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  • Vintage amp running too hot...

    I've run into a snag with my Flot-A-Tone amp restoration. The 5U4G that came in it was a bit worn, according to my tube tester, and, when I installed a NOS 5U4G (not a GB), the B+ shot up about 50V.

    It's a 2 x 6L6 amp with a 200 Ohm common cathode resistor. With a fresh 5U4G, it's runn ing 387V on the plates and 339V on the screens (measured from cathode, not ground). Cathode voltage is 27V. That means I'm seeing about 26W plate dissipation on an amp that was apparently made to use metal 6L6s or 5881s.

    I'm guessing that part of it has to do with higher AC mains voltages. The high voltage secondary PT winding reads ~370-0-370VAC.

    This is one of my least favorite problems in older amp repair--how to tame an amp that's running too hot. Any advice would be appreciated :-)

    Would you play with the cathode resistor value? Try to bring down the B+ voltage somehow?

  • #2
    Since that's not really high voltage for 6l6's I would probably just tame it with the bias.
    "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, Chuck. That's what I'm going to try.

      Would you think about lowering the screen voltage at all? The voltage to the screens is dropped via a 7.5k resistor. Perhaps 10k?

      Oh, and the B+ voltage actually shot up 70 Volts, from 350 to 420V. I've never seen a 5U4 replacement make such a huge change in supply voltage before. Another known-good 5U4G gives me the same B+.
      Last edited by Rhodesplyr; 11-29-2011, 12:04 AM.

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      • #4
        Check the heater voltage.
        If it's hitting 7.0V, then it's time to build RG's bucker into it
        Vintage Voltage Adapter
        Pete.
        My band:- http://www.youtube.com/user/RedwingBand

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        • #5
          Originally posted by pdf64 View Post
          Check the heater voltage.
          If it's hitting 7.0V, then it's time to build RG's bucker into it
          Vintage Voltage Adapter
          Pete.
          I've been trying to make a habit out of ALWAYS checking heater voltage, B+, and main filter cap. voltage ratings now. Not just for old stuff. Worked on a solid state Eden that was less than 10 years old. At our line voltage (124-125V) it had 63VDC on the main filters which were 63VDC caps. So now we not only have to keep up with old caps, but one's that may be stressed due to designs that run things too close to their tolerance. Had another newish (Chinese) tube amp, heater voltage up near 7V, 6BQ5's running real hot due to high B+. Transformer clearly marked 115VAC. Why aren't modern transformers built for 125V?
          And those hot heater voltages also reduce tube life expectancy.
          Time for RG to start mass producing his vintage line adapter, not just for vintage stuff anymore.
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


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          • #6
            I ended up bumping the cathode resistor up to 270 Ohms from 200, and I made a couple of other minor tweaks to the circuit, like a shunt to the screen supply to lower the screen supply. It's now at 70% plate dissipation with SED 6L6GCs. I went back and forth between 250 and 270 Ohms and chose 270 by ear.

            I've heard rumors that Flot-A-Tone amps were basically built with whatever parts they had on hand and without a lot of thought given to the results. The stock 200 Ohm cathode resistor value comes right from the RCA 6L6 datasheet, but the screen voltage is a good bit higher than RCA specified, so the resulting bias current is also higher than specified. I don't think that the excessively hot bias is just the result of higher AC mains voltages in this case. This amp must have had quite an appetite for the old metal 6L6 output tubes.

            Of course, it's not the first circuit I've seen from that era that has 6L6/6L6G tubes idling at or above their maximum dissipation. Even the RCA datasheet shows 6L6s running at 56mA per tube with a 400V plate voltage.

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            • #7
              Originally posted by Rhodesplyr View Post
              I went back and forth between 250 and 270 Ohms and chose 270 by ear.
              How did you reduce the screens? Is that the resistor in the B+ rail you changed? If you liked the tone prior to the change you may recover some of that by reducing the value of the next resistor in the rail that feeds the preamp (to bring the preamp back up to a higher voltage as before the screen voltage reduction).

              How does it sound?
              "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


              • #8
                Originally posted by Chuck H View Post
                How did you reduce the screens? Is that the resistor in the B+ rail you changed? If you liked the tone prior to the change you may recover some of that by reducing the value of the next resistor in the rail that feeds the preamp (to bring the preamp back up to a higher voltage as before the screen voltage reduction).

                How does it sound?
                It uses a 7.5k wirewound resistor to drop the voltage to the screens (and another 15K wirewound to drop voltage to the preamp section). I calculated that ~50k to ground on the lower side of this resistor would be a good way to increase the current draw and, thus, the voltage drop, improving screen regulation at the same time. (It's how they did it in my Scott 299.) What's slightly odd about the original component choice is that they used a 10W 7.5k wirewound resistor that, with the stock circuit, only dissipates a third of a Watt. A 1W carbon comp resistor would have been fine.

