76 Jmp 50 Bias Questions

Discussion in 'The Workbench' started by Dblgun, Jan 6, 2019.

  1. Dblgun

    Dblgun New Member

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    The power, output transformers and choke are original. PV unloaded with the 150k dropper and the 110v tap is 425vdc.
     

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    Last edited: Jan 10, 2019
  2. Dblgun

    Dblgun New Member

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    Additional pics
     

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  3. charveldan

    charveldan Well-Known Member

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    Somewhere with a Friedman Marshall & Les Paul.
    425 sounds more reasonable.
    Hope you get it sorted out.

    Choke is a Dagnall. I cant tell if the PT & OT are Dagnall or Drake. They're usually clearly marked.

    One thing about 60's & 70"s Marshalls is the parts could be fairly inconsistent month to month.
    They used what they had on hand.
     
    Last edited: Jan 10, 2019
  4. Dblgun

    Dblgun New Member

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    To my knowledge they're both Drakes.
     
  5. neikeel

    neikeel Well-Known Member

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    All original Drakes.
    Wonder why your output valves are dragging the voltages down so much when at bias point (or was your meter playing up before it died?)
    I expect plate voltage of 380v when biased up.
     
  6. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    An unloaded amplifier like this will have a higher B+ by 35-45VDC on average. That is normal. I would expect 380-390VDC when loaded but the thing is he is having to use the 110V tap instead of the intended 120V tap at primary.
    The power transformer in this amplifier may just be on the low side of tolerance.
     
  7. neikeel

    neikeel Well-Known Member

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    Indeed, 35-45v voltage drop (10%) from tubes out to tubes in is totally normal but the OP described a 100v drop after tubes in and the current draw set to approx the correct voltage which is excessive IMO.
    Makes me wonder if the output valves are on their way out (I have come across some sino tubes that do that, such that I chose not to use them).
    Hopefully more info to follow.
     
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  8. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    I noticed that too. But he did say "with a set of known good EL34" in the first post. I would suspect he has used these before, in another amplifier.

    Lets ask.

    Dblgun, have you tried other sets of known good power tubes and measured results. This will give indications about the amplifier and the power tubes.

    Also passing through my head is the thought that you better watch this power transformer closely to make sure it does not nose dive and is just operating below normal.

    Just one of them check it things, make sure output and cabinet impedances are matched while using a speaker cable for hookup. Test cable and cabinet for proper resistance with multimeter.
     
  9. Dblgun

    Dblgun New Member

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    The "known" good tubes are a quad taken out of an amp several years ago. I helped a friend retube an amp using the latest and greatest tubes and he gave them to me for helping him out. They were a quad of GT EL34's and didn't have much time on them. I figured to put the two that were closest matched into this amp. I do not have or have access to a tube tester. Just to be sure I put a set of JJ's I have in it and the readings are within a volt either way so I'm not thinking I have tube issues but let me know if I'm missing something.

    So here's where I'm at currently. As mentioned previously the pv unloaded looks to be 425vdc. With tubes in and bias pot maxed, I'm reading 417vdc and -53.2vdc, with the bias pot at the other end of the adjustment 375vdc and -32.7vdc. This is with the original 150k dropping resistor and a 47k bias range resistor. The issue I'm having now is I've lost my bias in that my cathode current runs in the range of 1.4-15.5ma. This thing is starting to bug me.
     
    Last edited: Jan 14, 2019
  10. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    The GT and the JJ both run under 16mA idle when loaded?
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    On the wrong side of the tracks.
    The bias you have is more like 6550 / 5881.
    Not the bias you would see with EL34s.

    The issue I'm having now is I've lost my bias in that my cathode current runs in the range of 1.4-15.5ma.

    Start by testing the speaker load.
    What is the DC voltage on pin 5 when you have the bias adjusted?
     
  12. Dblgun

    Dblgun New Member

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    Mickey, yeah the current read at the cathode is the same with either set of tubes and can be adjusted no higher than 19.8ma by way of bias pot.

