Help interpreting bias rite results?

Discussion in 'Amp Central Station' started by decibel, May 25, 2021.

  1. decibel

    decibel Tele-Meister

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    Hey all,

    I have no idea what I'm doing with amps, so thanks in advance. Can I get some help interpreting bias rite results? Tube B is actually the first power amp tube. Tube A is the second one (this was confusing and stupid, but I wasn't planning to post a question when I did it). This is a fixed bias '62 Princeton. Here's the images of results:

    https://imgur.com/a/yINA1y4

    My questions:

    1. How much do these numbers matter? I put in some tubes that were a closer match (both around 19), and the amp sounded worse.

    2. Is it correct to say my first power amp tube is on the hotter side, and the second is running very cool?

    3. Is this a normal configuration? I did these by ear, and this combination sounds the best.

    Thanks!
     
  2. DavidP

    DavidP Friend of Leo's

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    No idea what you are doing with amps?? If a true '62 Princeton, you have a fixed bias resistor so I wouldn't venture under the hood to play around with that if you don't know your way around!!
    Given the values in your link, one (22.4/353) is running cool @approx 60%; the other (31.3/369) is running waay to hot!
    These are mismatched, some folks like the sound of that.
     
  3. Wally

    Wally Telefied Ad Free Member

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    Those voltage readings are odd, ime. One would not expect different voltages of that type, ime. I am going to think that this amp needs to be on the bench of an experienced tech. Has it ever been recapped?
    What were the voltage readings with those colder tubes...the ones that were drawing 19ma?
     
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  4. hepular

    hepular Tele-Afflicted

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    going to defer to Wally on this one.
     
  5. decibel

    decibel Tele-Meister

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    I'm not sure, but I will check tomorrow.

    It had caps put in in 2016 and everything was okay'd at the time, but I did travel cross country with it twice since then. A lot of bumpy roads. Could that cause it to fall out of spec?

    Thanks!
     
  6. Lynxtrap

    Lynxtrap Tele-Afflicted

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    The tube that draws more current on the cathode (higher Ik) should have lower voltage (Va).
    Try swapping the tubes A and B between the sockets and take a new reading.
     
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  7. fiveightandten

    fiveightandten TDPRI Member

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    Those tubes appear to be much too far off from each other to be run together. You need a matched pair.

    And if the are a matched pair, that amp needs to be looked at by an experienced tech.
     
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  8. decibel

    decibel Tele-Meister

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    The weird thing is they sound amazing.
    I have a much closer matched pair, and these sound better.
     
  9. fiveightandten

    fiveightandten TDPRI Member

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    Yeah, some imbalance can sound good, but it may not actually be the imbalance that you prefer. Tubes draw different amounts of current and you can hear the difference between them.

    The issue is that it’s drawing uneven amounts of current through the OT and the rest of the output section. When it’s uneven enough that the plate voltage is varying, that’s pretty significant. Especially in a vintage amp, I wouldn’t risk it, personally.
     
  10. Wally

    Wally Telefied Ad Free Member

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    Re: matched power tubes. I have seen exactly one perfectly matched set of power tubes in a guitar amp. A few months ago I bought a dead 1972 Princeton Reverb. Torn grille cloth with KILLERS spray painted. Bass woofer 10”. Recap, service, install an Oxford 10J4 from the second half of 1974...reconed by Weber 15 years ago and unused since. Got lucky.....great amp with the original trannies.
    For testing purposes, I pulled out a random set of used RCA 6V6s. When I checked the current draw, they were pulling a perfect match...27.6ma. It now has a set of NOS RCA 6V6GTs that are running a bit cooler than that. The Tremolo was good to go even at that 27.6ma, which was around 81% of max plate dissipation. One of the best Prin Rev amps I have come across. It was pulled out of an attic....it had been dead for years.
     
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  11. 2L man

    2L man Tele-Holic

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    Some difference between anode/plate voltages is normal when OT is wound "vintage" and other half of the primary fully first to the bottom and second over it where longer length of wire is needed, although winding count is the same, but there comes more resistance to it.
     
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