Changing power tubes w/o bias

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Norma Schock, Nov 27, 2019.

  1. Norma Schock

    Norma Schock Member

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    Got a new amp, it's vintage with two 7027a power tubes. The ones it came with are not matched as one is larger than the other. I have an ampeg vt22 that has four old 7027a tubes, so I took two of those and put it into the new amp to test it out before I bring it to a tech to have is formally biased with new tubes, etc. I played it for 15min and works with no problems. I just wanted it to have matched tubes to test before bringing to tech, as I bought off reverb without hearing it.
    Just wondering, because it wasn't biased for the for the two 7027a's that I put in, could this damage the amp in any way, and thus should I have the tech address any potential issues from that? For all I know, it could have been forever since last biased, or even biased for the ones it came with; one owner amp from 1966. Thanks
     
  2. xtian

    xtian Member

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    The only issue is having the tubes burn up, and the possibility of taking out other components when they go. If they're biased too hot, they will red plate (start glowing red or orange, but not just from the heater elements...the whole plate structure glows) and then they'll eventually die.

    BTW, having two different size glass bottles does not, in itself, mean the tubes are mismatched. It's a matter of how much current each tube draws, and that's different for each tube, even with the same exact manufacture.
     
  3. Tron Pesto

    Tron Pesto Member

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    The reality is that the amp is designed to run 7027A tubes (btw - I'm very familiar with VT-22 amps...). So what's the worst that could happen?

    But before that, let's discuss what "matching" means. First, it does not have anything to do with matching the manufacturer, size, or appearance of the tube. It has to do with the PERFORMANCE (typically measured in plate wattage dissipation) of the tube. Forget about what it looks like. So l let's explore things based on what is the effect on the amp, or performance if two tubes are dissipating significantly different wattages..

    1) In a push pull amp (which this amp is), such a discrepancy will likely result in a noisy amp with a loud hum. The hum will be due to the imbalance offsetting the "humbucking" noise cancelling effect of a nice balanced set. While there are other things that can lead to "hum", if the amp is quiet, it's likely the output tubes are well balanced.

    2) There might not be much of a "hum" but maybe may tubes are running too "cold". In this case, the signs would be an amp that lacks volume, punch, and dynamics. It may also exhibit "unpleasant" distortion at relatively low volumes. Most people would not be happy with the amp.

    3) The tubes are balanced, but running too hot. The amp may actually sound pretty impressive in this case, but that won't last for long as you output tubes will fail sooner than later. You would probably be able to physically see the effect of this with red/glowing output tube plates. This is called redplating. The plates are the large metal structures that are the main visible elements of the tubes. These structures will begin to glow.

    4) There is an imbalance and one tube is running too hot. See the above and imagine one tube is "red plating."

    So what does this all mean? If the output tubes are not red-plating, and the amp is quiet at idle, AND when pushed, the amp is nice an punchy and full of tone... you're good.

    That does NOT preclude the BEST way to approach this: get it to a technician that knows what they are doing and have them check the amp out front to back.

    On another note - there are manufacturers claiming to manufacture new 7027A tubes. The funny thing is that the stated specs hint that these tubes are simply "decent" 6L6 tubes labeled as 7027As... My experience it that the best 7027A substitute manufactured today is the Sovtek 6L6WXT+ tubes. I have installed them in two different VT-22 amps from the early 70s and they are killer and stand up to the abuse.
     
  4. Norma Schock

    Norma Schock Member

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    What other components would be at risk other than the tubes (just to know). In my situation I just played it for 15min, and didn't really drive the tubes hard as I had the volume at a lower level for the most part.
     
  5. Tron Pesto

    Tron Pesto Member

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    The risks to the amp in the situation you describe are minimal. Arguably a screen resistor could go south, but unless you are really pushing that amp HARD for an extended period, not much could go wrong other than some early tube death.

    HOWEVER, in these amps, the most common thing to go wrong in an imbalanced output or an amp that is started up after a long time (and suspect to an unusual voltage peak), are the frying of the "flyback" diodes.

    The flyback diodes are essentially safety components installed in these amp connected between each side of the output transformer primary and ground. If the connection to the speakers is interrupted (or not connected at all and a large transient is introduced), the output transformer could sorta have a "reflective high voltage reaction". These components (diodes) are there to keep these transients from damaging the output transformer, but when exposed to repeated abuse, they can fail (short) and the amp will continually blow fuses until the diodes are replaced (or removed).
     
