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    6CA7 or KT77

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    Pillo69

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    6CA7 or KT77

    Post by Pillo69 on Wed Feb 01, 2017 5:35 pm

    I have been using EH-6CA7 for 6 years on an ST70.
    A tube has broken, and thinking of a new quartet I have heard that the kt77 have a similar sound improving the treble.
    Can anyone inform ?.
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    Bob Latino
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    Re: 6CA7 or KT77

    Post by Bob Latino on Wed Feb 01, 2017 8:14 pm

    Pillo69 wrote:I have been using EH-6CA7 for 6 years on an ST70.
    A tube has broken, and thinking of a new quartet I have heard that the kt77 have a similar sound improving the treble.
    Can anyone inform ?.

    The Genalex Gold Lion KT66 is a nice tube in any ST-70 type amp. I have not heard their KT77 but they also get good reviews.

    Bob
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    Pillo69

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    Re: 6CA7 or KT77

    Post by Pillo69 on Thu Feb 02, 2017 6:47 am

    Thank you for your attention, Bob.
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    Dave_in_Va

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    Re: 6CA7 or KT77

    Post by Dave_in_Va on Thu Feb 02, 2017 11:11 am

    This is all so subjective but I've tried several sets of power tubes in my VTA ST70. I ended up with a very nice quad of Mullards from around 1970 but the second best sounding set were the (WAY cheaper) Valve Art tubes that came with it.
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    Bob Latino
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    Re: 6CA7 or KT77

    Post by Bob Latino on Thu Feb 02, 2017 11:29 am

    Dave_in_Va wrote:This is all so subjective but I've tried several sets of power tubes in my VTA ST70. I ended up with a very nice quad of Mullards from around 1970 but the second best sounding set were the (WAY cheaper) Valve Art tubes that came with it.

    Those Valve Art EL34's as well as the Ruby EL34's are actually "rebadged" Shuguang EL34's. Shuguang is probably the best Chinese manufacturer of vacuum tubes. Shuguang also makes a real premium line of output tube with their "Treasure" series tubes.

    For those that want an EL34 tube set with their the VTA ST-70 amp kit, I ship a matched quad of Shuguang EL34's.

    Back in 2009 I listened extensively to the Shuguang EL34's. Excellent sounding, durable and inexpensive. See link below ..

    Shuguang EL34 post from March of 2009

    Bob
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    Pillo69

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    Re: 6CA7 or KT77

    Post by Pillo69 on Thu Feb 02, 2017 2:02 pm

    Bob Latino wrote:
    Pillo69 wrote:I have been using EH-6CA7 for 6 years on an ST70.
    A tube has broken, and thinking of a new quartet I have heard that the kt77 have a similar sound improving the treble.
    Can anyone inform ?.

    The Genalex Gold Lion KT66 is a nice tube in any ST-70 type amp. I have not heard their KT77 but they also get good reviews.

    Bob


    I got a quartet of KT66 Genalex Gold Lion.

    Which Bias setting is necessary ??.
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    Pillo69

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    Re: 6CA7 or KT77

    Post by Pillo69 on Thu Feb 02, 2017 2:37 pm

    I found the thread.

    Required 45mA., For original ST70 (resistance of 15.6) the BIAS setting is 1.4VDC.

    http://dynacotubeaudio.forumotion.com/t3043-new-bias-recommendations-for-all-vta-amps?highlight=bias+kt66


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    tubes4hifi
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    Re: 6CA7 or KT77

    Post by tubes4hifi on Thu Feb 02, 2017 2:38 pm

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    Pillo69

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    Re: 6CA7 or KT77

    Post by Pillo69 on Thu Feb 02, 2017 2:58 pm

    I tried the Bias setting and the minimum allow is 1.65v (53mA per valve).

    Do you have to make any changes ?.
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    Pillo69

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    Re: 6CA7 or KT77

    Post by Pillo69 on Thu Feb 02, 2017 8:10 pm

    The inability to adjust the bias to 1.40-1.56v. (KT66). It may be due to the placement of resistors (10k and 20k) in parallel to the existing 10k resistors in the 7-pin terminal ?.
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    tubes4hifi
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    Re: 6CA7 or KT77

    Post by tubes4hifi on Thu Feb 02, 2017 8:51 pm

    yes, those resistors affect the range of adjustment
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    Pillo69

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    Re: 6CA7 or KT77

    Post by Pillo69 on Fri Feb 03, 2017 11:30 am

    I have left only the two 10k resistors in the polarization circuit.

    The minimum setting for the bias potentiometers is 1.58v., I think it's still a little high.

