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Circuit changes for different power transformer?

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  • #1

    Circuit changes for different power transformer?

    Hello,

    I want to build a variation of this low wattage amp with a Vox Normal channel.
    The kit includes a transformer with 200V secondary (30VA), but I want to use a bigger transformer with 250V (50VA), that I have lying around. What changes would I have to make to the circuit?

    Amp schematic: https://www.tube-town.net/cms/userfi.../low-molly.pdf
    PSU schematic: http://www.tube-town.net/info/datenb...s/psu-mini.pdf
    kit transformer: https://www.tube-town.net/ttstore/Tr...ml?language=en
    my transformer: https://www.tube-town.net/ttstore/Tr...50VA::527.html


    regards, Immo
    Tags: None
  • #2
    I would get the proper transformer and keep the one you have today for future use, that poor double triode output is already stressed to the max being forced to deliver *power* , a 250V transformer will go even further.

    Or choose another more suitable project.

    In any case, triodes make very poor *guitar* power amplifiers , they are bland and lack bite ; the "tube guitar sound" we all love **always** came from pentodes.
    Juan Manuel Fahey

    Comment

    • #3
      +1

      I installed a triode/pentode switch on exactly one amp, ever. I didn't like the tonal change in triode mode. This is probably why I never prioritized building a micro amp. I remain hopeful that when I do get around to that project that I won't be disappointed by the tone. There are a lot of guys building these things now. Including a couple of commercial offerings. But I've never heard anyone say "The tone is 'as good/better' than my pentode amp." More often it's "It sounds pretty good." or "It doesn't sound too bad." And that from DIYers that are building them. And you know how people love to love what they've built. So from that perspective "It sounds pretty good." is like damning with faint praise I've been considering a pair of 6ak6's instead even though I have a bunch of test good, used 12au7's from an organ pull.
      "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

      Comment

      • #4
        Tube-town.de has many of those small amps based on the ECC99, and they sound REALLY good. Much better than similar amps that use a 12xx7. I have heard some of them in reality, and many more in demo recordings.
        The point is: There is no better way to make a low wattage version of an AC30. Other amps, like the AC4, use a single ended power amp with an EL84. Not bad, but a quite different sound, esp. if overdriven.

        And why shouldn't the tubes stand the higher voltage? A "real" Vox uses these higher voltages, too, that's why I would think that the preamp and phase inverter will sound better with the bigger transformer - they will run at exactly the same voltage as in a vintage AC30 then. The ECC99 can handle higher voltage than the EL84, that shouldn't be a problem. tube-town.de uses the bigger transformer in their hifi-amp "Petite Aurie", but with 560R/100uF at the cathodes: https://www.tube-town.net/cms/userfi...urie-schem.pdf

        My question was: What would I have to change to bias the tubes so that the amp really sounds close to a "real" Vox? Maybe the cathode resistor for the ECC99 should be a bit bigger than 270R?

        Comment

        • #5
          I've seen a few projects involving PP EF86s, and maybe that would be more appropriate. There just comes a point where the whole micro-amp thing gets pointless. And it seems to me that the whole point of Triode/Pentode switching is to castrate an otherwise perfectly good amp. Like choosing your gladiator to fight for the empire, then cutting and arm and a leg off and still sending him out to battle. Look, just use a smaller pentode, and suck up the idea that "bedroom triode tube amp" just doesn't make any sense! Unless 80dB is the lower limit in your bedroom...

          As a more serious question, aren't a lot of these 12--7 output amps preamp distortion grinders, anyway? Though this particular one does not seem to be so...

          Chuck, re: "it sounds pretty good," that's what the intended buyer for my last build said about his new Kemper "amp" when put up against his Matchless. Damning praise, indeed. I'm not willing to settle for that

          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

          • #6
            Roseblood11,

            This particular amp seems a good bit different from most of the micro-amp offerings I've seen, so I'm willing to withhold judgement. I like the fact that it's a two-knobber - most of those tiny amps seem to be preamp-grinders.

            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
              The ecc99 sounds great for low power applications. Much better than the 1W AU/AT based amps I've heard.

              Comment

              • #8
                To be fair I've never compared the inter electrode capacitances of a triode operated pentode and a real, dedicated triode tube. This could be a big reason triode operated pentodes don't ring my bell. So there's that. Also, good tone is for each player or listener to judge on their own.

                As to the AC30 and it's plate voltage being compared to class A single ended el84 operation... Not the same thing at all since the AC30 isn't strictly a class A amp. It's really not. In my observation, a good bit before the AC30 reaches it's maximum power that design also starts bias shifting into class AB1. This is partly why the AC30 can have those voltages. If such a design were biased hot enough to remain in class A all the way into clipping it would have a meltdown. This is why AC30 voltages cannot be considered analogous to single ended el84 operation. Though in reality I think you COULD run a single ended el84 at some 330Vp without too much difficulty as long as you don't overdrive it so much as the AC30 can it's power tubes. JM2C on that.
                "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

                Comment

                • #9
                  Whatever bias you use, those will still be bland low impedance triodes driving a speaker.
                  deserving the "not bad" to "quite good" (for a triode) labels they usually get.
                  You donīt like single ended amps? Fine with me.
                  But if you want to get closer to AC30 sound, a pentode amp if I ever saw one, because it uses NO NFB, so the very high impedance drive offered by pentodes is fully developed, then at least use a couple of low power pentodes.
                  Hereīs a couple examples, not to consider the full project but just the output stage, drive it with any preamp and phase inverter you like:






                  You can search for NOS "table radio" or "B/W tube TV" power tubes which are cheap, available and of course, NOS, because "nobody wants them" .
                  Once you get them, proper biasing can be calculated, and all you need to add is 1 resistor and 1 capacitor to feed screens, nothing else, everything else stays the same.
                  Juan Manuel Fahey

                  Comment

                  • #10
                    Yes, yes, yes, but what's even smaller than a 6AQ5? Which I actually have a few of...

                    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

                    • #11
                      Originally posted by Justin Thomas View Post
                      Yes, yes, yes, but what's even smaller than a 6AQ5? Which I actually have a few of...

                      Justin
                      EL95/6DL5
                      nosaj
                      soldering stuff that's broken, breaking stuff that works, Yeah!

                      Comment

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