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    Bias options

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    Gregg R.

    Posts : 80
    Join date : 2014-02-20
    Age : 71
    Location : Land of 11,842 Lakes

    Bias options

    Post by Gregg R. on Thu Jul 17, 2014 9:04 pm

    I recently purchased a matched quad of inexpensive Shuguang EL34s from eBay to try in my VTA-70. They show no signs of distress after about 40 hrs. and all have biased the same at about the 1:30 position on the bias pots for a setting of 0.40v.

    Shuguang EL34

    I have tried bias settings from 0.35v to 0.45v, with the higher setting giving a more detailed sound, but the lower setting seems to have more of that "tube magic." How high/low can I go before I create problems for the VTA-70? At only $34 for the EL34s I'm not really concerned that much about extending the life of the tubes.

    I marginally prefer the Shuguangs warmth and openness to the cryoed EH6550s I had been using ($159). I also like the looks of the smaller bottle. The 6550s look a little too chubby for the chassis.

    Thanks for looking,

    Gregg R.

    I hear larger differences swapping driver tubes than in switching power tubes. Is that to be expected?


    Last edited by Gregg R. on Fri Jul 18, 2014 1:37 am; edited 1 time in total (Reason for editing : Added Question)

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Bias options

    Post by Bob Latino on Fri Jul 18, 2014 10:54 am

    Hi Gregg,

    Roy Mottram and I both recommend a .40 VDC bias point for each EL34 output tube in a VTA boarded ST-70. You can probably use anywhere between .35 and .50 VDC per each output tube. Yes - running the output tubes at higher bias settings will shorten tube life. Running the output tubes at higher bias settings will also make the amp run a little warmer than normal. It's your amp and you are free to try different bias settings.

    Those Shuguang EL34 output tubes are a steal for $34 for a matched quad. Shuguang is IMHO the best Chinese manufacturer of output tubes. These tubes last a long time and sound great. These same Shuguang EL34 output tubes are also rebadged and sold under the Valve Art and Ruby brand names.

    Yes - swapping driver tubes (especially the center driver tube) has more effect on the sound of the amp than swapping output tubes.

    Bob

    Gregg R.

    Posts : 80
    Join date : 2014-02-20
    Age : 71
    Location : Land of 11,842 Lakes

    Re: Bias options

    Post by Gregg R. on Sat Jul 26, 2014 8:11 pm

    Thanks for the info, Bob:

    1. Setting the bias point at 0.425v seems to sound best for me in my 4.1 system(described in an earlier post). The bias is stable and the Shuguang EL34s are my preferred output tubes for now.

    2. 6L6 power tubes are common in guitar amps but rarely used in hi-fi amps. The same eBay seller offers Shuguang 6L6GCs for only $30/quad. Have you tried 6L6s in your amps?

    3. I measured my VTA-70 amp temps with a laser thermometer. The results are at the end of my Picasa build album:
                    VTA-70 temps

    Gregg R.

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Bias options

    Post by tubes4hifi on Mon Jul 28, 2014 6:02 pm

    nice set of photos Gregg!
    I've had several customers try 6L6GC tubes in their ST70s, many people like the sound of them. For $30 a quad you can't go wrong.
    Just FYI, the 6L6 is a beam tetrode tube, as compared to the EL34 /6CA7 which is a pentode tube. The 6L6 is the most popular power tube EVER used,
    it's been used as the primary power tube in guitar amps for 80 years now!!! It's drop in replacement, the pinout is the same, just missing a grid,
    which in the pentodes is usually tied to the cathode. Another variation of this tube is the KT66. (also FYI the KT designation means "kinkless tetrode).
    The KT series of tubes is very popular, the KT66, KT77, KT88, KT90, KT120

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