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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


    Tube question

    DavidR
    DavidR

    Posts : 139
    Join date : 2017-08-10
    Location : MetroWest, MA

    Tube question Empty Tube question

    Post by DavidR Tue Jun 29, 2021 9:07 pm

    I have an old Mullard 12AX7 i61 tube that measures strong on both triodes with no shorts or grid leakage on my Sencore TC142.

    The heater and cathode have always been quite dim compared to the other 2 tubes on my phono board which are variants of 12AX7.

    Is this any indication as to the tube condition or just the way it is and it means nothing?

    There are no issues with sound reproduction.
    DavidR
    DavidR

    Posts : 139
    Join date : 2017-08-10
    Location : MetroWest, MA

    Tube question Empty Re: Tube question

    Post by DavidR Tue Jun 29, 2021 10:54 pm

    Found my answer>>>

    Filament Glow
    The filament in a tube has the job of heating the tube. It is required to bring the tube up to a temperature where electrons will get excited and start flowing. The filament is also commonly called a heater for this reason. All common audio tubes will have a heater filament although in some tubes they are well hidden behind the plates and other elements making them difficult to see, even in a dark room. Regardless, if the tube is warm and sound is coming from the amplifier, you know the heater is doing its job no matter how bright or dim it appears. An excellent example of this is the reissue Tungsol 12AX7 tube shown in Photo 5. Its heater is extremely well hidden and casts little light. The Chinese 12AX7A also shown casts much more light even though it is operating in exactly the same circuit. On occasion we have people wonder why one tube appears brighter than another in a matched set they have. In almost all cases it’s simply a matter of one heater being more exposed than the other and therefore casting off more light. This is not a sign of a problem. Remember they are all hand made and a variance like this is normal. Photo 6 shows two EL34 tubes where one is brighter than the other due to the filament height. Both tubes test and perform perfectly.

    Another thing that needs mention is red-plating or "cherry" plate glow. This will happen when a tube is incorrectly biased, causing the plate to overheat. Generally speaking, tubes do not like this unnecessary stress and will not last long if rebiasing is not done. See Photo 7. In this case the plate itself is actually casting a red glow.

      Current date/time is Wed Jul 28, 2021 4:36 pm