Deluxe Reverb Bias Problem...Need Advice

Discussion in 'Amp Central Station' started by zoppotrump, May 23, 2015.

  1. zoppotrump

    zoppotrump Tele-Afflicted

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    Hi folks,
    i just bought an old DRRI that has been completely rebuild PTP and with Mercury OT and had been run so far with 6L6s and a solid state rectifier.....
    I want to turn it back to 6V6s and Gz34 and tried to bias it according the manual and some sites on the net....
    My problem is, that there is a huge difference on the voltmeter between metering silent or while strumming a chord....
    the meter reads around 17.5mV silent and 37mV strummed.....
    i was told to bias it around 23-25, but nobody mentioned whether silent or with load.....
    what should i do.... i play traditional country music and would like the amp stay rather clean with a slight touch of hair ...in between marty stuart and redd volkaert.....
    any advice or opinion greatfully taken.....
     
  2. Cleeve

    Cleeve Tele-Holic

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    Silent.
     
  3. Middleman

    Middleman Friend of Leo's

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    Silent and yes, 23-25 is the range for a little distortion. It will depend however on your power voltage read from pin 3 of one of your power tubes. Use that to plug in to the bias formula and that will find you the accurate range for your amp.
     
    Last edited: May 23, 2015
  4. telex76

    telex76 Doctor of Teleocity Ad Free Member

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    Yes, what they said.
     
  5. orangedrop

    orangedrop Friend of Leo's

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    It is neat that you have left the meter on there long enough to observe your amp's circuit under demand.

    Cool, right!
     
  6. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

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    Silent. But what you were told (23-25mv) means absolutely nothing as far as the sound of the amp goes. You can't pick a number out of the air and say "use this" - the best setting varies by plate voltage and tube type.

    If it's been running with modified parts that number's a crapshoot, especially if it had ben set up for 6L6's. "A" Mercury output transformer doesn't mean it's their direct-substitute for a stock DRRI OT....I would venture to guess that it specifically is NOT, and is meant for 6L6 output tubes.

    I can't tell you whether the bias should be set higher or lower. But a number (even a few mv...and what you are actually setting is the ma, but using a mv method) doesn't have a *sound*. It could be too cold, too hot...wow, sounds like the Three Bears! ;-)

    Seriously - (I always mention this first) be sure you understand the safety precautions if you are inside an amp, like properly discharging filter caps - which can kill you with the amp unplugged. *If* you understand amp safety, then first measure the plate voltage. Then check one of the many web sources (or the specs that came with your *specific* tubes, as each manufacturer's specs vary....sometimes by rather large amounts.

    THEN set the bias. A good starting point is 70% of the plate dissipation (the number you need from the tube specs. There's some math involved, as not all tube spec charts list guitar amp type plate voltages (with NOS tubes the plate voltages listed in the RCA tube manual, for example, are usually 50% or less those found in a guitar amp, as most TV and hi-fi equipment....the bulk of the tube usage except of the military...hits power tubes with very low plate voltages.

    So you need to figure out the dissipation at YOUR plate voltage, then set the bias at 70% of that dissipation figure....THEN use your ears. Try going 2-5mv/ma higher, then lower. Listen for the "sweet spot". As long as the best tone is within 10-20 of max plate dissipation you're safe. And that's all the 70% number is - "safe". A colder bias will make the amp play cleaner with more headroom, but OTOH it may make the amp sound a bit sterile. Most amps sound best biased a bit on the hotter side - warmer tone, earlier breakup. But also your tubes don't last as long (how long depends on how high you go and how robust the tubes are.

    But again - a small range of number is useless. I don't even know how anyone who knows what they are doing could possibly make that type of recommendation, especially if they know there's been a transformer change!

    Review:

    1. Safety
    2. Measure plate voltage
    3. Set bias at 70%
    4. Adjust bias for tone
    5. Verify the setting is in a safe zone (depending on the tube's reputation no higher than 90% of max dissipation, although I usually go no higher than 85% except for short studio sessions).

    Hope that helps! Feel free to email if you have any questions, or if you got hold of NOS power tubes and don't have an RCA tube manual to look up the specs (they can be found online, but if you don't know what you're looking for exactly they can be hard to locate).
     
  7. jazzguitar

    jazzguitar Tele-Afflicted

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    Yes, the Deluxe Reverb, like any guitar amplifier except single ended ones such as the Champ, uses a Class AB power section which means, under load, the the current will go up.

    Sorry but nope.

    That "set the bias to 70%" is a widespread internet rumor that has become almost a religion.

