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    Retubed ST-120

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    arledgsc

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    Join date : 2012-11-30
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    Retubed ST-120

    Post by arledgsc on Tue Jul 25, 2017 10:27 am

    I have racked up 1,000s of hours on my ST-120 over the past 5 years. I have not worked since March '17 and the ST-120 amp is on from 6am to late in the afternoon every day. So the two year old KT-120s were getting a little tired.

    I just installed my 3rd set of KT-120s received from brother Jim McShane. They bias right up with not much change from the old ones. More bass is the first improvement! Sound great right out of the box. I am very happy with KT-120s in the ST-120.
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    Bob Latino
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    Re: Retubed ST-120

    Post by Bob Latino on Tue Jul 25, 2017 5:48 pm

    arledgsc wrote:I have racked up 1,000s of hours on my ST-120 over the past 5 years.  I have not worked since March '17 and the ST-120 amp is on from 6am to late in the afternoon every day.  So the two year old KT-120s were getting a little tired.

    I just installed my 3rd set of KT-120s received from brother Jim McShane.   They bias right up with not much change from the old ones.  More bass is the first improvement!  Sound great right out of the box.  I am very happy with KT-120s in the ST-120.    

    If properly built, all the VTA amps are very reliable. The problem at times are the tubes. All tubes are consumables. After 3000 - 4000 hours the output tubes should be replaced. Some tube amp users try to milk out additional hours above and beyond the normal life of an output tube. If you do, sound quality will slowly erode and you may be setting your self up for a catastrophic failure. Sometimes the sound goes downhill so gradually that you may not even be aware of the change .. bass is reduced, the top end loses that sparkle but you slowly get use to it. How many times have you changed out the output tubes and noticed that the amp just sounded better? Driver tubes will last a little longer - maybe 5000 hours .. Also what Arledgsc said is quite true .. Buy your tubes from a reliable vendor. Jim McShane is one of a few tube vendors out there who really pretests the tubes and does a close match for his customers ... Bob
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    arledgsc

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    Re: Retubed ST-120

    Post by arledgsc on Tue Jul 25, 2017 6:53 pm

    And as luck would have it I had a KT-120 failure.  Sounded great for 2 hrs. then I heard a horrific, loud zap and no cathode current on one of the new tubes.  Put an old KT-120 back in and no problem with the other tubes. Jim was apologetic and said send it back.   

    And no fault of Jim it looks like the bad tube has an extra internal weld that I do not see on other KT-120s.?  It's pin 7 which is one side of the filament (heater).  A working tube pin 7 has one internal connection to the filament.  But the bad tube has a 2nd connection from pin 7 to a support post.  Take a look at the photo and zoom in.  On the upper, working tube you will see no connection from internal pin 7 to the indicated post.  In the bottom the bad tube has an extra welded bar from pin 7 to the support post.  Manf. error???

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    arledgsc

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    Re: Retubed ST-120

    Post by arledgsc on Wed Jul 26, 2017 11:02 am

    Jim McShane tells me the extra connection on internal pin 7 is a production change. KT-120s 2016 and above have the connection. Before this date no connection.
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    Peter W.

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    Re: Retubed ST-120

    Post by Peter W. on Wed Jul 26, 2017 11:26 am

    arledgsc wrote:Jim McShane tells me the extra connection on internal pin 7 is a production change.  KT-120s 2016 and above have the connection.  Before this date no connection.  

    I know that OEM Dynaco as often as not would use non-active socket connectors as bridge-connection points in some of their amps - and why it is that some metal 6L6 tubes will fail spectacularly in some Dynaco amps where glass ones will not.

    If pin 7 is normally inactive, and socket connector is 'dry' there should be no issues. I do not know whether VTA uses inactive connectors for bridging purposes.

    Jim McShane

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    Re: Retubed ST-120

    Post by Jim McShane on Wed Jul 26, 2017 1:31 pm

    Let me clarify a few things...

    1. It appears that sometime after 2014 there was a production change made. Why the change was made I don't know. It could have been performance related or it could have been related to supplier issues.

    2. Pin 7 is one end of the heater, it cannot be inactive. The new little tie bar is not a connection to anything electrically since what it attaches to at one end is just a support rod. If you look very carefully in both photos you'll see the small gauge wire connection going over to the bottom of the cathode sleeve and then going up inside it. That's the wire connection from pin 7 to the heater.

