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Sano 500R tube conversion (8417 to EL34 or ???)

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  • #1

    Sano 500R tube conversion (8417 to EL34 or ???)

    I've got this as a project amp. Had EL-34s in it when I got it, I assumed that they had done a conversion from the original 8417s. NOPE! They just stuck them in and let them redplate. Put my bias probe and nearly fainted as the current shot up to something like 150mA for one tube. Plate voltage is supposed to be 500V but don't know how much it came down. Anyway, figured I should probably do something about it. This is cathode biased and had a 125 ohm 10 watt resistor shared between the two output tubes. Put in a 350 ohm resistor, that got the current down to about 95mA. Put in a 470 ohm resistor (highest 10 watter I have currently) and the current is down to 65mA. Still too high but less heart-attack inducing. The cathodes also share a 40uF cap so I can't just separate the tubes and give each their own resistor.

    The question is should I try higher resistor values or is this just chasing my tail? I read somewhere about issues with cathode biasing at high plate voltages.

    Would there be any advantage to go with something more like a 6550?

    BTW, I have no idea what the plate or screen voltages are during operation. Because the power amp gets the mains power from an umbilical from the preamp chassis it is going to pretty unwieldy to arrange it for probing.

    This schematic is supposed to be pretty close as far as the power amp, but it shows 200 ohm for the cathode resistor instead of the 125 mine had.

    TIA,
    Greg

    SANO-schematic.pdf
    Attached Files
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  • #2
    The cathodes also share a 40uF cap so I can't just separate the tubes and give each their own resistor.
    Why can't you? Is two caps beyond your budget?

    I have no idea what the plate or screen voltages are during operation
    Pull the power tubes, turn it on, what does the plate voltage rise to unloaded?
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Looking at your schematic, it seems they expect 23v across a 200 ohm cathode resistor, and I get 115ma for that, or 58ma per tube.

      I also find it interesting they show 8417 for a 50 watt amp. A pair of 8417 is capable of 100 watts a pair.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        Originally posted by Enzo View Post

        Why can't you? Is two caps beyond your budget?
        No, but since the 40uF is part of a multi-cap can it requires a little extra rerouting. That cap shouldn't need a high voltage rating, so probably not a big deal to put a couple electrolytics inside the chassis. I'm sure I could also find a higher value resistor, the question was more if that the best way to approach it.

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        • #5
          IMHE there aren't many el34 tubes that will handle 500Vp when clipping made today. Anomalous circuit particulars that affect impedance at frequency relative to transformer specs will often cook the screens on current el34 offerings in most amp designs. I've had my best luck with the Ruby branded el34B tubes. Though they tend toward microphony in combo amps they are the most voltage tolerant available I think. And...

          Remember that if you're cathode biased you can subtract the cathode voltage from the plate voltage when calculating total current. Subtracting screen current not considered here.

          If you want to stick with el34's I think it can be made to work with the path you're taking (ie: bigger cathode resistor) and it wouldn't hurt to add 1k individual screen resistors. With no choke mitigating HF spikes on the screens you'll need to bring that down for modern manufactured el34's IMHE.

          Sorry, I don't know enough about current 6550 offering to comment on that option.
          "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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          • #6
            Originally posted by Enzo View Post


            Pull the power tubes, turn it on, what does the plate voltage rise to unloaded?
            480V unloaded

            Regarding using 8417s for a 50 watt amp, since this was an accordian amp I wonder if they were marketing it as 50 watts super clean.
            Last edited by glebert; 07-24-2022, 04:04 AM.

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            • #7
              Still seems like overkill.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                Excessive "idle" currents could be due to oscillation.
                Pull the PI tube and check again.
                - Own Opinions Only -
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                • #9
                  Originally posted by Helmholtz View Post
                  Excessive "idle" currents could be due to oscillation.
                  Pull the PI tube and check again.
                  Did this and the current dropped down to almost nothing. I'm like "yeah for Helmholtz" but also "dang, now I have to undo my rework AND fix an oscillation issue." Before I did that though, I tried a different set of tubes, in case the ones in it were cooked out. Put in some fresh-ish JJs and got 32mA on one and 38 on the other. Put the PI tube back in and current stayed exactly the same. : Headscratch:

                  OK, so assuming that the power amp can now operate without eating tubes, I have a question on driving them. Since the transconductance of the 8417s is about 2x any current tube can (and should) I consider replacing the 12AU7s with something with higher gain?

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                  • #10
                    Pretty sure I still have a few pairs of 8417 in stock...
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #11
                      Originally posted by Enzo View Post
                      Pretty sure I still have a few pairs of 8417 in stock...
                      NOW you mention this

                      Seriously, those are worth decent money.

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                      • #12
                        Yes, they are. I no longer will be using them or any of my other parts, so market rates, but available.
                        Education is what you're left with after you have forgotten what you have learned.

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                        • #13
                          So no more oscillation with the fresh EL34s?

                          Originally posted by glebert View Post
                          Since the transconductance of the 8417s is about 2x any current tube can (and should) I consider replacing the 12AU7s with something with higher gain?
                          Typically instability gets worse with higher loop gain.

                          But EL34s really should have screen resistors. Try 2.2k at each socket.


                          - Own Opinions Only -

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                          • #14
                            Originally posted by Helmholtz View Post
                            So no more oscillation with the fresh EL34s?

                            That's what it seems like, but if that was oscillation it is different than I have seen in other places. Audio still came out clear and strong. I know that is possible with HF oscillation but with the tubes running as hot as they were it seems strange.

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                            • #15
                              Do you have a scope?
                              - Own Opinions Only -

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