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Swapping rectifier tubes; 5AR4 vs 5R4 vs 5V4 vs 5Y3

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mherrcat, Jan 29, 2014.

  1. mherrcat

    mherrcat Member

    Messages:
    22
    Joined:
    Sep 2, 2013
    This has probably been covered here before, but...

    My Allen Accomplice Jr. currently has a 5AR4 rectifier and RCA 6V6GTA power tubes in it. I want to see how different rectifiers affect plate voltage, cathode current and sound, but don’t want to do anything to damage the amp. (Duh!)

    I found this site with a lot of specs on various rectifier tubes:

    http://www.fourwater.com/files/fullrect.txt

    Admittedly I do not know what a lot of these numbers mean, except that using a rectifier that draws a higher filament current could cause transformer damage, so I was looking at tubes with ~2-amp current.

    Here are the specs of the tubes I was looking at:

    [​IMG]

    Here is what the current setup looks like. The first plate voltage is with maximum negative bias voltage applied. The rest of the numbers are after the bias voltage was set and is pretty self explanatory.

    [​IMG]

    So here’s the question: Considering the specs on the 5R4 and 5V4 and 5Y3 rectifiers, how will the numbers I am seeing in my amp change using each of them and how might it affect the sound?
     
    Last edited: Jan 29, 2014
  2. wyatt

    wyatt Member

    Messages:
    4,150
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    Oct 11, 2005
    Of those, the 5Y3GT is ill-suited for the application for two reasons. 1) The Accomplice uses a 40uF first node filter cap, the 5Y3GT datasheet recommends the tube be followed by no more than 20uF. 2.) It has supplies a maximum current of 125mA (MaxPmA), which is borderline when cranking the Accomplice, and probably not enough if you switch to 6L6GC's. The tube will burn out prematurely if the amp demands more than 125mA on a regular basis.

    The Vdrop is the voltage drop across the rectifier, those numbers (-17, -25, -60, -67) are over-simplified, the actual Vdrop will depend on the actual incoming AC voltage becing rectified. The 5AR4/GZ34 has the least Vdrop, then the 5V4, then the 5R4 and finally the 5Y3. Higher voltages mean more headroom and tighter feel. Because the plate voltage changes, you should check bias after swapping rectifiers.

    A side-effect of the voltage drop is the sag it adds. The more Vdrop, the less efficient the rectifier is, meaning it takes longer to supply the current. This adds Sag. When you pick a note or strike chord, the amp demands more current and the less efficient rectifiers can't supply it right away, so you're note/chord sort of drops in volume before ramping back up again. The more the amp is cranked the prominent the sag.

    Ultimately, the differences will be subtle.
     
    Last edited: Jan 29, 2014
  3. mherrcat

    mherrcat Member

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    Sep 2, 2013
    So, the 5R4 would have more "sag" but still be OK in terms of the current it can supply? David Allen did say the 5V4 would be OK in that amp, but I didn't ask him about the others at the time.
     
  4. Diablo1

    Diablo1 Member

    Messages:
    620
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    Nov 22, 2013
    Yes, the 5R4 would be OK to use, as it can supply all the current and more than your amp could use. The amount of sag is a function of the voltage drop at the current rating. The 5Y3 would have the most sag, because it's 60 volt drop occurs at .125 amps. The 5R4 drops 67 volts at .250 amps. So, if we assume the voltage drop is roughly linear with current, the 5R4 would drop 33.5 volts at .125 amps. When you measure the B+ voltage in your amp, you'll be doing so at idle, when the amp draws something like .05-.075 amps. Expect to measure less voltage drop because you're drawing less current at idle. Play the amp to judge the sag effect with each rectifier.
     
  5. mherrcat

    mherrcat Member

    Messages:
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    Sep 2, 2013
    Thanks! I ordered a couple of different brands of 5R4 and 5V4 to try. Also looking at different 6V6 tubes as well. If anyone can point me to a comprehensive comparison that would be great!
     

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