Tube Bias Question

Discussion in 'Audio Hardware' started by Guitarded, Oct 10, 2018.

  1. Guitarded

    Guitarded Forum Resident Thread Starter

    I have a Cayin A50T-ii and recently had them (Cayin) send me a matched set of Gold Lion KT-77 tubes to replace the factory EL34 tubes.

    I switched the amp over to KT-88 Mode, installed the KT-77 set in the order it was marked and set about to Bias Adjustments per their suggested setting of .30v.

    The problem I am running into is that the Left Channel - Tubes(V1 + V2) will only come up to .28v + .26v respectively, at max. adjustment.

    So. My question is :

    Should I bring all the tubes to the same common level of .26v? Or, should I be running as close or at the reccommended .30v for all the tubes that will achieve it and just leave the ones that won't at their max settings?

    Thanks in advance for any helpful tips or knowledge.

    I know some stuff. But nothing about this sort of thing.
  2. Helom

    Helom Forum Resident

    I had this problem with one of the tubes in my Cayin. I just swapped positions until I could get them all to bias at the correct voltage. Set the bias, wait 30 min and check again.
    Guitarded likes this.
  3. Rolltide

    Rolltide Forum Resident

    Vallejo, CA
    KT 77 is a drop in EL34 replacement, not KT 88 despite the similar name. So unless I'm missing something switching it to KT 88 seems a mistake.
    progrocker, HiFi Guy and Helom like this.
  4. Guitarded

    Guitarded Forum Resident Thread Starter


    Thanks for that...that would probably be my problem.
  5. chervokas

    chervokas Forum Resident

    Yup, almost certainly. I don't know what KT-88 mode setting does on your amp, but a KT-77 is not a KT-88. A KT-77 is a beam tetrode that's a substitute for an EL34 pentode; a KT-88 is is a substitute for the 6550 beam pentode. Probably you should be running a KT-77 as if it was an EL34 with whatever the appropriate setting is for the amp.
    HiFi Guy likes this.
  6. Guitarded

    Guitarded Forum Resident Thread Starter

    Thanks, guys. This was the issue.
    Switched it back and dialed the bias in last night.

Share This Page