Low B+ Voltages...bad PT??

Discussion in 'The Workbench' started by bmwfreq, Feb 23, 2020.

  1. bmwfreq

    bmwfreq Member

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    I've noticed that my amp just doesn't sound great anymore. I measured the voltages inside my amp and I'm getting lower than spec'd voltages. The Marshall JCM 800 2203 Service manual says I should be getting 468VDC B+ with a 120VAC supply voltage. I'm only getting 450VDC B+ at 120VAC.

    Now, my amp at one time did have the spec'd voltage of 468VDC, but anymore I'm only getting around 450VDC, and my amp doesn't sound anything like it used to.

    Is this low voltage due to a PT that's going bad? I don't know. That's what I'm asking here. I know there's a lot of great minds here, so I thought I'd put it out there and see what everyone thinks my issue may be.

    Thanks in advance to everyone that replies.
    Voltages for Coffee 022320.jpg
     
  2. neikeel

    neikeel Well-Known Member

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    Wall voltage variations
    Hotter bias (output tubes on the way out)
    Tired filter caps

    Most unlikely problem is your PT. They typically just fail by shorting or going open circuit. Dropping 18vdc is nothing.
     
  3. RickyLee

    RickyLee Well-Known Member VIP Member

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    What is the heater voltages?

    I had about 3 old generic USA made amps from same make that had what I thought was bad power xfmrs. They were 100W rated amps running quad of 7581 and 6L6. I tried running them with two power tubes pulled to see if it helped much but did not raise them to proper spec. I do not remember if they were running hot to the touch though.

    The heaters in all these amps was running just over 5VAC. One was a bit higher around 5.7VAC. One of them I remember having a very high B+ of over 560VDC. They all had the same make/company power xfmrs. And this was with my wall voltage 123VAC - 125VAC.

    I figured the power xfmrs were bad and possibly could fail soon and ended up changing out two of them.
     
  4. bmwfreq

    bmwfreq Member

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    All five filter caps measure around 50uF/50uF
    Bias has ALWAYS been set to 40mA
    Tried three quads of 6550, nothing changed
    Tried 8 different preamp tubes, nothing changed
    Tried a new OT, nothing changed (re-installed original)
    Tried a new Choke, nothing changed (re-installed original)
    ...I don't know what else to try...?
     
  5. bmwfreq

    bmwfreq Member

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    Heater voltage at origin is 3.104VAC/3.097VAC = 6.201VAC (@120VAC)
    However, it drops to 3.00VAC/3.01VAC at V1 (this may be normal)?
     
  6. RickyLee

    RickyLee Well-Known Member VIP Member

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    V1 is the end of the heater string, or should be anyway. So you should see a slight drop at the end from the load/demand.

    This is a JCM 800 2203? What year?

    I know I would love to have those voltages in many of my amps lol. I have some 800's and then my DSL and TSL I have seen almost 7VAC heaters and 480V - 490V plates on EL34's. That was back when I noticed my wall voltage was over 125VAC all the time, like 125.5VAC or so. Seems like our supply here is now around 123VAC last few years. And it is not like they are plopping in new homes/businesses near me.

    For myself, I like my running voltages more around 450V on my old JCM 800 era amps. I heard a bit of a sweet spot in that range vs the higher 470V on up. Plus, much easier on your tubes/screens of these new production glass.

    But . . . . if you are not thinking the amp sounds right, that is not right. So many things it could be from old resistor/caps drifting ect.

    One thing regarding power xfmrs, is we are discussing what you have at idle. But we have no clue what is happening when you start playing it and it is under load. I guess if you could temp swap in a known good one or new one, obviously that rules that out.
     
  7. bmwfreq

    bmwfreq Member

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    MY REPLY IS WITHIN THE BODY OF THE QUOTE ABOVE...
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    If you want the amp to sound better get some fresh hi gain preamp tubes.

