5U4 vs 5AR4/GZ34

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by slhguitar, Feb 27, 2006.

  1. slhguitar

    slhguitar Member

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    Are these comparable, and close enough in voltages that a Bias for 5881/6L6s would not be necissary? Its for a fixed-bias mesa dual rectifier. Any thoughts on which sounds/feels better?
     
  2. phsyconoodler

    phsyconoodler Member

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    If you bias with the 5AR4,you can sub in the 5U4 and not worry.The 5U4 has more sag and can be spongier feeling.6L6's are biased quite a bit different that 5881's,so a rebias will probably be necessary.
     
  3. Swarty

    Swarty Member

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    5AR4 = about 20V over a 5U4.
     
  4. slhguitar

    slhguitar Member

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    That there is the problem. I'm fairly new to the whole amp tech world (only a few months in), and my amp is fixed-bias, with no pot for bias adjustment. What I suppose I was trying to get at is, would 5AR4s run a voltage that is within the safe range for regular Sovtek 5881s? If not, I have no problem with using 5U4s, I have simply heard good things about GZ34s in Mesa Rectifiers. Thanks.
     
  5. Shea

    Shea Member

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    Those Sovtek 5881s can take rather high plate voltages, so I would't worry about that aspect. But I would check the biasing, because the increase in plate voltage is going to change the power dissipation at idle.

    IMO Sovtek 5881s sound better when biased a bit on the hot side, so it might turn out that you don't have to change anything in the bias circuit. But I would still check it.

    Shea
     
  6. donnyjaguar

    donnyjaguar Member

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    If your amplifier is designed for GZ34 you shouldn't have any problem running 5U4G tubes. The difference between these two tubes, IIRC, is the GZ uses an indirectly heated cathode whereas the 5U4 uses a directly heated. That said, the HT out of the GZ34 can be on only pin 8, but it could be on pin 2 of a 5U4G amplifier. The power-up time of a 5U4 is much shorter than a GZ34 tube. Some will argue this is a bad thing, and I'll go along with that for the most part. The difference between the two times is probably as a result of the direct vs indirect heating of the cathode. The GZ34 is generally accepted as the best vacuum tube rectifier made.

    DJ
     
  7. Shea

    Shea Member

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    Yeah, it seems to be the most sought after, but I think amps of 50 watts and below sound a little sweeter with a 5U4. I can't hear any difference between a GZ34 and a solid-state rectifier in those amps.

    Shea
     
  8. brad347

    brad347 Member

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    i dont know the mullard 5AR4 I have in my Deluxe sure sounds sweet. It had a Sovtek/Groove Trash in there and I'm amazed at the difference in sound just between the two brands. I sure wasn't expecting it...

    I think 5U4s can sound "mushy" in lots of amps... I had one in a pro reverb and the amp totally opened up once I put the 5AR4 in.
     
  9. Swarty

    Swarty Member

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    Not true. A 5U4 draws substantially more fillament current than a 5AR4 and thus could smoke a power trans in an amp designed for 5AR4s (although one would hope the manufacturer would over spec the trans to accomodate either tube). I certainly would not advise putting a 5U4 in vintage amps (BF, Tweed, Marshall) that call for a 5AR4.
     
  10. Blue Strat

    Blue Strat Member

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    Since the question was about changing from a 5U4 to a 5AR4, often this is ok especially in Mesas which tend to be biased cold.

    On the other hand, not knowing how the 5881s in the amp are biased now ( I don't know the history of the amp/tubes), it's impossible to draw any conclusions about this particular setup.

    The only way to know is to get a bias probe, or similar, and run a test with the new rectifier. The Russian 5881s (which are neither 5881s or 6L6s, just similar) will handle the voltage increase, but possibly not the resulting change in bias that the switch from a 5U4 to 5AR4 will provide.
     
  11. donnyjaguar

    donnyjaguar Member

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    Ah! That is correct. The 1.1A of additional current would tax the transformer to the tune of 5.5Watts. I suspect most filament windings could tolerate this additional current, but I'm guessing the added demands on the transformer core would lower the HT a little more. This added to the higher inherent voltage drop could explain the softening up of the amplifier.

