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    ST120 KT88 bias for long life

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    deepee99

    Posts : 1337
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: ST120 KT88 bias for long life

    Post by deepee99 on Tue Dec 22, 2015 11:48 am

    Skizzy,
    Did I not read that the current and voltage needs of those guys were pushing the VTA iron a bit to the hot side? Please correct me.
    Tnx
    d

    Tubes4ever

    Posts : 61
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: ST120 KT88 bias for long life

    Post by Tubes4ever on Tue Dec 22, 2015 12:50 pm

    deepee99 wrote:Skizzy,
    Did I not read that the current and voltage needs of those guys were pushing the VTA iron a bit to the hot side? Please correct me.
    Tnx
    d

    The 5V3 uses 3.8 amps at 5V.  The 5V3A uses 3 amps, just like the 5U4GB.  However, the 5V3A is rated at 350mA instead of the 275mA of the 5U4GB.  The 5V3A is actually a better choice as it has a lower voltage drop at the currents we use than a 5U4GB.
    The 5AU4 is not electrically identical to the 5V3A.  You can use a 5V3A in place of a 5AU4. They are similar in plate current ratings.
    The 5AU4 uses 3.75 Amps.


    Tim


    Last edited by Tubes4ever on Tue Dec 22, 2015 12:57 pm; edited 1 time in total (Reason for editing : Additional info)

    jasn54

    Posts : 30
    Join date : 2015-04-13

    Re: ST120 KT88 bias for long life

    Post by jasn54 on Sun Feb 28, 2016 5:48 pm

    Bob Latino wrote:The Mullard CV4003 (12AU7) is a really great tube and built to standards set up by the British military...To save money, as Gregg has mentioned, you only need one for the center position. Bob

    Hey Bob, would you or some other, kind and knowledgeable person, please provide a little more info about why only the center position would benefit from a better tube?  How might one center Mullard match with a pair of the Beckman 5963's I acquired with the original kit?

    Kentley

    Posts : 335
    Join date : 2015-03-06
    Age : 64
    Location : Worcester, MA

    Re: ST120 KT88 bias for long life

    Post by Kentley on Sun Feb 28, 2016 5:59 pm

    jasn54 wrote:
    Bob Latino wrote:The Mullard CV4003 (12AU7) is a really great tube and built to standards set up by the British military...To save money, as Gregg has mentioned, you only need one for the center position. Bob

    Hey Bob, would you or some other, kind and knowledgeable person, please provide a little more info about why only the center position would benefit from a better tube?  How might one center Mullard match with a pair of the Beckman 5963's I acquired with the original kit?
    The center tube is the driver which amplifies the signal - as it is a dual-triode, it does both channels. The two outer tubes are "merely" phase inverters, in which one of the dual triodes supplies one of the push-pull output tubes with an exact out-of phase signal so that the output tubes can work in tandem. Thus, the two outer tubes are not critically involved with modifying anything in the signal besides phase. I hope that is accurate and it helps you understand.
    Kind, I am. Knowlegeable, perhaps not so much.... Laughing

    jasn54

    Posts : 30
    Join date : 2015-04-13

    Re: ST120 KT88 bias for long life

    Post by jasn54 on Sun Feb 28, 2016 6:28 pm

    You are all of the above Kentley, thank you. So...do others merely swap the center? (I suspect not...

    Kentley

    Posts : 335
    Join date : 2015-03-06
    Age : 64
    Location : Worcester, MA

    Re: ST120 KT88 bias for long life

    Post by Kentley on Sun Feb 28, 2016 6:52 pm

    jasn54 wrote:You are all of the above Kentley, thank you. So...do others merely swap the center? (I suspect not...
    Many of us have found, IIRC, that the center driver tube is the single user-switchable element in the ST-xxx amps that has the most influence on the sound. And since every system is different, and every set of ears has different preferences, there is no predicting how this one critical tube will fare in any situation. Try a 12BH7 as well - for many it gives a slightly darker and ballsier sound. GE made a great 12BH7.
    In the outer, splitter holes, just about any pair of good-functioning, matched 12AU7s will do fine. I've personally never heard and discernable differences between anything I've used there. I use RCA 5963s that Bob L. originally supplied, simply because they are heavy-duty, reliable tubes.
    In short, there's a lot to be said for having several choices for that center baby. You never know what you like until you try for awhile. There's plenty of inexpensive choices on the market. Go for diversity until you find something that clicks.


    Last edited by Kentley on Sun Feb 28, 2016 11:17 pm; edited 1 time in total

    jasn54

    Posts : 30
    Join date : 2015-04-13

    Re: ST120 KT88 bias for long life

    Post by jasn54 on Sun Feb 28, 2016 7:01 pm

    Bravo Kentley! Well done! (I have Downtown Abbey reruns on right now ☺)

    deepee99

    Posts : 1337
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: ST120 KT88 bias for long life

    Post by deepee99 on Mon Feb 29, 2016 2:24 pm

    Tubes4ever wrote:
    deepee99 wrote:Skizzy,
    Did I not read that the current and voltage needs of those guys were pushing the VTA iron a bit to the hot side? Please correct me.
    Tnx
    d

    The 5V3 uses 3.8 amps at 5V.  The 5V3A uses 3 amps, just like the 5U4GB.  However, the 5V3A is rated at 350mA instead of the 275mA of the 5U4GB.  The 5V3A is actually a better choice as it has a lower voltage drop at the currents we use than a 5U4GB.
    The 5AU4 is not electrically identical to the 5V3A.  You can use a 5V3A in place of a 5AU4. They are similar in plate current ratings.
    The 5AU4 uses 3.75 Amps.


    Tim

    I'm sorry, I was thinking of 6SN7s; there are a couple of European variants that pull way too much filament current. Brain pharte on my part.

    Tubes4ever

    Posts : 61
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: ST120 KT88 bias for long life

    Post by Tubes4ever on Mon Feb 29, 2016 8:22 pm

    No problem Deepee.  It happens to all of us. drunken

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