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Bassman replacement OT Q...

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  • #1

    Bassman replacement OT Q...

    I figgered with all the OT questions floating around, I'd ask my own. I am weak in math, but grasp concepts. So, here's the story.

    For a long time I hunted for a 4ohm cab for my 50W Bassman head. I always knew the OT was a relacement from a Twin Reverb, nlt an100W "upgrade." Original Fender branded part from 1970, Schumacher. It didn't hit me til yesterday that I've probably been running the wrong speaker load for years! So if the Bassman would be a ~5600:4 ratio, then wouldn't the Twin Reverb be a ~2800:4 ratio, with two pair of || PP 6L6s? If it was a Super Twin with 6x6L6, wouldn't it be ~1900:4? I'm aware those may or may not be the ACTUAL specs on the OT, but I'll use idealized specs from the 6L6 data sheet.

    So as I figure it, to keep the same ratio with 2 6L6s to speaker load, should I be running 2ohms, ideally? Normally it wants more power tubes... Thing is, I'm glad it's oversized, cuz I ran it and still do on 8ohms sometimes, usually 4. Occasionally 2 if I feel like using 3 15s... So if anyone asks, I reckon Fender trannies are indeed quite robust!

    Thanks,

    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 -
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  • #2
    If the OT is from a 4 tube TR it would be best to run an 8R load to your Bassman. Increasing the speaker load increases the load on the tubes. And, as you've observed, you can fudge.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #3
      Thanks! Okay, now that I have the answer, maybe I'll be able to grasp the math... back to the books. Of course, I'm not going to buy a new 8R replacement speaker. Just won't run it with two 4R. I COULD just put in that proper tranny I have lying around, but I like the tone of the amp too much.

      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

      • #4
        I ran my Twin with two 6L6 pulled back in the days, Mesa have switch to cut the cathode of two tubes to run only a pair of 6L6. Nobody worry about it. Just don't run 2 ohm speaker load.

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        • #5
          Originally posted by Justin Thomas View Post
          Okay, now that I have the answer, maybe I'll be able to grasp the math... back to the books. Of course, I'm not going to buy a new 8R replacement speaker. Just won't run it with two 4R.
          Don't know if this will help, but I think of the OT in terms of a mirror reflecting back a ratio. You have a 2800:4 ratio from the twin. You are doing the equivalent of pulling a pair of tubes. The tube side I think of like resistors, pull 2 tubes you are doubling the impedance, add 2 tubes you are halving the impedance.
          So when you pulled 2 tubes, you doubled the source impedance, now you want to reflect (mirror) back double the load impedance so you increase the load from 4 to 8 ohms. This effectively doubles the OT's primary Z so it is again matched to the tubes.
          Your 2800:4 ratio could also be called/labelled as 5600:8, 11200:16, or 1400:2, it's all the same ratio of 700:1
          As you say, you don't want to change speakers so you will stick with the 1 step mismatch and avoid the 2 step. If you are not power hungry and could stand to shed some volume, you might consider a 4R load resistor in series with your speaker. It won't act like a speaker impedance, but you may or may not like it. Only your ears can tell you.
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


          ​

          Comment

          • #6
            Thanks g-one, it'll take a minute to digest... The analogy of pulling two tubes makes more sense - a better way to look at it. In all my adventures and years of mismatching impedqnce, I've never noticed enough of a difference volume or tonewise that I really cared. When I got it I actually only had an 8R cab, so I made up a dummy box to put another 8R load in parallel, to run it at what I thought was correct. Playrd it loud, nothing ever cooked. There's a reason I gravitate towards oldies, I guess...

            The nice part is, IF I ever burn it, a former customer gave me the exact same one, and I have a new correct one on the shelf that I got when my favorite music shop closed. Trannies for a lifetime.

            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
              Originally posted by Justin Thomas View Post
              When I got it I actually only had an 8R cab, so I made up a dummy box to put another 8R load in parallel, to run it at what I thought was correct. Playrd it loud, nothing ever cooked. There's a reason I gravitate towards oldies, I guess...
              Actually if you have an 8 ohm cabinet, that would be perfect. You have an OT from the Twin that is about 2K:4ohm, if you use an 8 ohm cabinet, the transformer will become 4K:8 ohm. That bring the primary impedance close to the optimal impedance for two 6L6!!! Remove the parallel resistor and you are home free.

              Comment

              • #8
                It sounds optimal no matter what I plug into it... It's a BF Bassman. Sounds a hell of a lot better than the only other BF Bassman I ever played; I think I enjoy the SF heads better. Mine is an AB165 when they started getting "funky." I love em. But a blondie would be prety sweet just for the looks.

                And I've run it @ 2, 4, & 8 ohms. It doesn't seem to care. Amd if it doesn't care, I don't care as long as it's loud and crunchy.

                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

                • #9
                  Remember that most of the old vintage Fender's with "ext spkr" jacks had no impedance switching. When you added an ext. cab, you ended up with a mismatch. They didn't care either .
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  ​

                  Comment

                  • #10
                    Yep, if it is not broken, Don't fix it!!! They are one of the most reliable amps around. Even mismatch and all, the power tubes might last slightly shorter!!!

                    I just got myself a 73 SF Pro Reverb a year ago and loving it. Think of it this way, the expensive Tone Master or Vibro King is no better, just newer......and they cost $3K!!!!

                    I for one like the SF better than BF. I actually took the chassis out, put a few switches that I can switch between BF and SF with just one power down. So I can A/B compare with everything else being truly equal. I like the SF better!!! The only thing I follow the BF is I remove the two 2000p cap from the plate of the two power tubes. I find the amp gain a little more sparkle without it. If it does not oscillate, I am happy.

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