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Low Power from Cathode Biased 2x6V6 Magnatone

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  • Low Power from Cathode Biased 2x6V6 Magnatone

    Hey guys, how's it going around here? It's been awhile since I've been around. I've been playing music so much for the past few years I haven't spent much time working on amps. But I have an old Magnatone Troubador here that belongs to a good friend that I'm freshening up for him. It's a rather simple circuit, that uses a 6SJ7 pentode for the preamp, a 6N7 twin triode for a paraphrase style PI, and a pair of cathode biased 6V6's for power. When I got the amp it had the original cap can, plus a couple of 10uf individual were added to replace 2 of the original caps, but the main 20uf caps in the multi cap can were still being used. The power tubes were a mixed pair of old tubes that both tested weak. I found a new cap can at CE Distribution with 2 x 20Uf & a x 10uf sections, so I used that to replace all of the filter caps. I replaced all tubes with new, and gave it a general clean up, resoldered some questionable joints, cleaned up the tube sockets, jacks, etc. The amp soounds really good, but the power is really low, like around 4 - 5 watts output, and that's pushing it.

    You can view a schematic here, though it's hard to read, the resolution is real low: https://www.magnatoneamps.com/schema...one_M192-5.jpg

    Voltage readings are:
    B+ = 300
    Power tube plates = 299
    Screen grids = 298
    Kathode resistor (150 ohm) = 14.8

    Voltage readings at the output is abut 4.2vac at onset of distortion. Interesting, the voltage doesn't really change when I change the resistance of the load, from 4 to 8 to 16, the voltage remains about 4.2 volts. The speaker in the amp is an 8 ohm speaker, but it is not original, it's an old Ampro labeled speaker, must be out of an Ampro cabinet.

    What do you think? It looks like I have plenty of time to fiddle with this thing now, what with the Covid lockdown, I'm not going anywhere.

    Thanks for any help or advice you might have.

  • #2
    Here's a slightly better/tweaked schematic. Did you check or change the cathode bypass cap at the output tubes?

    Click image for larger version

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    "Yeah, well, you know, that's just, like, your opinion, man."

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    • #3
      Originally posted by The Dude View Post
      Here's a slightly better/tweaked schematic. Did you check or change the cathode bypass cap at the output tubes?

      [ATTACH=CONFIG]57430[/ATTACH]
      Replaced with new 25uf 25 volt electrolytic.

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      • #4
        Also note: since the field coil speaker is gone, it's using a 1k5 dropping resistor, between the 2 20uf caps. Maybe I should upload my drawing of that part of the schematic

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        • #5
          You could have a loss of gain before the power tubes. Do you have a scope? If so look at the power tube grids and see if you're clipping there. Are plate voltages good on your preamp and PI tubes? Possibly a bad plate resistor?
          "Yeah, well, you know, that's just, like, your opinion, man."

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          • #6
            I do have a scope. I'll check the power tube grids. Volts at the preamp & pi plates are high compared to the schematic you posted. That schematic does help. I can see the original power tube cathode resistor is 250 ohms, not 150. But that's not going to significantly change the power output. Still, the tubes are a little hot, so I might try changing that resistor. I'll see if I have one in my stash.

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            • #7
              Don't confuse loudness with power. The amp isn't as loud as you expect, but that may have nothing to do with the power tubes. Dude is on the same wavelength I think.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                Speaker efficiency?

                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 -

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                • #9
                  Actually, the volume isn't bad, it's the actual power measurements that's bothering me. The amp sounds fine to me, very nice actually.

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                  • #10
                    As the the OT center tap is connected after the 1k5 dropping resistor, available supply current and power are strongly limited. As a consequence B+ may drop to 150V at full power (please measure). Such low B+ will not allow for more than maybe 5W output.

                    I would connect the OT CT before the 1k5 resistor. If you want to reduce the increased B+, insert a dropping resistor of a few hundred Ohm between the two 20 reservoir caps and take B+ from the second cap.
                    Last edited by Helmholtz; 03-18-2020, 05:17 PM.
                    - Own Opinions Only -

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                    • #11
                      I'm sorry, I probably didn't describe the B+ and the filtering very well, or correctly. Here is a quick drawing I made of the current arrangement. This is how the amp came to, except with fresh caps & tubes. The voltages are with new caps/tubes. Mains voltage at my bench was 117 vac Click image for larger version

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                      • #12
                        Looks like the rectified voltage at the 5Y3 cathode is considerably lower than in the schematic.

                        - Is the 5Y3 good?
                        - What is AC voltage at the HT winding?
                        - What are heater voltages (for reference).
                        - What is B+ at the OT CT at output clipping?
                        - Own Opinions Only -

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                        • #13
                          Originally posted by The Dude View Post
                          You could have a loss of gain before the power tubes. Do you have a scope? If so look at the power tube grids and see if you're clipping there. Are plate voltages good on your preamp and PI tubes? Possibly a bad plate resistor?
                          Signal at the power tube grids is clean and smooth when the output starts to clip. Voltages at the girds at 6.46vac at one, and 4.84vac at the other. Slightly asymmetrical, but I don't think that's so unusual with a paraphrase PI is it?

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                          • #14
                            Originally posted by Helmholtz View Post
                            Looks like the rectified voltage at the 5Y3 cathode is considerably lower than in the schematic.

                            - Is the 5Y3 good?
                            - What is AC voltage at the HT winding?
                            - What are heater voltages (for reference).
                            - What is B+ at the OT CT at output clipping?
                            Tested with new 5Y3 and existing rectifier tube, no difference.
                            A/C voltage at the rectifier socket are 311/313
                            HEater voltage is 6.4vac at the pilot lamp
                            B+ voltage at the OT center tap at the start of clipping are essentially the same as at idle, maybe 1 volt higher. From that point on the voltage goes up as clipping increases.

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                            • #15
                              Someone please correct me if I'm wrong, but with a cathode voltage of 15VDC, I think you should be able to get around 10VAC at grids before clipping?

                              Originally posted by hasserl View Post
                              Signal at the power tube grids is clean and smooth when the output starts to clip. Voltages at the girds at 6.46vac at one, and 4.84vac at the other.
                              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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