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Marshall Artiste with low P/T secondary and low output

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  • Marshall Artiste with low P/T secondary and low output

    A pre-emptive thanks to my learned colleagues.

    This is a problem with a Marshall Artiste 50w from about 1978.

    It came in with several problems which I have worked through (V1 plate resistor, and Bias rectifier diode). The amp is now going but with a very, very low output (4.8VRMS across the speaker).

    The B+ voltage was very low (360V where I was expecting 420V). I replaced all the original filter caps and the main rectifier diodes. B+ is still 360V). Next I measured the P/T secondary (with rectifiers out of circuit) and it was reading 570VRMS (Fluke 87V) where the specs on the Classic tone site had me expect 690V http://www.classictone.net/40-18023.pdf . That’s down about 20%.

    Question #1: I thought Rectified DC = 0.9xVRMS. Clearly I am wrong. I don’t understand why a secondary voltage of 690VAC would result in a B+ voltage of only 420VDC according to Marshall. Removing the HT fuse didn’t see any notable increase in B+.

    Here are relevant readings with all tubes installed:
    Click image for larger version

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    (and in text for, just in case picture doesn't load)

    Measured location Expected Value Actual Value
    P/T Secondary Vrms 690 570
    V @ rectifier VDC 360
    V5plate 420 358
    V5 screen 420 349
    V5 grid -30 -25 (maxed)
    A4A plate 175 153
    A4B plate 170 141
    A3A plate 210 171
    A3B plate 125 98
    A2A plate 140 113
    A2B plate 100 88
    A1A plate 140 67
    A1B plate 105 92

    Question #2: Could having a V1 plate as low 67V be the cause of the low output?. Looking at the 1KHz sine wave at the speaker output shows a slight distortion on the top half of the sine wave.

    Question #3: It looks like the P/T is a couple of windings short of a picnic. Is there anything you recommend I check before breaking the bad news to the customer.
    Attached Files

  • #2
    I wouldn't be too alarmed there were quite a few Marshalls from 74 that had lower then normal voltage, I in fact have one.
    came from the factory this way sounds great but not at all a 50 clean watts amp.
    I can bias it colder and get the plates up to 390 but sounds better biased hot.

    Yeah the low output is definitely a problem.
    Last edited by dstrat; 07-12-2018, 12:42 PM. Reason: added comment

    Comment


    • #3
      Heater voltage? Line voltage? Mains selector position?
      - Own Opinions Only -

      Comment


      • #4
        !. You have a cap input filter, so rectified is 1.414 times RMS. (0.9 x RMS is for choke input filters) 570vAC, which is 285-0-285. 285 x 1.414 is 402v, about 5% low from the print. Any chance the amp is set for 240v and your mains are 220v?

        The Classic Tone transformer looks too high a voltage spec. It would result in almost 500v B+. Is that the transformer actually in your unit? Or are you just assuming the specs from Classic Tone are correct?

        Your B+ is 60v low, so is there a ton of ripple or is it smooth but low? And you measured the transformer with no rectifiers, but what does teh transformer measure with the circuits connected?
        Education is what you're left with after you have forgotten what you have learned.

        Comment


        • #5
          An Artiste'? Wow!
          never saw one in person, can you post some pictures?

          Love to see the insides as well as the outside.

          Comment


          • #6
            Originally posted by Helmholtz View Post
            Heater voltage? Line voltage? Mains selector position?
            Hi Helmhotz. The heater is 6.29-6.30VAC. Line voltage is currently 241.3VAC (at 7:30am) Mains selector is on 240.

            Comment


            • #7
              Originally posted by Enzo View Post
              !. You have a cap input filter, so rectified is 1.414 times RMS. (0.9 x RMS is for choke input filters) 570vAC, which is 285-0-285. 285 x 1.414 is 402v, about 5% low from the print. Any chance the amp is set for 240v and your mains are 220v?

