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Marshall 1959 head - OT vs. EL-34's

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  • Marshall 1959 head - OT vs. EL-34's

    I've got this 100 watt 70's Marshall head that was run into a bad speaker cabinet that took out every amp that was plugged into it. I have not seen the insides of this cabinet yet, so I'm not sure what's wrong with it. The Marshall ended up with a shorted power tube from that experience with the cabinet. I retubed and biased the amp with new JJ EL-34's. When I tried to measure the power output of the amp, I noticed that the 2 leftmost power tubes were glowing bright blue and the output waveform folded up a lot. I powered down the amp, swapped the tubes around, and reapplied power. With no input, and with a load resistor on the output, the same tubes glowed bright blue. So, is it possible that the OT is compromised and took those 2 tubes out? Or is it most likely the tubes were just bad?

  • #2
    Whats the plate voltage on each side of the output xfmr primary? Sounds to me like half the primary is open.
    The farmer takes a wife, the barber takes a pole....

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    • #3
      And pull the power tubes from the sockets and power up. Measure voltages. Is there full B+ on pins 3 AND 4 of EVERY power tube socket? Is there proper bias voltage on every pin 5? When old tubes die they often damage circuit resistors.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        Yeah, I checked all of the voltages at the power tube pins, everything looked fine. That's why I started to wonder about the OT. I'm going to dig into it tonight.

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        • #5
          I ran 3.1Vac into the 8 ohm tap of the OT secondary and only got 22.9Vac out of each half of the primary side of the OT. Isn't that really a low value for the voltage on the primary?

          I've only tried this once before on another amp and I thought I got around 300Vac at the primary.

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          • #6
            That OT should produce a load resistance of (about!) 1750 Ohms for 4 x EL34s.

            1750/8 = 218.75:1 impedance ratio on an 8 Ohm secondary

            the square root of 218.75 is ~ 14.8 (i.e. 14.8:1 turns ratio)

            So the voltage across the primary with 3.1VAC on the 8R secondary should be:

            3.1 x 14.8 = about 46VAC (which is 2 x 23 - i.e. what you got on each half of the primary)

            So unless I'm way wrong its probably somethin' other than the OT.
            Last edited by tubeswell; 11-21-2008, 09:01 AM.
            Building a better world (one tube amp at a time)

            "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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            • #7
              I powered down the amp, swapped the tubes around...the same tubes glowed bright blue
              Have you tried different tubes? I know the blue does not necessarily indicate a bad tube, but when you relocate the tubes and still have the same problem the first thing I think is possibly bad tubes.

              Just a guess, these guys have all probably forgotten more about amps than I'll ever know, but that's the first thing I thought...Kind of like me with computers, I've kicked myself a dozen times for overcomplicating things and overlooking something simple, which eventually turned out to be the culprit.
              Why do I drive way out here to view the wildlife when all the animals live in town?

              My Photography - http://billy-griffis-jr.artistwebsites.com/

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              • #8
                You can also just meaure the primary resistance & get somewhat of an idea of it's static condition. What you would be looking for is a desparity (like >50 ohms) from the centertap to either end that go to the plates. some OT's do have a difference of 20ohms or so & work perfectly fine as it's impedance that matters for proper operation. It's not the most definitive method, but easy enough.

                Also obviously if you had another OT laying around, you could sub it in for a temp test. I'd think the primary impedance would have to be very far off for it to cause blue tubes when idling. glen

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                • #9
                  I suspect the blue glowing tubes are the good ones, and the two not glowing are doing nothing. Check the screen resistors on those two sockets. I know you measured the voltages, but maybe the crispy remains of the resistor still conduct enough to give a reading on a DMM.

                  (When EL34s melt down and short from a loose speaker connection, they can take the screen resistors with them, since the short is usually between screen and cathode/control grid/whatever.)
                  "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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                  • #10
                    I figured it out, it turns out that one of the tubes is raising the bias voltage from -43V to -22V, if you hit the amp hard enough to start it clipping. The amp's output folds up at that point, and the pair of tubes with that bias voltage will start to glow blue if you let them go long enough. Even after I removed the input signal, the bias voltage for the pair with the bad tube was drifting back and forth from -22 to -43V repeatedly. I took that tube out and tried it on the other side of the bias circuit (there is a separate feed for the push and pull sides) and it did pretty much the same thing.

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