Bias and wall voltage

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by slegros, Sep 19, 2008.

  1. slegros

    slegros Member

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    I have a Marshall Bluesbreaker RI I just retubed and(tried) to bias.

    Im in Japan where wall voltage runs 100V 50Hz

    The plate voltage measured between 330-340V, and the hottest I could Bias the amp to was 33mA by maxing out the bias pot.

    Theres a Japanese sticker on the amp which says 100V 50Hz, but also an English sticker on the chassis which reads 120V 60Hz.

    My guess is that Marshalls imported to Japan come in with North American spec 120VAC transformers.

    Am I correct in assuming the 20% wall voltage drop would equate to the same 20% plate voltage drop? If so would a 20% increase in wall voltage give me a 20% increase in plate voltage, putting my plate voltages in the 400V range and allowing me to bias to a better spec?

    Any suggestions besides using a step up transformer to go from 100V to 120V wall voltage to plug the amp into, and biasing for a wall voltage of 120V using the step up transformer?

    Anything else I might be missing?

    Thanks!

    As a footnote My Ampeg Superjet which I know has a 100V transformer in it read 400V and I biased it just fine to 42mA.
     
  2. mooreamps

    mooreamps Senior Member

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    Yes, a couple of things.

    First, running 50 cycles into a transformer that may only be rated for 60 cycles, and the primary coil may over-heat. Also, at 100 volts, it will tank the filament voltage.

    So, if the transformer is rated for 50/60 cycles, you can pull the voltage back up with a variac.

    -g
     
  3. slegros

    slegros Member

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    Yes, 50/60 cycle transformer, but seems to be 120V. It is a Japanese market amp with the import sticker from Yamaha music(the official importer.)

    Not thinking of using a variac, but just a heavy duty step up plug-in wall transformer to go from 100-120V.

    Thanks!!!
     
  4. phsyconoodler

    phsyconoodler Member

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    No.You need to change the bias range resistor to be able to bias the amp hotter.If the amp is set up for your country's volyage,by increasing the input voltage you will over-volt the heaters.
    At the rather low plate voltage you can bias it much hotter than 32ma.More like 45ma or so.The range resistor is the key,or try another set of power tubes and see if they draw more current.No two are alike especially from a different manufacturer.
     
  5. slegros

    slegros Member

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    I got it running hotter(up to 33mA) by swapping tubes. Thats the thing, the combination of the chassis sticker saying 120V and the fact that Im reading 340V on the plates leads me to believe that Marshall doesn't spec 100V transformers in the Japanese exports, but uses the 120V units for North America causing the low plate voltages. Any other reason Id be getting only 340V on the plates?
     
  6. mooreamps

    mooreamps Senior Member

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    As the "most annoying troll on TGP", see if you can check the data sheet of this transformer, or the schematic, for a 100 volt primary tap.

    -regards
    -g
     
  7. phsyconoodler

    phsyconoodler Member

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    What rectifier tube do you have?If you swap it for a diode rectifier pulg-in you will get the voltage up.
     
  8. mooreamps

    mooreamps Senior Member

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    Aren't you forgetting about filament volts ???

    -g
     
  9. phsyconoodler

    phsyconoodler Member

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    He already has a rectifier tube.The transformer already supplies the correct filament voltages.It has nothing to do with the B+.
     
  10. stratman_el84

    stratman_el84 Member

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    Measuring the heater voltage would be the decider on if the PT is a 120VAC unit or not. *IF* the heater volts are low, ONLY then would an increase of line voltage to 120V instead of 100V be justified OR safe. Otherwise, Phsyconoodler is correct, changing the bias range resistor would be the ticket to getting the tubes to idle in the proper dissipation range for the plate voltage supplied.

    Cheers!

    Strat
     
  11. slegros

    slegros Member

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    I had 2 rectifier tubes, and 2 prs of 6L6's. I used the combo that got me the highest current reading(33mA). Voltages ranged from 330-340v from pin 3 to ground. Depending on the combo of power tubes and rectifier, readings ranged from 28-33mA. Did I miss anything?

    I read both stratmans, and phychonoodlers posts about reading heater voltage and changing the bias resistor-both make sense. Where would I take a reading to check heater volts?

    A SS rectifier is maybe a good option-certainly the increase in voltages would be welcome!

    Many thanks for the responses! Its been very very helpful!!!
     
  12. stratman_el84

    stratman_el84 Member

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    Again, before you increase the AC voltage to the amp, you really really need to confirm the heater voltage is low. If the heater voltage is not low and you increase the AC voltage, you're going to have some serious problems, not the least of which is premature tube death and possibly other problems. Heater voltage needs to be quite close to spec for proper tube life and operation.

    Cheers!

    Strat
     
  13. slegros

    slegros Member

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    Which pins do I measure across to check that?
    What is good range for 6L6 heater voltage?
    Thanks!!!
     
    Last edited: Sep 20, 2008
  14. stratman_el84

    stratman_el84 Member

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    You'll see a pair of wires (usually twisted together, usually green) going in a daisy-chain from tube socket to tube socket. The proper reading is 6.3VAC.

    Cheers!

    Strat
     
  15. slegros

    slegros Member

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    Thanks! That settled it! Heaters are reading 5.0V It definitely has a 120V tranny in it instead of a 100V unit for Japanese wall voltage.

    Looks like a step up 100-120V wall transformer and re-bias for 120V is the fix!

    Disappointing that Marshall is either too cheap or too lazy to put proper 100V transformers in their products destined for Japan. I guess the logic is the market is not big enough to warrant the expense of speccing a dedicated transformer, and/or the 120V transformers will work without damaging the amps. Marshall users in Japan must see some AWESOME tube life..... Built in Brown sound!!

    Many thanks to all who posted here and helped me figure this out conclusively without blowing up my amp!!!
     
  16. phsyconoodler

    phsyconoodler Member

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    I hope you didn't measure that 5v at the rectifier tube.If you did,then that's correct.Measure at the power tube sockets or the preamp tubes,pins 4,5 and 9.
     
  17. slegros

    slegros Member

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    I measured across the power tube. Pins 2&7 if I remember correctly. The wires which were daisy chained across all tubes. Is that OK?
     
  18. phsyconoodler

    phsyconoodler Member

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    Yes that's correct.It is strange that the proper tap is not hooked up inside the amp.Especially if it's a japanese model.You should get a schematic and find out where the 100v tap is and get it hooked up.Then you don't have to worry about step-up transformers and band-aid fixes.
    Assuming there is a 100v tap inside???
     
  19. mooreamps

    mooreamps Senior Member

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    My opinion, I don't like to see them drop anything below 6.0 volts.

    -g
     
  20. mooreamps

    mooreamps Senior Member

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    We don't believe this power transformer has a primary tap for 100 VAC, and his filament voltage appears to be measuring at 5.0 volts dc. Therefore, I suggested he run the amp from a variac, to see if he can increase his line voltage.

    -g
     

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