              Thanks for rectification multiplier. I need to hit the books everytime you help me Enzo. I must be getting old The amp is set for 240V, the mains fuse has 241.3v sitting on it and the filament is 6.29-6.30 VAC


              Originally posted by Enzo View Post
              The Classic Tone transformer looks too high a voltage spec. It would result in almost 500v B+. Is that the transformer actually in your unit? Or are you just assuming the specs from Classic Tone are correct?
              I just assumed that Classic Tone know what they're talking about compared to me.

              Originally posted by Enzo View Post
              Your B+ is 60v low, so is there a ton of ripple or is it smooth but low? And you measured the transformer with no rectifiers, but what does the transformer measure with the circuits connected?
              When connected to the circuit and all tubes in place

              P/T secondary: 275-0-275VAC
              DC side of rectifier:
              HT Fuse IN: 358V with 11.4 v p-p ripple .http://music-electronics-forum.com/a...0&d=1531434075
              HT fuse OUT: 393VDC with 138mV p-p ripple http://music-electronics-forum.com/a...9&d=1531433994

              Is a 35v drop excessive with a load on the power supply? I expect the ripple to increase, but is 11.4V too much?
              Attached Files

              Comment


              • #8
                Originally posted by drewl View Post
                An Artiste'? Wow!
                never saw one in person, can you post some pictures?

                Love to see the insides as well as the outside.
                I can't wait to hear this much-maligned baby sing again. Photos as requestedClick image for larger version

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                Comment


                • #9
                  What is happening on the grids on the output tubes when it starts to clip on the output?

                  BTW, I had the 100W version of one of these here recently.
                  Last edited by nickb; 07-13-2018, 08:31 PM.
                  Experience is something you get, just after you really needed it.

                  Comment


                  • #10
                    Originally posted by nickb View Post
                    What is happening on the grids fn the output tubes when it starts to clip on the output?

                    BTW, I had the 100W version of one of these here recently.
                    Hi Nick, I'm just looking at the Channel 2's V2 with a 100mV 1Khz signal at the input. The signal on the grid of V2 is 106mV and its plate is 1.27VAC. On V4 the pin 2 Grid is 1.08V and the pin plate is 19.2v. On V5 (EL34) the signal on the grid is 18.2v (I can't get any more negative than -24VDC bias).

                    HERE IS THE WEIRD THING: With the reverb pot on minimum, the signal across the speakers is 8.17Vrms ( 8 watts). When I turn the reverb pot to maximum the signal across the speakers goes to 19.4vrms (47watts). The B+ and Plate voltages are still all low. I have NO reverb tank connected to the reverb leads. At maximum volume with maximum reverb the output signal is clipping as one would expect to see. If I turn down the reverb the signal cleans up as it reduces. Its like the reverb control is acting as a Master Volume.

                    Is it normal to have the terminals on the reverb drive transformer exposed like that? Probing fingers can touch the supply voltage line.

                    Comment


                    • #11
                      Originally posted by christarak View Post
                      Hi Nick, I'm just looking at the Channel 2's V2 with a 100mV 1Khz signal at the input. The signal on the grid of V2 is 106mV and its plate is 1.27VAC. On V4 the pin 2 Grid is 1.08V and the pin plate is 19.2v. On V5 (EL34) the signal on the grid is 18.2v (I can't get any more negative than -24VDC bias).

                      HERE IS THE WEIRD THING: With the reverb pot on minimum, the signal across the speakers is 8.17Vrms ( 8 watts). When I turn the reverb pot to maximum the signal across the speakers goes to 19.4vrms (47watts). The B+ and Plate voltages are still all low. I have NO reverb tank connected to the reverb leads. At maximum volume with maximum reverb the output signal is clipping as one would expect to see. If I turn down the reverb the signal cleans up as it reduces. Its like the reverb control is acting as a Master Volume.

                      Is it normal to have the terminals on the reverb drive transformer exposed like that? Probing fingers can touch the supply voltage line.
                      As DC voltages look reasonable for an open loop PT secondary voltage of 570VRMS and heater voltage is spot-on, it is most probable that the PT is O.K. (having had lower than typical voltage from the first day). Shorted secondary turns would eat power, heat up the PT and cause heater voltage to drop as well. The same with shorted primary turns. In case of doubt you may do R.G. Keen's transformer short test:
                      https://www.premierguitar.com/articl...ormer-tester-1

                      There is also no sign that the low voltages are caused by some fault in the amp. (Not saying that there is no other problem.) In other words I see no chance to noteably increase voltages with this PT - except when using the 220V primary tap. This would raise heater voltage to around 6.9V (can be dropped by a suitable small value resistor if you care) and increase supply and bias voltages by 9%.