    I have done swaps between these two without issues, but I agree that's not to say it would be wise to do on all amplifiers. On the Donny Jaguar amplifier the transformer came from equipment that used a 5U4 anyway so my GZ34 should be fine.

    DJ
     
  12. slhguitar

    slhguitar Member

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    Where can I find a bias probe? Is it essentially just a multi-meter that people use to check bias?
     
  13. Blue Strat

    Blue Strat Member

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    Check the Bias Rite at www.tedweber.com

    You CAN measure bias with just a multimeter, but it's not always easy knowing where to connect it, and there's potential danger to you, the meter and/or the meters fuse.

    Using a bias probe removes all the danger and guess work. I've biased amps in every concievable way and now use a bias probe only.
     
  14. phsyconoodler

    phsyconoodler Member

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    Yes,the 5U4 draws more filament current.You need to be wary when using it in place of the 5AR4,but many amps including Fenders used the same power transfomers for both tubes.You need to know what transformer you have in your amp and then you will be able to determine the filament current rating.You may find that a large resistor in the B+ line will give you the 'sag' you are probably seeking.It also creates heat,so it needs to be in a place that won't cause grief for other components.
    I guess the real issue here is why you want to change and why people think there's some magic in a simple rectifier tube that converts AC to DC.It's the actual voltage that's the key here.One guy talked about how much better his amp sounded with the Mullard rectifier tube vs the sovtek one.The resulting voltage is the key to that 'improvement',not the change in brands.Both are capable of performing admirably.The bias in that particular amp is probably the key to the percieved 'performance' change and not the tube itself.
    Also,popping a different rectifier tube into an amp without being aware of the changes in voltage to the whole amp is not advised.That means you have to be able to check the bias and see what is happening in there before you forge ahead.some amps there is no probalem at all and others can cause you grief.As the original question seemed like he wanted to put in a 5U4 in place of the 5AR4,it was not really an issue.He also did not say what amp he wanted to do this on.I retract my statement saying it's OK to do this.We need more info before making a judgement on it.
     
  15. brad347

    brad347 Member

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    I'm the guy who noticed the improvement in tone with the Mullard 5AR4 over the Sovtek. I can't explain it and I too was surprised... for the record I re-biased the amp to the proper bias point with each tube.

    Can I ask your opinion about power transformers? Do you think that two different power transformers can sound different all else being equal?
     
  16. billdurham

    billdurham Member

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    You didn't ask me, but I would like to respond. The "sound" of the power tranformer, from a technical standpoint is an apples and oranges question. When you think about the AC voltages that the power transformer provides...the high voltage for the tube plates gets rectified into DC and is AC isolated from the audio side of things by the choke. I don't think that a power tranformer has a "sound". It can definetly effect the sound of an amp if it is unable to provide enough current to the amplifier, but as far as having a sound..the only sound it has is a pretty ugly 60hz.
     
  17. phsyconoodler

    phsyconoodler Member

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    The only way you could notice an improvement in sound by changing a rectifier tube is if the old one was on it's way out.Or if the final rectified voltage is different than the original tube.A rectifier tube ONLY converts AC to pulsing DC voltage.Nothing more and nothing less.
    Billduram amswered the question about the power transformer.
    The phenomenon of IMPROVED sound you are hearing is an direct relation to what the voltage was before and after the tube change.Yes,a mullard is more durable and much,much better tube than the Sovtek is.
    So if the voltage is less,then your amp likes to be there.If it's more,the same applies.The resultant voltage does more than change the power tube bias,it also raises or lowers the preamp side too.That can be good or bad.In your case it's good from a sonic point of view.
     
  18. brad347

    brad347 Member

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    cool. Thanks for the reply.
     
  19. VintageJon

    VintageJon Member

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    The level of DCV a 5AR4/GZ34 provides is bit higher than a 5U4, and this effects the output stage as having slighly more head room. The same applies to pre-amp stages as we are talking about B+ here.

    It 's not apparent right off the bat, depending on your ears, but it will make a diffference in tube life and sublte tone.

    -Jon
     
  20. VaughnC

    VaughnC Supporting Member

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    Another word of caution....I replaced a 5U4 in one of my amps with a 5AR4 and it increased the high voltage on my first 450 volt rated filter cap from 435 volts to 465 volts. Safe, maybe....but I didn't want to risk it with Chinese caps.
     

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