    3. In the case of metal 6L6s the metal shell is connected to pin 1. Many of the data sheets or circuit design notes caution against using pin 1 as a tie point because of that connection. It can be hazardous to connect anything to pin 1!!

    What happened to Scott's tube was simply bad luck - in spite of very careful testing and 24 hour+ burn in tubes can fail. And usually it happens in the first few hours of operation - "infant mortality" is the term used to describe it. Of course I'll be replacing the tube for him!!
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    arledgsc

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    Re: Retubed ST-120

    Post by arledgsc on Wed Jul 26, 2017 3:20 pm

    Thanks Jim! Still a mystery as to why the change to connect one side of the 6.3V heater circuit to the support rod. I'm guessing it used during manf. for a new process to flash the getters perhaps.

    Jim McShane

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    Re: Retubed ST-120

    Post by Jim McShane on Wed Jul 26, 2017 3:29 pm

    arledgsc wrote:Thanks Jim!  Still a mystery as to why the change to connect one side of the 6.3V heater circuit to the support rod.  I'm guessing it used during manf. for a new process to flash the getters perhaps.

    My best guess is that it was designed to provide more support for the connection but nobody outside the plant may know for sure!

    FYI - getters are usually flashed using RF induction coils to heat the tube internals so it wouldn't affect that.
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    peterh

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    Re: Retubed ST-120

    Post by peterh on Wed Jul 26, 2017 4:03 pm

    arledgsc wrote:Thanks Jim!  Still a mystery as to why the change to connect one side of the 6.3V heater circuit to the support rod.  I'm guessing it used during manf. for a new process to flash the getters perhaps.
    My guess is that the rod in question was "floating" without any connection. And this is
    not a good thing in a tube. Thus the fix was to connect it to a nearby electrode ( it does not
    matter what as long as it don't is left floating)

    Jim McShane

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    Re: Retubed ST-120

    Post by Jim McShane on Wed Jul 26, 2017 4:24 pm

    peterh wrote:
    arledgsc wrote:Thanks Jim!  Still a mystery as to why the change to connect one side of the 6.3V heater circuit to the support rod.  I'm guessing it used during manf. for a new process to flash the getters perhaps.

    My guess is that the rod in question was "floating" without any connection. And this is not a good thing in a tube. Thus the fix was to connect it to a nearby electrode ( it does not
    matter what as long as it don't is left floating)

    Why is it not a good idea? There were MANY very high quality/performance tubes made with extra supports that weren't connected to anything. Many  power tubes with 7AC or similar base connections have the metal shell connected to pin 1 - which has no internal connection inside the tube. Many miniatures were produced with extra supports as well - 5687 types, 6C4s, a number of other dual triodes, and so on.

    Can you cite a source where I can find corroboration for your statement? Thanks!
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    peterh

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    Re: Retubed ST-120

    Post by peterh on Wed Jul 26, 2017 11:58 pm

    Jim McShane wrote:
    peterh wrote:
    arledgsc wrote:Thanks Jim!  Still a mystery as to why the change to connect one side of the 6.3V heater circuit to the support rod.  I'm guessing it used during manf. for a new process to flash the getters perhaps.

    My guess is that the rod in question was "floating" without any connection. And this is not a good thing in a tube. Thus the fix was to connect it to a nearby electrode ( it does not
    matter what as long as it don't is left floating)

    Why is it not a good idea? There were MANY very high quality/performance tubes made with extra supports that weren't connected to anything. Many  power tubes with 7AC or similar base connections have the metal shell connected to pin 1 - which has no internal connection inside the tube. Many miniatures were produced with extra supports as well - 5687 types, 6C4s, a number of other dual triodes, and so on.

    Can you cite a source where I can find corroboration for your statement? Thanks!
    I have seen this discussed in one of the books i have , i'll have to come back later.
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    arledgsc

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    Re: Retubed ST-120

    Post by arledgsc on Thu Aug 03, 2017 12:40 pm

    The replacement KT-120 arrived from Jim McShane and the ST-120 is back to former glory! The bass quantity and quality restored, thick textured mid-range (overtones), good vocals with normal sibilance, and clear, open trebles with plenty of sparkle. Soundstage huge - deep and wide. Great tubes Jim and super amp Bob!

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