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  • #16
    Originally posted by Randall View Post
    plate = 135v, screen = 127v and cathode = 9.5v. Since these now all look good, would this point to a look back at the OT?
    Check the peak signal voltage at the grid first.
    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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    • #17
      OK. Here is what I see for voltages driving the input with 45mV as measured with a non true RMS Fluke:

      preamp

      grid - 0.125v p-p
      plate - 11v p-p

      output

      Grid - 11v p-p
      Plate - 100v p-p

      3.9 ohm load - 1.5v p-p
      It's weird, because it WAS working fine.....

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      • #18
        I sure could use some help with why this amp is producing such low output, cause I'm pretty in the weeds here. I'm back to suspecting the OT again, I wish I had one hanging around to try.
        It's weird, because it WAS working fine.....

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        • #19
          Have you tried the neon bulb tester on it? If you haven't built one it will come in handy for transformer testing:
          https://www.premierguitar.com/articl...ormer-tester-1
          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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          • #20
            That ^^ and/or try a substitute OT. Most hot chassis amps only turn out a watt or two anyway. A Champ OT, maybe even a reverb drive transformer clipped in as a substitute may pass a better wattage figure, then you know what you have to do. Also do you your power test at say 500 Hz on up as low frequencies drop precipitously with tiny iron.
            This isn't the future I signed up for.

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            • #21
              When I try the neon bulb test it does flash a bit, but not as much as a DRRI PT I have laying around. I suspect more iron means more flux and more neon power?

              Shame but I do not have another single ended transformer to clip in.

              I tested power at 1 KHz.
              It's weird, because it WAS working fine.....

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              • #22
                Originally posted by Randall View Post
                Plate - 100v p-p

                3.9 ohm load - 1.5v p-p
                suggests a primary impedance in the 17k Ohm range. Is this reasonable? Could the load actually be a 2R speaker?
                If it still won't get loud enough, it's probably broken. - Steve Conner
                If the thing works, stop fixing it. - Enzo
                We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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                • #23
                  Originally posted by eschertron View Post
                  suggests a primary impedance in the 17k Ohm range. Is this reasonable? Could the load actually be a 2R speaker?
                  Hmm, don't rule it out. I've seen 2 ohm speakers in some 50's Premier amps. When it comes to funky old amps, anything goes.
                  This isn't the future I signed up for.

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                  • #24
                    Originally posted by Randall View Post
                    When I try the neon bulb test it does flash a bit, but not as much as a DRRI PT I have laying around. I suspect more iron means more flux and more neon power?

                    Shame but I do not have another single ended transformer to clip in.

                    I tested power at 1 KHz.
                    Could you verify the 180 Ohm DCR of the OT primary, as shown in the schematic? Is the 1.5Vpp across the 3.9R load just below clipping?
                    - Own Opinions Only -

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                    • #25
                      "Could you verify the 180 Ohm DCR of the OT primary, as shown in the schematic? Is the 1.5Vpp across the 3.9R load just below clipping?"

                      The OT primary measures 265 ohms, the secondary is 0.3 ohms. The top of the wave is squashed a little with reference to the above measurement.

                      OK, I actually do have a single ended transformer in my shop, it is the reverb drive tranny from a Vibrolux Reverb. I subbed it and did indeed get a higher wattage measurement, but still it is not right. With the reverb tranny I get 0.5 watts, with the "original" I only get about 0.2 watts.
                      It's weird, because it WAS working fine.....

                      Comment


                      • #26
                        Originally posted by Randall View Post
                        OK, I actually do have a single ended transformer in my shop, it is the reverb drive tranny from a Vibrolux Reverb. I subbed it and did indeed get a higher wattage measurement, but still it is not right. With the reverb tranny I get 0.5 watts, with the "original" I only get about 0.2 watts.
                        That shows you're heading in the right direction. You could try a number of different load R's, see which one gives you max power, then that's your target speaker nominal ohms rating.

                        What a lot of trouble to go to for such a peewee amp. But it is somebody's pride and joy. Chalk up the hassle as a "learning experience."
                        This isn't the future I signed up for.

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                        • #27
                          Have we looked at the signal at the power tube grid? I mean is the tube being driven fully or is the signal there already weak?
                          Education is what you're left with after you have forgotten what you have learned.

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                          • #28
                            No, it's not the original speaker, it is in fact 8 ohm. But I am not testing with the speaker, I am testing with a 3.9R 5w resistor.

                            "Have we looked at the signal at the power tube grid? I mean is the tube being driven fully or is the signal there already weak?"

                            Yes, as previously stated:

                            OK. Here is what I see for voltages driving the input with 45mV as measured with a non true RMS Fluke:

                            preamp

                            grid - 0.125v p-p
                            plate - 11v p-p

                            output

                            Grid - 11v p-p
                            Plate - 100v p-p

                            3.9 ohm load - 1.5v p-p
                            It's weird, because it WAS working fine.....

                            Comment


                            • #29
                              You are NOT driving power tube to full power.
                              You pulled "drive it with 45mV at the input" out of the blue.
                              Rated power is measured at output reaching clipping, all you got so far is "The top of the wave is squashed a little with reference to the above measurement." which in the Guitar World means "still clean"
                              Increase drive signal to 100mV ... or even 150mV if necessary, so *bottom* peak visibly clips, let devil take care of the top one, and remeasure.
                              Please post a screen picture here.

                              Single ended amps, and doubly so *cheesy* ones, hardly clip symmetrical.

                              That said, you have 138V +V so maximum plate can swing down is to some 40 or 50V above ground, (tube saturation voltage) and high twice that, so maximum *possible* swing is some 200Vpp or so.

                              About twice what you are measuring so *potentially* you could have 4X the power you are measuring now.
                              In practice I doubt you can get more than 1W RMS.

                              Which is ample power for an AC/DC table top radio, your amp is not different from that.

                              Way back then it was more than enough for a Kansas or Oklahoma picker, relaxing at home after a hard day´s labour, or a kid trying to learn Rock licks he heard on the radio.
                              Juan Manuel Fahey

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                              • #30
                                Juan, thank you for joining in, I always learn something from you.

                                Here is the waveform driven at input with 1KHz at 120mV (measured with Fluke non true RMS), output is measured across a 3.9 ohm 5 watt resistor with mystery OT. Scope is set at 0.5v/div for approx. 2.5v p-p. My calculations put that at .20 watts. 2.5 v p-p / 2 = 1.25 v peak, 1.25v * .707 = 0.88 v rms, (0.88 * 0.88)/ 3.9 = 0.20 watts.

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                                It's weird, because it WAS working fine.....

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