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I have some general Q's about tubes!

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  • I have some general Q's about tubes!

    I purchased my first tube amp in December, and fell in love with it. I just got another one this past weekend. I have some questions about them. Feel free to respond to all or one of these following questions:


    What is the purpose of the metal covers that go over the preamp tubes? Does it hurt anything to use the amp while these are removed?

    I have heard that if you replace a power tube with another tube of the same hardness rating, the tubes don't need to be biased. Is this true?

    I have a 5 watt tube combo. I have been searching and searching, and I cannot for the life of me find a bias pot. Could it be that this amp, although cheap, simple, and featuring only one power tube, does not need biasing?

    If you don't connect a speaker load to a tube amp, you can damage the amp, as I am told. What if, instead of connecting a speaker, I connect a resistor of precisely the same wattage and resistance of the speaker that the amp is made to use? EX: It's a 5 watt tube combo with a built in 8 ohm speaker. I hook up a 5 watt 8 ohm resistor instead of the speaker. Theoretically, would this work? Would there be any damage?

  • #2
    The metal covers are shields. They keep the tubes from picking up noise from the environment they're in. They're not critical, but you'll likely want to leave them.

    Your amp is probably cathode biased, which means there's a fixed resistor between he cathode and ground which biases the amp. Translation, there is no bias pot like you're thinking, which is used in large push-pull power amp designs. What kind of amp do you have?

    Yes, you can replace the speaker with a resistive load, this is done alot when diagnosing an amp because no one wants to hear a 1kHz sine wave at full volume. You'll want to oversize the resistor some. So for your 5 watt amp, I'd do a 10 watt resistor minimum, and if it's not too expensive, look at 15 or 20 watt. The resistance you'll want to be the same as the speaker impedance. So if you've got an 8 ohm speaker, find an 8 om resistor. Mouser, Digikey, Allied, Jamco, Newark, will all have these kind of wire wound power resistors.


    • #3
      OK, here goes...

      1. The shields stop the tubes from picking up hum, radio signals, etc. In an amp where the tubes hang upside down, the spring inside the shield also stops the tubes from falling out. If your amp doesn't make weird noises with the shields removed, and the tubes stay in, go ahead.

      2. If you mean the rating system used by Groove Tubes, the answer is "Probably". Different manufacturers' rating systems don't correspond, though.

      3. If your amp is only 5 watts, then it's probably single-ended and cathode biased. There's no bias pot because no bias adjustment is needed, just stick in any tube of the same type and rock out. Cathode bias is self-adjusting and will compensate fairly wide variations in tubes.

      4. Substituting a dummy load for the speaker like that is just fine.
      "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"


      • #4
        4. If you can't find an 8 Ohm resistor, 8.2 Ohms or even 10 Ohms will work OK.


        • #5
          And consider that a guitar amp is not a precision instrument, and none of these paramaters are critical. If the impedance of the speaker load is reasonably close, the amp will be fine. And while some people chant "bias bias bias" like a mantra, it just is not all that critical. I liken it to tire pressure in your car. No one would worry if the tire pressure was a pound off from the recommended amount, unless you were a professional race driver. SO don;t worry if your bias is not some exact number.
          Education is what you're left with after you have forgotten what you have learned.


          • #6
            ok, thanks for all your help everyone!


            • #7

              I have an Allen Hot Fudge w/Nuts -which is like a cross between a Deluxe/Princeton but with a more robust output of about 35 watts with 6L6's/EL34's or 20 watts with 6V6's. this is a fixed bias amp but with a potentiometer on the underside of the chassis to set the bias. for someone that can't wrap their head around that concept, I think that some amps have a variable bias circuit that depends on amp fluctuations. i am not sure but i think that's the jist.

              anyway, I adjust the bias on my amp by ear. and in the case of the 6L's/EL34's adjust very hot. i have to back off a little on the 6V's or they fry.