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Plate voltage higher than B + ?

Discussion in 'Amp Tech Center' started by Dr Derek, Jan 10, 2021.

  1. Dr Derek

    Dr Derek TDPRI Member

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    Before I go into detail about the problem, here's a little background.
    I was so happy to find this Dynacord Da 20 amplifier going for around 100€. It was just about working but didn’t sound at all healthy. I considered myself lucky to find the amp without the cabinet as , in my opinion the DA 20 combo did not have the flair and charm of the earlier DA16. The actual amplifier being almost identical.>
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    Received_262704978523728 by Dr Derek posted Jan 10, 2021 at 11:23 AM

    I replaced all the electrolytics and all the coupling caps (two at a time, testing in between) and slowly the amp came back to life. One of the EL 84 s had a plate pin missing so I guess it was working on one valve when I first tested it. Replaced both output valves. It now sounds quite nice although I feel it should have more power for an 18w push pull amp.
    Anyway, to get to this strange thing of the high voltage which is on one side of the O/P transformer. All the voltages are correct according to the schematic except for the plate voltage on valve 7. The B+ is 365v , the plate on valve 6 is about 361v but the voltage on valve 7 is up around 370v.. The resistance on this side of the transformer measures 138 ohm , a lot higher than on the other side (v6) which is 104 ohm. Cathode voltages and resistors are almost identical and Ok. I have googled this problem but nowhere can I find that the plate voltage is higher than the B+. I can only imagine that it could be caused by induction on the one side of the o/p transformer. Another thing, this doesn't happen everytime I turn the amp on. Sometimes it decides to act completely normal, despite me tapping all over to try and get it to misbehave. I think I have uploaded a schematic below. (The tremelo connection to the cathode of v7 has was disconnected when I received the amp and is still disconnected, , read somewhere that this can be quite hard on the output valve,,,the tremelo now works solely on the screen of valve 5, as far as I can see.)
    Any theories anyone.?
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    Last edited: Jan 11, 2021
  2. 2L man

    2L man Tele-Meister

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    There are at least two ways to wound push pull OPT and "bifilar" method produce almost the same resistance when both primary coils are wound same time. Another obviously vintage method is to wound using single wire and when first coild is narrower deeper and there comes less wire so its resistance comes smaller than following coil although both have same count of turns.

    OPT primary resistance is very small when compared to operative impedance so coil resistances should have only small effect. There are more variables on OPTs which also effects its operation.

    On Hammond datasheets there are coil resistances but I don't know others who build guitar OPTs publish them. HiFi OPT manufacturers publish data better.
     
    Last edited: Jan 10, 2021
  3. timfred

    timfred Tele-Meister

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    Imbalanced DCRs with a difference of dozens of ohms in an OT is quite normal and by themselves nothing to worry about.

    What does normal mean in this case? B+ greater than both plate voltages?

    What are the screen and cathode voltages? Confirm grids are 0 volts?

    Could you tell us what the other nodes of the power supply are reading? Is the screen supply normal (slightly less than B+?)

    One thing to try: Swap power tubes to see if the voltage follows the tube or the socket.

    edit: How much AC ripple is on your B+? Are you measuring with no input signal/volume at 0? I’m having a hard time figuring out how pure DC can be higher at the plate.
     
    Last edited: Jan 10, 2021
  4. Dr Derek

    Dr Derek TDPRI Member

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    Yes normal means b+ is higher than both plates,

    Screen voltages are the same as each other , slightly lower than (correct) plate voltage , as in the schematic.

    Cathode voltages also the same, and correct as in schematic.

    Swapped tubes, high voltage is always on same socket.

    Not sure what ac ripple is. When I play a guitar chord , the normal socket stays relatively stable but the 'high voltage ' socket drops in voltage. I guess there's a clue there.

    It's like a ghost voltage which doesn't seem to have any effect on anything else.
     
  5. Dr Derek

    Dr Derek TDPRI Member

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    Also , confirm grids are 0 v.
     
  6. Dr Derek

    Dr Derek TDPRI Member

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    Th
    Thanks for the reassurance about the opt resistances. It's just this higher than b+ voltage which gets me.I've googled this in so many ways but nothing comes up.
     
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  7. Dr Derek

    Dr Derek TDPRI Member

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    Will measure ac ripple tomorrow, could be something there. Would any ac in the opt windings cause the excessive DC voltage. ( am I clutching at straws here ? )
     
    Last edited: Jan 10, 2021
  8. timfred

    timfred Tele-Meister

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    Yeah me as well. I wouldn't be surprised to see 5-10V of residual AC ripple at the OT. That might get "rectified" somewhere - OT, tube, or even on the way to or in your meter - and appear as DC.
     
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  9. elpico

    elpico Tele-Afflicted

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    Do you have a different meter you can try to replicate the measurement with? As handy as they are, they're not magic oracles. They do make mistakes.
     
  10. Dr Derek

    Dr Derek TDPRI Member

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    I have a spare meter but it's the old vu/needle style. I guess I could try with thst,, I'm open for anything . But it's hard to imagine that a meter would only give incorrect readings over and over again on one plate and always give stable readings everywhere else.
     
  11. Jon Snell

    Jon Snell Tele-Holic

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    Incorrect readings of DC on the anode using a generic DVM (not wishing to be rude but cheaper possibly a meter than say Fluke) is quite common. Your moving coil meter may be more accurate in this case as it won't add noise to the reading like your DVM will.
     
  12. 2L man

    2L man Tele-Meister

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    Please measure OPT primary both side voltages against OPT voltage input and put red to input and black to primary outputs or power tube anodes? Then reading is smaller less than 10V. Also wound multimeter cables at least ten times and magnetic stray to multimeter lessens.

    Then if other side comes positive and other side is negative it is very strange.
     
  13. Dr Derek

    Dr Derek TDPRI Member

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    Just to keep you updated, I check the amp today and it was acting ' normal ' , that is anodes on both el 84s were 3 to 4 volts under the b+. .I measured the ac on the b+ , it was 4.4v,,,,(also the same on both plates.). Played guitar through it for about 20 mins, Sounded ok. . The mystery continues. ( I'll Try and keep my mind open ) See what happens.
     
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