    There is nothing wrong w/ the transformer.
    The voltages on the schematic are approximate only and vary according to which brand tubes you install.
    Getting wide variations is normal in a tube amp.
    The actual voltage is rarely what's shown on the schematic, that's normal.
     
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  9. RickyLee

    RickyLee Well-Known Member VIP Member

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    Yeah, that is a tough one for sure. The 18V could be caused by a 2VAC - 4VAC dip in your wall voltage, but sounds like you are somewhat close on keeping an eye on that. You can ballpark multiply X 4 the change in your B+ in relation to the wall voltage fluctuation. For example, in my home during summer when my air conditioner is running along with other appliances, I have seen my wall voltage go under 120VAC. This translated to a much lower B+ in my amps of course. If my mains drops from 123VAC to 119VAC, then that lowers the amps B+ aprox 16VDC. Then the demand from other households pulling from same transmissions lines in my area, so you get that one.

    Then factor in the bias idle. A hotter bias idle setting will pull down B+ slightly. I would imagine aging and failing power tubes could affect this as well.

    But one thing I would say from my experience, is the actual B+ voltage drop itself of 10VDC to 20VDC from mains fluctuation in an amp that has no issues, should not cause bad tone. That is with no issues of dirty power/noise/interference ect.

    I would change those filter caps if they are original/OEM. Try that one. Many people claim they hear a big improvement putting in new filters. Though I don't know why I have never been lucky and heard an improved tone with new filters lol. But that is important.

    It is starting to sound like the experience I was having with my '82 4010 vs my '84 4010 and '83 2204. That '82 just would never sound as good as the others and never did figure that one out. I never swapped out xfmrs though.

    Bottom line is, if you had a good tone and now you don't, something is not right.
     
  10. bmwfreq

    bmwfreq Member

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    I've tried brand new preamp tubes...many. Also brand new power tubes.

    I changed almost everything with the exception of the filter caps. They're only 7 years old, and all measure 50/50uF. Could they possibly be bad even though they measure the expected cap value?? Is there a way of determining this besides measuring the cap value?
     
  11. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Have you tried measuring your AC HT voltage, when the DC seems lo to you? Maybe a slightly weak rectifier diode?
     
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  12. bmwfreq

    bmwfreq Member

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    D1 Voltages 022320.jpg
    Yes. D2~D5 seem to have the expected voltages.
    D2 & D3 = 450/450VDC
    D4 & D5 = 225/225VDC.

    Diodes tested at (w/diode tester):
    D1 = .559v
    D2 = .525v
    D3 = .565v
    D4 = .539v
    D5 = .561v

    On D1 I have -68VDC on one side, and .6VDC on the other side.

    Anything look out of sorts here???
     
  13. thetragichero

    thetragichero Well-Known Member

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    d1 is for the bias i presume?
     
  14. bmwfreq

    bmwfreq Member

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    Correct!
     
  15. South Park

    South Park Well-Known Member

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    If the plate voltage is 450 than what is the bias you may have to turn it up a little . You could be getting a low voltage off the grid
     
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  16. mickeydg5

    mickeydg5 Well-Known Member

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    Try checking your cables with a multimeter and cleaning all cable plugs and amplifier jacks with an electronics lubricating cleaner.

    Any issues with any of that will make your guitar signal dull.
     
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  17. MickeyJ

    MickeyJ Well-Known Member

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    Based on what you told us there's nothing wrong with the bitch.

    Don't forget that taste can change over time, and hearing ability as well.
     
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  18. Matthews Guitars

    Matthews Guitars Well-Known Member

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    How old are the filter caps? If they're original, you can expect that they're not operating at peak efficiency. Their ESR will be increasing, providing more resistance, and lowering the output voltage. Very possible that this is the situation.
     
  19. mickeydg5

    mickeydg5 Well-Known Member

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    :thumb:
     
  20. MickeyJ

    MickeyJ Well-Known Member

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    higher esr causes bigger ripples, and that = hum. if the amp doesn't hum, his filters are a-ok.
     
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