    Last edited: Nov 28, 2019
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  6. pdf64

    pdf64 Member

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    It's worth noting that for a plate to glow red hot, it would normally have be dissipating somewhat above its absolute max limit continuously for more than a few seconds. Metal structures can change when heated to that level, and importantly for tubes, gas molecules normally trapped within the metal crystals may become released and so degrade the vacuum. Hopefully its getter flashing can catch them before they cause trouble, but they can only cope with so much.
    So, whilst red plating is a fairly sure indication that the plate is overdissipating, the absence of red plating should not used as an indication that the plate isn't overdissipating.
    My understanding is that a tube's plate doesn't red plate when it reaches 100% of its design centre limit (maybe ~30% safety margin), or 100% of its design max limit (maybe ~15% safety margin) or 100% of its absolute max limit, but at some level above that.

    As described above, a plate that is red hot will release gas molecules; if positively charged, they will be attracted to the negative voltage of the tube's control grid. That will reduce the negative voltage, the plate will pass more current, and the tube may spiral into self destruction / catastrophic failure. Hopefully a fuse would blow and so protect the output and power transformers from the fault current drawn by the tube, but there's no certainty in that, and vintage transformers may not cope with too may such incidents before they fail (and who knows how many they've already been exposed to?).
     
  7. Norma Schock

    Norma Schock Member

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    Thanks for all this info, concerning the Sovtek 6L6WXT+ tubes, you can just put those (or any 6L6 tube) in place of a 7027a?
    Also, should an amp be regularly biased even if the tubes are not replaced?
     
  8. crumb

    crumb Member

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    If you're going from 7027a to 6l6gc there is no problem at all.

    Going from 6l6gc to 7027a can be a different story so if you're reading this and want to try that, there is a small thing you have to check first.

    regularly bias? naaah, just a ' top up' a few days maybe 10 hours use after you put your new ones in, they can drift a bit after they're put in.
    If you drop the amp while it's hot, or it gets hit hard when it's hot, I'd check the bias current then too.
     
  9. Vanyu

    Vanyu Member

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    Are you sure your old 7027’s need replaced? They’re tougher than nails. Just because they’re physically different doesn’t mean they’re not matched. 7027’s are getting scarce, save that matched quad for something special :)
     
  10. Norma Schock

    Norma Schock Member

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    Is there a way to test how much life you have left in a tube? The quad of 7027 will stay in the ampeg vt22. Maybe then I'll stay with the two different sized 7027a 's that came with the new addition.
     
  11. Vanyu

    Vanyu Member

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    Not definitively, as long as your amp works good and sounds good and the tubes aren’t super microphonic, then you’re good. 7027’s are pretty stout, they probably still have some years left in them
     
  12. HotBluePlates

    HotBluePlates Member

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    If you have to ask, you're probably not set up to measure/estimate tube life. :D

    Some Hickok tube testers include a "Life Test" function. You test the tube's transconductance ("Gm") normally, then throw the Life Test switch. This lowers the heater voltage feeding the tube, and you continue to monitor the tube's Gm reading. If the reading stays in the same place (or drops only a little and/or slowly), this is interpreted as the cathode having plenty of reserve emission capability, and lots of remaining life. If the reading falls away quickly and/or very far, this is interpreted as the cathode not having much reserve emission. The reduced heater power is actually lowering the cathode's ability to emit electrons.

    The classic test for power output tubes (which no one seems to say they do) is to put the tubes in a test amplifier setup (with a specified power supply, transformer impedance, bias and test signal size), attach a dummy load, apply a test signal, and measure the actual power output of the tubes. Here, you're testing the tube in the manner in which it will be used, and you get direct apples-to-apples info on how the tube performs. The catch is you need to make/have your own amplifier for the power output test, and you need to have developed a baseline of what new tubes will do in the amp.

    A way around that is to decide on a drive signal, perhaps one that peaks maybe 2 volts less than the bias voltage of the output stage. Measure peak a.c. volts at the grid of a tube from one side of the output stage to the grid of a tube on the other side of the output stage (this lets you know you're applying a signal that's as big as the output tubes can handle without overdriving). Measure volts across the dummy load, and calculate power output. The power output of old tubes can be directly compared to the power output of new tubes.

    It's important to note the above test will tell you how much power your tubes are making, and whether that's much below the power output you could be getting with new tubes. It will not tell you when to declare the old tubes are too weak to continue to use. That is a judgment call you have to make, as tubes that test very weak in a tube tester or in a power output test may sound just fine in your amp (albeit not making as much power output as you could get with new tubes).​

    I think I remember even the Hickok tube tester instruction manual saying, "The best test for any tube is the socket/equipment in which it will be used." If the tube seems to provide subjectively acceptable performance when you plug it in an amp, then it's still useful.
     
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