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    ST70, Gold Lion KT-77's low bias

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    mmmalmberg

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    Join date : 2012-10-07
    Location : Orinda, CA

    ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Fri Mar 02, 2018 5:13 pm

    One of my four winged-C Svetlana's was flaring and my ST-70 was blowing fuses. I had first tried swapping rectifier tubes having blown a fuse before seeing the tube flaring.

    So ordered Gold Lion Kt-77's, can't bring the bias up past about 1.38VDC. Is this a risk of going to KT-77's? Bias was fine with the Svet's and whatever tubes I had before those.

    Thanks for any tips. I did a few searches but there are so many variety of low bias threads, I hope this exact question hasn't been discussed before (probably has...) if so just let me know.

    Any thoughts appreciated.
    -Mark M.

    p.s. No it's not stock, different driver board and upgraded p.s. and power xformer, but has been running fine for years 'til now...
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    Bob Latino
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    Re: ST70, Gold Lion KT-77's low bias

    Post by Bob Latino on Fri Mar 02, 2018 5:49 pm

    mmmalmberg wrote:One of my four winged-C Svetlana's was flaring and my ST-70 was blowing fuses. I had first tried swapping rectifier tubes having blown a fuse before seeing the tube flaring.

    So ordered Gold Lion Kt-77's, can't bring the bias up past about 1.38VDC.  Is this a risk of going to KT-77's? Bias was fine with the Svet's and whatever tubes I had before those.

    Thanks for any tips. I did a few searches but there are so many variety of low bias threads, I hope this exact question hasn't been discussed before (probably has...) if so just let me know.

    Any thoughts appreciated.
    -Mark M.

    p.s. No it's not stock, different driver board and upgraded p.s. and power xformer, but has been running fine for years 'til now...

    If your amp still has the original Dynaco bias system, try paralleling each of the two 10K resistors on the 7 lug terminal strip with another 10K resistor. This will alter the bias range to the point where you should now be able bias your tubes properly. This has come up before from people who have replaced the selenium rectifier with a diode. A diode has more forward voltage than the selenium rectifier and requires somewhat lower value resistors on the 7 lug terminal strip.

    Bob

    BNR_1

    Posts : 104
    Join date : 2013-06-11

    Re: ST70, Gold Lion KT-77's low bias

    Post by BNR_1 on Fri Mar 02, 2018 5:57 pm

    In case you wanted a bit more info to follow on to Bob's suggestion. I just read it yesterday so the timing was perfect.

    http://dynacotubeaudio.forumotion.com/t43-selenium-rectifier?highlight=selenium+rectifier+resistor+mod

    mmmalmberg

    Posts : 8
    Join date : 2012-10-07
    Location : Orinda, CA

    Re: ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Fri Mar 02, 2018 6:20 pm

    Thanks Bob and BNR. So, if I haven't replaced the rectifier with a diode, this is still a solution, yes?

    If I go back to EL34's am I likely to need to undo this mod?

    Thx again...

    BNR_1

    Posts : 104
    Join date : 2013-06-11

    Re: ST70, Gold Lion KT-77's low bias

    Post by BNR_1 on Fri Mar 02, 2018 6:48 pm

    mmmalmberg wrote:Thanks Bob and BNR. So, if I haven't replaced the rectifier with a diode, this is still a solution, yes?

    If I go back to EL34's am I likely to need to undo this mod?

    Thx again...

    Just to be clear we are speaking of the selenium rectifier. If the existing is still wired it should be replaced with a diode. 1N4007 silicon diode is adequate.

    Regardless of the tube you choose to use (EL34, KT77, etc.) the selenium rectifier should be removed and replaced with a modern diode.

    Separate of those questions, I would assume your selenium rectifier was replaced given the modifications you described on this amp.

    mmmalmberg

    Posts : 8
    Join date : 2012-10-07
    Location : Orinda, CA

    Re: ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Fri Mar 02, 2018 11:30 pm

    BNR_1 wrote:

    Just to be clear we are speaking of the selenium rectifier.  If the existing is still wired it should be replaced with a diode.  1N4007 silicon diode is adequate.

    Regardless of the tube you choose to use (EL34, KT77, etc.) the selenium rectifier should be removed and replaced with a modern diode.

    Separate of those questions, I would assume your selenium rectifier was replaced given the modifications you described on this amp.

    I don't remember replacing it but it doesn't mean I didn't, that was probably 12-14 years ago. I'll check - thanks:)

    Just to get further to the heart of things, is it typical or at least logical for some reason that the switch to KT-77's from EL-34's resulted in the inability to reach the correct voltage? (they were said to be "drop-in replacements")


    mmmalmberg

    Posts : 8
    Join date : 2012-10-07
    Location : Orinda, CA

    Re: ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Sun Mar 04, 2018 5:45 pm

    Checked both my st70's and selenium rectifiers are long gone. I have a CAE PC-S7UB power supply board in each.  There's no terminal strip as described, so I assume those parts may be on the p.s. board. Looks like maybe R15 and R16 per the docs http://www.curcioaudio.com/ST7U-B-DOC.pdf

    OK working as I write this, piggybacked 10K's on top of the 10K's (didn't have any 1W resistors of range to just replace). Happy Gold Lions.


    mmmalmberg

    Posts : 8
    Join date : 2012-10-07
    Location : Orinda, CA

    Re: ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Sun Mar 04, 2018 5:45 pm

    (thanks all:)

    mmmalmberg

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    Join date : 2012-10-07
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    Re: ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Sun Mar 04, 2018 7:59 pm

    Quick follow-up, is 1.56 (or measured resistor value / 10) still to be considered correct bias voltage with the kt-77's?

    mmmalmberg

    Posts : 8
    Join date : 2012-10-07
    Location : Orinda, CA

    Re: ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Mon Mar 12, 2018 1:06 pm

    Well in any case it's working and sounds as good as my ears can hear:) Now on to the intermittent hum/noise in my CJ PV-5...
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    Bob Latino
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    Re: ST70, Gold Lion KT-77's low bias

    Post by Bob Latino on Mon Mar 12, 2018 1:21 pm

    mmmalmberg wrote:Quick follow-up, is 1.56 (or measured resistor value / 10) still to be considered correct bias voltage with the kt-77's?

    If the board that is being used in your amp uses the original bias pots, then you would normally use the 1.56 bias point BUT > because of the state of today's output tubes, you should probably lower the bias point on your amp to about 1.25 VDC (actually 1.248 VDC) per each channel. This lower bias setting will give a bias current of 40 milliamps per each tube of the pair on each channel. Your tubes will last longer and the amp will sound the same.

    Bob

    mmmalmberg

    Posts : 8
    Join date : 2012-10-07
    Location : Orinda, CA

    Re: ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Wed Mar 14, 2018 11:00 am

    Thanks Bob - I had them at 1.5 and will drop to 1.25; much obliged.

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