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Bias - Class AB or Class A?

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  • Bias - Class AB or Class A?

    Hi,
    I'm repairing/renovating an old tube monaural Hi-Fi amplifier for a friend. It's a low powered amp with 2x 6V6 power tubes, a 5Y3 rectifier tube (and a couple of 12AX7 preamp tubes) and is cathode biased. The amp was not working when my friend gave it to me to fix. The cathode bias resistor on the power tubes is toast and unreadable and unmeasurable.
    I cannot find any documentation of any sort on this 1958 manufactured unit and the company has declined to provide any info, documentation and/or a schematic. But OTOH, it's a straightforward power amp and power supply, like many I've seen before. My question: How can I tell if the manufacturer intended this amp to run in Class AB or in Class A. It'll certainly affect how I choose a new cathode bias resistor and how many mAs the power tubes will be pulling.

    Thanks for your expertise,

    Bob M.

  • #2
    you need to figure out the primary impedance of the OT,and the plate and screen grid voltages coming out of the supply,then it's possible to do some math or use a loadline calculator,there are some online

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    • #3
      I've heard of brave souls getting a ballpark value for burnt out resistors by carefully removing part of the outer coating to expose the carbon / wire spiral. You then put your ohm meter between the lead and the spiral and find the highest resistance ( not open) spot. Repeat from the other end and add the two results. Won't work for carbon composition resistors.

      Also it might help if you can mention the make and model. Someone might have one or seen one.
      Experience is something you get, just after you really needed it.

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      • #4
        Full class A in PP amp is very unlikely and has no advantage over well biased class AB. Typically cathode biased amps run in class AB even if biased at 100% idle dissipation. Does the amp have a cathode bypass cap? If yes, that would indicate class AB. True class A PP amps don't need a bypass cap.

        As biasing above 100% plate dissipation is not advisable I would bias it just below 100%.
        - Own Opinions Only -

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        • #5
          And really, it doesn't matter what class it becomes, all you really want to know is a reasonable value for a cathode resistor.

          And I agree, if we knew what the heck make and model we were talking about, it would help.

          SAMS Photofact might well cover it.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            Hi all,
            And thanks for the replies. The amp in question is a Realistic AF-12 monaural Hi-Fi amplifier, 12 watts, built in 1958. The cathode bias resistor (pin 8) just disintegrated into dust the first time I touched it. I wasn't kidding - unreadable, unmeasurable. Yes, it had a cathode bypass cap (but blown). After repairing the power supply and testing the tubes, I jumpered wired in a few cathode resistors to see what I might have in the way of numbers here and the usual suspects yielded pretty large mAs. A cathode bias resistor of 250 ohms gave me a cathode bias voltage of 20.8Vdc, mAs - 41.5 - B+ 347Vdc, Plate 341, Screen 336. I thought that seemed too high. The tubes were throwing off alot of heat but still not red-plating. I tried a (combination) resistor of 343 ohms and of course my numbers changed to: Bias - 23.7Vdc, mAs - 34.0 - B+ - 361 Vdc, Plate - 355, Screen - 351.

            I read a few things and consulted some bias charts for 6V6 tubes that said for 355 Vdc plate voltage, I should probably be running these tubes around 24~25 mA per tube but then I started thinking if this were designed to run in Class A it would be higher, more like 90% dissipation. So, I thought I might get some opinions here.

            My intuition would be to run the amp with a bigger value cathode resistor , like a 470 ohm, to drop the mAs into a more reasonable range for a long-lived amp, putting out less heat.

            I won't solder in any parts until I'm happy with how this amp is running and numbering out.

            Bob M.

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            • #7
              Originally posted by Bob M. View Post
              My intuition would be to run the amp with a bigger value cathode resistor , like a 470 ohm, to drop the mAs into a more reasonable range for a long-lived amp, putting out less heat.
              470 . . . You could do that, at the risk of having such a low bias current that you'll have severe crossover distortion. If it's for guitar, and you like a gritty sound that never cleans up then go ahead. My seat-of-the-pants guess (intuition if you will) says 250 to 330 ohms, right where you've been experimenting. Is there a bypass cap across this cathode resistor? If not, you can add one, and that will help squeeze out a couple more clean watts. Anywhere from 10 to 50 uF should do, and I'd recommend a 50V rating at minimum.

              New JJ 6V6 will handle the plate power no problem. If you have old RCA, Sylvania, GE I'm sure they'll do fine as well. This kind of design, whether in guitar, PA, or hi fi amps, usually ran output tubes above their plate power ratings by 20%, sometimes more.
              Enjoy. Every. Sandwich.

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              • #8
                First of all: The 70% max dissipation rule only applies/ makes sense with fixed bias.

                Cathode biased amps can be and often are biased at around 100% plate dissipation at idle. This is possible because with cathode bias (other than with fixed bias) total plate dissipation continuously decreases with increasing signal.
                If cathode biased tubes don't replate at idle, they won't redplate at any output. Reason is that the increasing cathode voltage actually makes them run cooler at higher output levels.

                So a 250R cathode resistor will be safe, resulting in 13.3W plate dissipation for a 14W (design-max) tube.

                E.g. in an AC30 the EL84 tubes typically run at 120% plate issipation at idle. Still it's class AB.
                Last edited by Helmholtz; 05-05-2020, 09:04 PM.
                - Own Opinions Only -

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                • #9
                  What’s the wall and heater Vac?
                  My band:- http://www.youtube.com/user/RedwingBand

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                  • #10
                    Sounds like something you would eat in a pub over there. "I'll have the wall and heater, please. And a pint to wash it down."
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #11
                      If it helps, the Sams number is 503-16. I don't have access to Sams any more, but if someone does......
                      "Yeah, well, you know, that's just, like, your opinion, man."

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                      • #12
                        Cool, I had a feeling SAMS would have it.
                        Education is what you're left with after you have forgotten what you have learned.

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                        • #13
                          Thanks for the good discussion. It's the rare time when I don't learn something here. Dude, thanks for that SAMS number; I'd forgotten about them lately.

                          Yes, there is a cathode bias bypass cap and I've replaced the non-working one with a new 22uf@50V.

                          The house electricity here is normally between 120 and 122 VAC where I live and today it's at 121.1 VAC. The heaters are just at 6.3VAC and 3.15VAC each leg of the heater string and the rectifier has its own 5V winding, as you would expect to see.

                          Yes, I thought 250 ohms to 300 ohms for the power tube cathode bias resistor until I saw the mAs were over 40 per tube. I guess I'm so much of a fixed bias guy that those numbers got me wondering. I don't think I have a cathode biased amp (except for a couple of Champs I have around). I guess I've built a few over the years (like tweed Deluxes) and I probably kept it pretty close to a stock circuit. Anyway, I'll stick with convention and your sage recommendations and keep a close eye on the plates of those 6V6 tubes (for red-plating) and I should be good to go. Thanks for the discussion.

                          Bob M.

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