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Silicon-Free Fixed Bias?

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  • #1

    Silicon-Free Fixed Bias?

    Hey all,

    Just a random silly question I got while going through my tube stash...

    I have lots of tubes with an extra "small-signal diode," as they are called. Was wondering, as bias supplies are usually well under 100V, and consume "minimal" current," any reason why one couldn't be wired up to have a fixed-bias supply? We never use more than half-wave rectification for that, anyway... Just jokingly thinking of an "EMP-proof" fixed-bias amp. Probably would only consider with low-powered amps, so I won't be toasting anything more than EL84s or 6V6s, and tiny transformers... Just a wacky idea I had.

    Thanks,

    Justin

    Edit: already seeing how this may not work, at least not in a multi-section tube... Or at all, for that matter. But, I'll take ideas, anyway!
    "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
    It would be a lot of work and overhead to replace a 10-cent diode! Voltage drop, limitations on in common tube elements, making sure the diode heats up before the output tubes(!) etc...

    Comment

    • #3
      "No silicon" bias in Tube Amps?

      Sure, google https://en.wikipedia.org/wiki/Cathode_bias

      Want to be even more traditional?
      just check this amazing new invention, the bias battery, published in Popular Science, 1929
      https://books.google.com.ar/books?id...attery&f=false
      Juan Manuel Fahey

      Comment

      • #4
        And there's always good old selenium .
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        ​

        Comment

        • #5
          Originally posted by g1 View Post
          And there's always good old selenium .
          And even gooder older back-bias. I have a 1953-ish Stromberg-Carlson PA amp here that's a back bias design. You can find similar designs in the back pages of your RCA vacuum tube handbook. Of course it has a somewhat limiting effect on the power you can achieve and may require some whomping big power resistors but it will give you the satisfaction of doing it "the old fashioned way" and maybe even some of that "old fashioned tone," which isn't a bad thing at all.
          This isn't the future I signed up for.

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          • #6
            You can skip not only the silicone stuff but all resistors and capacitors as well:

            A High Voltage No Resistor No Capacitor Power Supply

            Reasonably powerful SE No R No C Amplifier

            Comment

            • #7
              Gee whiz, way to take all the fun out of using all my extra oddball tubes!
              Duh, back bias would be nice... And, double duh, cathode bias...
              Okay, we can let this one die a slow death, unless y'all wanna keep having fun with it!

              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

              • #8
                I figure, any significant power, even a milliamp or two for a bias supply, being drawn thru those "small signal diodes" would likely be enough to infiltrate your audio in the same tube with inescapable buzz. Those diodes were more meant for radio reception not power. Of course, you can give it a try & report the results. I figure if it was a useful method someone would have done it by now - - - but sometimes surprise - - - nobody has.
                This isn't the future I signed up for.

                Comment

                • #9
                  Well, you know,I'm sure they were ALWAYS faithful to the tube data sheets! :P

                  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

                  • #10
                    OK Justin, I'll give you a straight answer.

                    Those little added diodes in some tubes wouldn;t probably be the best idea, since they usually share a cathode with something else. But there is no reason you can't use a tube rectifier for a bias supply, just like you can for B+. The little 6AL5 is a dual diode. In a 7-pin miniature socket. SHort little thing. has current ability for bias. A common application for them was as detectors in AM radio circuits.
                    https://frank.pocnet.net/sheets/093/6/6AL5.pdf

                    In the olden days, back when tubes were octal, like god intended, there was the 6H6. The 6AL5 more or less took its place. The metal 6H6 was super short, it looked more like a plug than a tube. I always thought they were cool. Ther are glass 6H6s, they are not so short, they just look like a squatty 6SN7.
                    6H6, Tube 6H6; Röhre 6H6 ID596, Double Diode

                    If you want to stick with 9 pin miniature sockets like your 12AX7, you could use one side of a 6T8. it is three diodes and a triode, but note there is only one diode not sharing cathodes.
                    6T8, Tube 6T8; Röhre 6T8 ID3910, Triple Diode-Triode

                    6BC7 is three independent diodes.
                    http://datasheets.hypertriton.com/6BC7.pdf

                    I am sure there are others.

                    And if you wanted to be creative, I'd wager you could strap a 12AT7 into diode mode and use that.
                    Education is what you're left with after you have forgotten what you have learned.

                    Comment

                    • #11
                      Yep! All thermionic devices are first diodes, then have some control of the diode current added with grids.

                      It's interesting to speculate that if we could use positron-based tubes for thermionic applications, we'd have perfect complementary thermionic devices.

                      Of course, that cheery possibility is somewhat tempered by the fact that the entire tube would have to be made from anti-matter to get thermionic production of positrons, so the whole device would have to be kept in a HARD vacuum, and connecting wires to the pins would be really dicey.
                      Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

                      Oh, wait! That sounds familiar, somehow.

                      Comment

                      • #12
                        Simple, just make the anti-matter tubes out of di-lithium crystals.
                        Education is what you're left with after you have forgotten what you have learned.

                        Comment

                        • #13
                          Think about how such a tube diode would have to be connected. If you had a Fender type transformer with just a tap on the HV winding, the cathode of the diode would have to be connected to the tap and the DC output would be the plate. If you had a fully floating bias winding like some Marshall amps, the cathode of the diode could be grounded, one side of the winding connected to the plate and DC coming off the other side of the winding. If you grounded one side of the winding, it would just be the same as a grounded CT.
                          WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
                          REMEMBER: Everybody knows that smokin' ain't allowed in school !

                          Comment

                          • #14
                            As a side comment:if you are close enough to a Nuke blast to feel EMP effects, then you are in bigger trouble than a dead amp.
                            Juan Manuel Fahey

                            Comment

                            • #15
                              Well, I <WAS> born about 40 miles from Three Mile Island, on the same night it blew up... the hospital was under advisement for evacuations... So, I think I'll be okay! :P

                              No, seriously - I say EMP-proof jokingly. There's just some internet lore that is just TOO insane to believe. But I guess anything's believable... keep having fun - I'm enjoying the anti-matter theoretics!!!

                              Justin

                              Edit: I think it's entirely within the realm of possibility that there could be a way to have an EMP without the radioactive side effects...
                              "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

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