    Can this be solved? Or should I keep it?


    stewdan

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    Re: 6CA7 or KT77

    Post by stewdan on Fri Feb 03, 2017 1:36 pm

    Hi --- for modern power tubes, 1.58v of bias voltage is high and will degrade the tubes faster.  On "stock" ST-70's or MK III's with a diode replacing the selenium rectifier I usually use 1.25v and adjust the setting accordingly.

    There were a couple comments made by Bob Latino back in January 2010 on the matter of Bias Resistor Values.

    If you do an Advanced Search looking for " Always, always, always replace the bias selenium rectifier with a silicone diode!!  " and Bob Latino's name as author and you will get the Forum Topic that is in quotes.

    Bob's recommendations are for replacing the Bias resistors on a ST-70.  Post Numbers 4 thru 7 or 8 have the info on resistor values that will work to lower the bias voltage from the 1.5v+ value.

    Hope this helps!!
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    Pillo69

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    Re: 6CA7 or KT77

    Post by Pillo69 on Fri Feb 03, 2017 6:07 pm

    I have 1N4007 diode, I also did the parallel resistor modification mentioned by Bob in message number 4.
    http://dynacotubeaudio.forumotion.com/t243-always-always-always-replace-the-bias-selenium-rectifier-with-a-silicone-diode?highlight=selenium+rectifier

    This modification only adjusts minimum of 1.65v with the potentiometer (KT66), with a 6CA7 the adjustment of 1,56v is placed in the center.

    I removed the resistors in parallel and got a minimum adjustment of 1.58v (KT66).

    The power transformer "is not original", it is new to 240v and extended current.



    stewdan

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    Re: 6CA7 or KT77

    Post by stewdan on Sat Feb 04, 2017 5:09 pm

    Pillo69 --- try paralleling a 10K resistor between lugs 1 and 2 of the 7-lug strip and see if that allows you to get lower than 1.58v.

    Two 10K resistors in parallel gives you 5K effective resistance. If the two 10K resistors drop the Bias voltage below 1.58v, and you are happy with it, then you are done.

    See DMTPARKER's Post #8 of Bob's "Always replace selenium rectifier with a diode" posting for his attempt at dropping the voltage.

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    Pillo69

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    Re: 6CA7 or KT77

    Post by Pillo69 on Sun Feb 05, 2017 5:58 am

    Thanks for your help, stewdan.

    To obtain a higher negative voltage, necessary for the kt66, I have placed a resistance of 15k in parallel to the resistance of 10k between terminals 3 and 4.

    Currently, the polarization setting can be perfectly realized.
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    MarcVBelgium

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    re : 6CA7 or KT77

    Post by MarcVBelgium on Sun Feb 05, 2017 1:10 pm

    Bob Latino wrote:
    Pillo69 wrote:I have been using EH-6CA7 for 6 years on an ST70.
    A tube has broken, and thinking of a new quartet I have heard that the kt77 have a similar sound improving the treble.
    Can anyone inform ?.

    The Genalex Gold Lion KT66 is a nice tube in any ST-70 type amp. I have not heard their KT77 but they also get good reviews.

    Bob


    I'm using Genalex Gold Lion KT77 in my rebuilt MkIV's......Wonderful.......I surely can recommend them.

    In my ST-120 I'm listening to Shuguang Black Treasure KT88, NOS 12AU7 and NOS GZ37........also a dream to listen to !
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    arledgsc

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    Re: 6CA7 or KT77

    Post by arledgsc on Sun Feb 05, 2017 1:31 pm

    I replaced EL34s in a Marshall style guitar amp with JJ KT77s. They bias right up and give the amp more balls. The KT77s compliment the rounder tones of a Fender Strat especially the wire wound strings. The Gold Lion KT77s are supposed to be even better.
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    Pillo69

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    Re: 6CA7 or KT77

    Post by Pillo69 on Sun Feb 05, 2017 2:17 pm

    The first impression with the Genalex Gold Lion KT66 is a very pleasant sound, higher treble and clearer.

    The polarization setting is set to 45mA, as recommended by Bob.
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    Pillo69

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    Re: 6CA7 or KT77

    Post by Pillo69 on Mon Feb 06, 2017 6:16 pm

    A potentiometer for the polarization with a wider range of adjustment, can lower the adjustment and allow the use of all types of valve without modifying circuit resistances ?.
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    Pillo69

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    Re: 6CA7 or KT77

    Post by Pillo69 on Wed Feb 08, 2017 7:00 pm

    A small musical fragment, after the polarization adjustment with the KT66.


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    Re: 6CA7 or KT77

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