if I may make a personal request, please don't change a bunch if stuff just because "the internet says it sucks as is." Give it a very fair shake first; theresnot many of these left.

    Justin
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

    Comment

    • #3
      Originally posted by KCman View Post
      2. There are 2 wire wound 1.5k 7w resistors. From pin 8 (6l6) to ground. one on each 6l6 tubes.
      Originally posted by Justin Thomas View Post
      Look for an AC568 schematic. I bet that will answer your questions.
      So those are 150 ohm, not 1k5 then. Did you measure them?
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


      ​

      Comment

      • #4
        That looks to be the one! Thanks so much!!

        I thought I tested the resistors and I wrote them down from what was written on them. I will recheck. I was about to ask if they should be 150ohm now that I've seen the schematic. Beat me to it g1.

        I wouldn't change it Justin. I don't buy into the hype.. It appears to be all original (if those resistors aren't 1k5 ha!)

        The only thing that is definitely changed is the fuse holder. Looks like the other one fried. It's gone and burnt marks are in its place. Someone wired an inline holder, than put a 20 amp fuse in it!! That's fixed now I also took out the death cap and disconnected the ground switch.

        The problem was one of the 70uf went. I had a 100uf to test it and it sounds great!!

        I was gonna recap it, that's why I wanted to know about the 5uf between the cathodes.

        Thanks for the quick replies!?

        Comment

        • #5
          One more thing not on any schematics is the last cap in line in the doghouse is a 16uf instead of 8uf. Is that a matter of "hey, we have a bunch of these, they will work, let's get rid of em" or is it a version change? I'm going put a 16uf in. It sounds good.

          Comment

          • #6
            16uF is fine. Yes, most likely a purchasing decision.

            One thing about electrolytic caps: don't waste money getting an exactly 16uF cap if a 20 or 22 is a lot less money. Most of those older values are no longer common & so the prices on those old values are horrendous. I'd just get a 20 or whatever & be done with it. Tbe tolerance on the old part is probably way more than +/-20% anyway. They're far from precision components. Just watch your working voltage rating & your polarity.

            Don't forget the bias supply in your cap job, make sure it goes in + to ground, and don't be afraid to move the voltage to 100V. Good safety margin.

            Justin
            "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
            "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
            "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

            Comment

            • #7
              Note the 5uf cap is a non-polar type. The two 6L6s work at opposite phase, so instead of individual 5uf caps to ground from each cathode. one cap suffices.


              Yes, please don;t waste time and money looking for vintage value caps like 8uf and 16if. Modern 20uf caps are perfectly fine here.
              Education is what you're left with after you have forgotten what you have learned.

              Comment

              • #8
                Thanks for the knowledge and advise!

                I've always raised the cap value to what is popular on Electrolytics. I understand the tolerance and drifting of old caps.

                How critical is the 50uf (bias) ? I found a 50u 100v, just curious about raising that one.

                I've been reading the threads on here for a couple years. I feel I know some of you by now and have learned tons. I wish I had contributions. I just started repairing for the local store, instead of just for myself, so maybe someday.

                Oh, when I re-read this thread, I noticed I put "thanks for the quick replies!?"

                Question mark was a typo, but reads kinda funny. Like, THANKS! I think? Or maybe?

                Well anyway, I think I'm coming down with anal blindness, I just don't see my a** going to work today.

                Comment

                • #9
                  Did you get snow yesterday? Then take a 'snow day' for work?

                  100uF/100V is what I often raise the bias caps to.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  ​

                  Comment

                  • #10
                    Ok. I've seen that (100uf) just checking if that was alright.

                    Yes it snowed a few inches and People are ice fishing already. I'll wait a couple weeks though. I am self employed at all I do, just a joke!!

                    Have a great holiday!!

                    Oh yeah, they were 150 not 1k5. I need bifocals and think that 45 is too young!!
                    Instead, I use magnifier Google's!! Ha ha

                    Comment

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