    This 70% value is, as a rule of thumb, a safe limit to the hot side of setting the bias in a Class AB amplifier. It is not a target value.

    And it is not applicable to all amplifiers and all output impedances.

    If we have an amplifier A with lower plate voltage – say 360V – than amplifier B – which let us say has 450V – we are told we must set the bias current in amp A higher than in amp B to get an idle plate dissipation that is 70% of the tubes' maximum value if both use the same type power tubes.

    Usually higher plate voltages go with higher output power of the amp, so we end up with the low power amp A idling at a higher percentage of its maximum power output than the higher powered amp B, thus reducing its maximum output power. This is obviously nonsense.

    zoppotrump, you can set your amp to any figure between say 15mA and 28mA which sounds good to you, assuming a plate voltage of 420V typical for a Deluxe. If you like country clean you probably do well with about 20mA (which assuming a 1 Ohm resistor in the bias rig means 20mV).

    In my humble opinion and caveat emptor and YMMV, of course.
     
  8. backline

    backline Tele-Afflicted

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    It always amuses me when a guy asks about biasing power tubes, and the first thing he hears is ......."Make sure you know how to Zero the caps." :)

    70% .?
    It is fine to run your tubes there if it sounds right, but that is the rule of thumb Max dissipation.....not a desired finalization of power tube biasing.
    good luck OP
     
  9. Johnny Cache

    Johnny Cache Former Member

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    Are you measuring volts or mA? There is a difference, volts should be somewhere around -32 to -38, mA should be around 19 to 23. Also you have to take into account the plate voltage. And unless your using a tone generator and oscilloscope, I'd say silent (idle). When you put a signal into your amp things go up as they should.
     
  10. kavalero

    kavalero Tele-Meister

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  11. kavalero

    kavalero Tele-Meister

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    70% of plate dissipation is the maximum biasing value, not the starting point.


    All my Fender B/F amps are biased between 55-60%.


    There is no need for going above this value, as there is no sonic benefit of doing so + you are cooking your tubes without any need, shortening its life(s).


    :D
     
  12. BobbyZ

    BobbyZ Doctor of Teleocity

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    Silverface makes a good point about checking to make sure those expensive transformers are ok with 6v6 tubes.
     
  13. orangedrop

    orangedrop Friend of Leo's

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    6v6 has a lower output than the 6L6 and if it has been running without problems, the Mercury Magnetics will certainly handle the output.

    The Silverface Deluxe is commonly rebiased and run on 6l6GC with no other changes.

    Depending on the PT that happens to be in it, they often get lucky.
     
  14. kavalero

    kavalero Tele-Meister

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    I've been running 6L6GC in many of my DeLuxe Reverbs in the past without any issues. In fact 6L6 sound way better than 6V6 in my opinion.
     
  15. BobbyZ

    BobbyZ Doctor of Teleocity

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    I was referring to the power transformer. If it's designed specifically for 6L6s it might eat up 6v6s.
    Just seems like a good idea to find out. It could be fine, probably is but we don't know that for sure.
     
  16. BobbyZ

    BobbyZ Doctor of Teleocity

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    Now that I reread the OP it looks like just the OT was changed so nevermind.
     
  17. zoppotrump

    zoppotrump Tele-Afflicted

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    Hey guys...thank you so far.....i think i have to state a few things clear......
    i have absolutely no experience with the inside of an tube amp....so wait, before you burn me....the amp i bought is completely rebuild.
    the tech did the following:
    complete rewired PTP
    bigger mercury OT
    added mid pot for both channels
    reverb and tremolo work on both channels
    he added on the backpanel a mesuring spot for ground and left & right Powertube and a trimpot that is accessible from the outside of the chassis. so you don´t have to open the chassis and the danger of electrocution is abolute minmal....
    i set the bias according to my ears and mesured on mV
    with the best sound the meter read: silent 19mV and on load 38mV
    I really would not dear to open the chassis ....i have no suicidal tendencies....unfortunately the next serious tech here lives 100 miles away.....
     
  18. King Fan

    King Fan Friend of Leo's

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    If it sounds good at 19, I'd say you've succeeded. Good job.
     
  19. orangedrop

    orangedrop Friend of Leo's

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    Are you referring to Mercury's "Tone Clone" transformers?

    IIRC, these are supposed to be clones from some of the vintage Fenders.

    You can stick a gigantic transformers in your amp so long as the voltages are right, and if you bias things so the tubes will be happy they'll run a long time.

    The two terrible things that do happen from seriously upsizing are weight and headroom.

    Who wants headroom?

    Just kidding!
     
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