                      The reverb transformer should isolate from supply voltage, I guess what you feel when touching the secondary terminals is a high frequency oscillation in V3a (scope) caused by the open transformer secondary (similar to a power amp without speaker load).
                      How is the performance with reverb tank connected?
                      How do you measure grid voltages? These are high impedance circuits (around 1M) and the meter/scope should have at least 10M input impedance..
                      Last edited by Helmholtz; 07-13-2018, 09:08 PM.
                      - Own Opinions Only -

                      Comment


                      • #12
                        Originally posted by christarak View Post
                        Hi Nick, I'm just looking at the Channel 2's V2 with a 100mV 1Khz signal at the input. The signal on the grid of V2 is 106mV and its plate is 1.27VAC. On V4 the pin 2 Grid is 1.08V and the pin plate is 19.2v. On V5 (EL34) the signal on the grid is 18.2v (I can't get any more negative than -24VDC bias).
                        106mVAC in and 1.27VAC out is only a gain of 11. That is way too low. Can you retest but use a 10nF 630V ( or so) capacitor in series with your meter lead? That said this is very much a clean amp. You can always mod it to make it more JCM800 like.

                        HERE IS THE WEIRD THING: With the reverb pot on minimum, the signal across the speakers is 8.17Vrms ( 8 watts). When I turn the reverb pot to maximum the signal across the speakers goes to 19.4vrms (47watts). The B+ and Plate voltages are still all low. I have NO reverb tank connected to the reverb leads. At maximum volume with maximum reverb the output signal is clipping as one would expect to see. If I turn down the reverb the signal cleans up as it reduces. Its like the reverb control is acting as a Master Volume.
                        The reverb mixer is rather, er, lacking. You can try on old trick and that is to swap the wires to the reverb level pot wiper and clock wise pins. It will help to stem the gain loss with no reverb.

                        I notice you can get 47 watts out, that's good. Therefore your issue is not the power supply or output stage. It's just gain and these don't have a lot of that. You can add cathode bypass caps to V1 on CH1 to give a bit more gain.

                        Is it normal to have the terminals on the reverb drive transformer exposed like that? Probing fingers can touch the supply voltage line.
                        Safety standards have evolved over the years. At the time that was OK. Feel free to protect more to your own satisfaction.

                        Here is a gut shot of the 100W version. Notice some mods around the input stages.

                        Attached Files
                        Last edited by nickb; 07-14-2018, 02:04 PM.
                        Experience is something you get, just after you really needed it.

                        Comment


                        • #13
                          If you can't get more negative than -24VDC at power tube grids, I think you should pull the power tubes and fix that before doing anything else.
                          The change in output level with the reverb tank disconnected makes no sense to me. There is no signal path through the reverb then. Makes me think it is just oscillating.
                          Originally posted by Enzo
                          I have a sign in my shop that says, "Never think up reasons not to check something."


                          Comment


                          • #14
                            Thanks for the pix, these are rare.

                            I actually have an Artiste' 2x12 cab I found on the early days of Ebay.

                            I built a Marshall 18w chassis for it back in the day and still use it alot.

                            Comment


                            • #15
                              Originally posted by g1 View Post
                              If you can't get more negative than -24VDC at power tube grids, I think you should pull the power tubes and fix that before doing anything else.
                              The change in output level with the reverb tank disconnected makes no sense to me. There is no signal path through the reverb then. Makes me think it is just oscillating.
                              I missed the -24VDC. g1 is right that is a problem. I'm surprised the EL34's aren't red plating. Might also explain you low volts.

                              The gain drops as you increase the reverb level due to interaction in the reverb / dry mixer.
                              Experience is something you get, just after you really needed it.

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