                With a simple circuit, one 6SC7 gain stage into a 6SC7 paraphase inverter, it has a clear, responsive, punchy tone that moves smoothly and gradually into distortion if driven hard. The speaker appears to be a close copy of a Jensen P12N, slightly honky, but in a good way.

                I checked all my tweaks by ear to make sure that they didn't have a major effect on the amp's character. The supply to the preamp stages is still 315V, plenty for 6SC7 stages.

                I've clearly documented all my changes so that if anyone in the future wants to turn it back into a tube-destroyer, it can easily be done. As I wrote before, my overall impression of this amp is that it wasn't carefully thought-out to begin with.

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                • #9
                  Just for reference....a 5R4 will drop more voltage than a 5u4.
                  The farmer takes a wife, the barber takes a pole....

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                  • #10
                    Increasing the cathode resistor value doesn't always get you to where you want, because while it does result in decreased current flow thru the tube it also results in increased plate voltage, which tends to offset the reduction in current and total watts dissipated remain the same or close to it. Often you have to increase the value of the rk so high that it negatively effects the tone of the amp. At 270 that probably is not a problem, but your ears are the judge of that. 26 watts dissipation is probably not too hot for 6l6GC's run in cathode bias, as the bias shift that occurs when the amp is pushed will keep the tubes from exceeding max dissipation. Tweed Deluxes routinely run their power tubes at max dissipation at idle and it seems to have very little effect on tube life. Attempts to cool them down with larger cathode resistors just tends to ruin the tone, IMO of course.

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                    • #11
                      Originally posted by hasserl View Post
                      Increasing the cathode resistor value doesn't always get you to where you want, because while it does result in decreased current flow thru the tube it also results in increased plate voltage, which tends to offset the reduction in current and total watts dissipated remain the same or close to it. Often you have to increase the value of the rk so high that it negatively effects the tone of the amp. At 270 that probably is not a problem, but your ears are the judge of that. 26 watts dissipation is probably not too hot for 6l6GC's run in cathode bias, as the bias shift that occurs when the amp is pushed will keep the tubes from exceeding max dissipation. Tweed Deluxes routinely run their power tubes at max dissipation at idle and it seems to have very little effect on tube life. Attempts to cool them down with larger cathode resistors just tends to ruin the tone, IMO of course.
                      In this case, adjusting the bias current with the cathode resistor was effective. It did raise the plate voltage, but within acceptable limits. B+ supply to the OPT ends up being 440V. Before all tubes warm up, the B+ on this amp hits over 520V anyway, so I used series wired 350V capacitors with voltage divider resistors for the first stage. The screen supply shunt resistor I installed keeps the second stage from going over 450V at turn-on.

                      I wasn't so much worried about plate dissipation here as I was about something else I noticed that correlated with the high bias current: gradually increasing control grid emission, with several different 6L6/5881/6L6GC types, after a few minutes in operation. (To answer the knee-jerk challenge before it's issued, no, it's not the coupling capacitors. They were replaced *and* tested in-circuit to rule them out.) With cooler bias, which is still on the warm side, this simply doesn't happen, using the same tubes. I'd prefer not to send out a vintage amp that's a time-bomb for thermal runaway. Tone, to my ear, was not significantly affected. If you want a tube that's running hot, you can use a 5881 or a metal 6L6. The latter is apparently what this amp was designed for. The reissue Tung-Sol 5881s sound nice in this amp, too.

                      I've read elsewhere that not very much thought went into the design of Flot-A-Tone amp circuits, and, after working on this one, I tend to believe it. As it's turned out, it's an amp I'd definitely keep if the owner didn't want it back.

                      In terms of the Tweed Deluxes, those, AFAIK, are all 6V6 amps, correct? I know that 6V6s and 6L6s technically have the same maximum grid-leak resistor values on the datasheets, but my experience has suggested that 6L6s are slightly more prone to thermal runaway when run hot than 6V6s.

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                      • #12
                        Rhodesplyr, I tried to respond to your PM, but your storage is full and you can't get any more messages.
                        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.

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                        • #13
                          My favorite method is to use a diode stack, drops .7 volts for every diode stacked. OR a variac to drop the voltage...
                          The diode stack was used by many manufacturers of tube radio transmitters, and it works just fine.
                          But you are correct. The unit was designed for 110 volts. The transformer is just about a 4.5 to 1 ratio, and so, you are over-voltage.
                          The 6L6 tube has undergone several improvements since the amp was built, and so you have a case of over efficiency, requiring a recalculation of screen resistors and bias.

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