    Ampmad, the DC voltage at pin 5 will vary from -34 to -53, again by way of adjustment at the bias pot. Measurements at pin 3 and 6 (PV) go from about 415vdc to 390vdc during the pot adjustment. I'm not sure where the speaker load should be tested in reference to your suggestion but the selector is set for 8ohms and there is an 8ohm dummy load in the output jack. Checking the across the jack and the orange wire from the OP it reads 8.4 ohms and my meter probes have about .3 in them. I have a feeling this is not what you are referring to so let me know and I'll check it and get back to you.

    Thanks
     
  13. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    First, the bias negative voltage for that amplifier should range below -30VDC.

    Second, it would be good to know the VAC output at each tap of the HT secondary.
     
  14. Dblgun

    Dblgun New Member

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    I think I've finally got it working quite a bit better. Ended up with 390vdc at the plates loaded using a 150k input/dropping and 43k bias range resistor. Thanks to all those who gave input and direction.
     
  15. myersbw

    myersbw Well-Known Member

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    I'm still a little curious about this and about the noted erratic behavior you mentioned earlier concerning bias. To recap a little...you initially changed the 150L's to 220K's...then back but let the resistor change to 47K?

    And, at one point noted fluctuation of bias, etc. So, you have a 43 year old amp thereabouts and changed a bias pot setting. Did you happen the clean the bias pot with DeOxit or similar? I'm curious mostly about the stability of the bias pot and as to whether you changing that pot took you into decades-of-gunk-build-up-land and that maybe it needs a good cleaning to work properly? Just a thought. (I use DeOxit D5 a lot....sometimes the fader cleaner/lube)

    Oxidation and other crap hovering around a pot can do strange things. If you do decide to go back and clean...definitely give it minutes to dry out per the manufacturers' suggestion.

    Cheers!
    Brad
     
  16. Dblgun

    Dblgun New Member

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    Brad, you're absolutely right. Upon picking the amp up it had 6550's, 100k bias feed/grid leak resistors, a 150k bias input(feed) and a 47k bias range resistor. I changed the 100k's to 220k along with changing the 150k to 220k and the bias range to 56k per the EL34 schematic. I did end up cleaning the bias pot and a bunch of other things with DeOxit, repaired some solder connections and replaced a 100k resistor that was cracked and acted funny when the amp warmed up. I went back to a 150k bias input resistor and a 43k bias range resistor to get the bias set. I'm still working on it but think it's definitely headed in the right direction. I checked both transformers to the best of my ability and they appear to fall within the specs I could find. After checking the OT impedance I was able to bias using the shunt and voltage drop to check my previous measurements and they checked out. I'm gonna try to get around to putting on a scope in the next couple of days and see how it looks. Please let me know if you have any thoughts or suggestions, I can use all the help I can get!

    Thanks,
    Brent
     
    Last edited: Jan 15, 2019
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  17. FourT6and2

    FourT6and2 Active Member

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    Sounds like you got it all squared away. But man... 390v Vp is still low IMO. I guess it could just be the normal set point for your amp though.
     
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  18. myersbw

    myersbw Well-Known Member

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    And, btw, I just had a 70's MkII 50 watt on my bench 3-4 weeks back hovering around 390V...you're not alone. So long as power ratings aren't exceeded and you're enjoying the tone (AND all is stable)...just do your bias calculations and be done with it. I would be tempted to replace that bias pot. Sometimes a thorough cleaning "stirs the pot" of gunk on the interior in a not so good way. That pot is one weak point that you don't want to fail...ever.
     
  19. Dblgun

    Dblgun New Member

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    Brad, nice to know there are others out there similarly afflicted! Actually heard of a few more in the last week or so.. I should have a bias pot so I'll change it before securing the board. Thanks
     
  20. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Stop worrying about it. Marshall used what was available or cost productive at the time. In those years there were lots of transitions with components. Some amplifiers got the 380VAC+/- HT and others got the 460VAC+/- HT power transformers. Either people like the higher or the lower offerings. Some transformer manufacturers knowing the situation even offer both or multiple voltage levels in their aftermarket models.
     

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