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    My Recent ST120 Tube Failure

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    Heaviside

    Posts : 6
    Join date : 2011-10-06

    My Recent ST120 Tube Failure

    Post by Heaviside on Sun Oct 09, 2011 4:12 pm

    Hi everyone,

    I recently had some problems with my ST120 after flawless operation for about a year. Bob Latino spent a fair amount of time with me on the phone and emailing to help me troubleshoot and get my amp running again. Thanks again Bob.

    I was using new Genelex Gold Lion KT88's. After a year and about 1000 hours of operation, the tube nearest the rectifier arced internally. This burned out one of the resistors in my Weber WZ68 rectifier and also the 10 ohm 2 watt cathode resistor on that tube.

    This experience and subsequent troubleshooting with Bob's help got me thinking, and I wanted to run a couple ideas by the group.

    First, why the back left tube? An unscientific study of this site and a couple others makes me think that the back left tube tends to fail more often than the others. If you look at a photo that Bob posted where he measured temperatures at each point in the ST120, you can see that the back tubes run significantly hotter than the front tubes due to (I assume) less airflow caused by their proximity to the transformers. In addition, the back left tube runs hotter than the back right. So, my first thought is, wouldn't it be a good idea to rotate the tubes on a periodic basis just as we rotate the tires on our cars?

    Second, I am wondering if the back left tube is under more thermal stress due to its proximity to the rectifier, which is also putting out heat. Many of us have gone to the Weber WZ68 to take advantage of its better current capability and reliability. But that thing still runs pretty darn hot. Most of that heat is being generated by two 18 ohm wire wound resistors that are in series with the output B+ connection. The purpose of those two resistors is to emulate the "sag" of a tube rectifier. Apparently that's important in guitar amplifiers, but what is it buying us in this application? Conveniently, Weber makes the WS1, which I believe is the same thing without the sag resistors. It still has the thermistor to reduce inrush current.

    Third, in the process of troubleshooting my amplifier, I burned out two additional rectifier tubes (fortunately they weren't NOS Mullards!) If you search this and other forums, you might get the impression that Dyanaco style amps are responsible for the relative scarcity of NOS 5AR4's today. In essence we're using them as fuses. Even the Copper Caps are being gone through on a regular basis. So, my next question is why we don't use a proper fuse on the B+ line? The main 5 amp fuse may or may not blow before the rectifier. Wouldn't it make sense to have (for example) a 500 mA fuse to protect the rectifier?

    Thanks,

    Dan



    Sal

    Posts : 221
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: My Recent ST120 Tube Failure

    Post by Sal on Sun Oct 09, 2011 9:08 pm

    I agree, fusing the B+ is a good idea.

    Sal

    ArlanB

    Posts : 52
    Join date : 2011-01-23
    Age : 69
    Location : Santa Cruz, California

    Re: My Recent ST120 Tube Failure

    Post by ArlanB on Sun Oct 09, 2011 9:35 pm

    Dan,

    I have gone through 4 sets of KT88's over the last three years in my VTA ST120. In every instance the left rear power tube closest to the rectifier has been the offending valve dying with an internal light show taking the GZ34 with it. Now as to whether heat is a factor I cannot say because I have always had a fan running whenever the amp is on. I do this because I just can't justify letting the power transformer carry so much heat even if it is designed to do so. The fan is set to blow air from the left front side of the amp across to the right rear encompassing all tubes and transformers. I have a Weber WZ68 that I have used in the past, however it got so hot even with the fan I was afraid that it might fail, so I use GZ34's because they are not as expensive. I have wondered why my failures were always in that location. Fortunately I have only lost the valves and no other components. I just resigned myself to the fact that newer tubes are just not designed to last as long as NOS. This is what would be called "Planned Early Failure". It assures the manufacturers a steady market for new valves.

    ArlanB

    Tom

    Posts : 166
    Join date : 2011-04-04

    Re: My Recent ST120 Tube Failure

    Post by Tom on Mon Oct 10, 2011 12:52 pm

    I'm considering a ST-120 after having built a SP-8 and really enjoying it.

    Question: If you're considering buying the kit without the chassis; and/or
    modifying the layout as baddog suggests (http://dynacotubeaudio.forumotion.com/t978-futuristic-st-120)
    couldn't you space the components out a bit more to help resolve the heat problem?
    The footprint I see for the ST-120 is about 13"x9" (right?);
    much smaller than my current amp at 17 3/8" x 15".

    Your thoughts?

    Heaviside

    Posts : 6
    Join date : 2011-10-06

    Re: My Recent ST120 Tube Failure

    Post by Heaviside on Mon Oct 10, 2011 1:08 pm

    I just want to make it clear that I am not suggesting there is "heat problem" or any other problem with the ST120. In my view, it's the best amp kit with the best support out there. I bought mine after a lot of research, and I'm glad I did. All of Bob's components are top notch, and the sound is fantastic.

    As others have pointed out, tube reliability isn't where it was back in the day, so I'm just exploring ways that we might improve it. I'm a tinkerer at heart and like to try things.

    I would and have recommended the ST120 without reservation.

    Best,

    Dan


    Tom

    Posts : 166
    Join date : 2011-04-04

    Re: My Recent ST120 Tube Failure

    Post by Tom on Mon Oct 10, 2011 1:25 pm

    Meh. Maybe "heat problem" was a bad choice of words? Mea culpa!!


    Sal

    Posts : 221
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: My Recent ST120 Tube Failure

    Post by Sal on Mon Oct 10, 2011 2:14 pm

    ArlanB wrote:Dan,

    I have gone through 4 sets of KT88's over the last three years

    ArlanB

    Hey, that means you have 9 good tubes from the first 3 sets, if they test good, you would have 2 quads to use... :-)

    Sal

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: My Recent ST120 Tube Failure

    Post by Bob Latino on Mon Oct 10, 2011 5:09 pm

    Tom wrote:I'm considering a ST-120 after having built a SP-8 and really enjoying it.

    Question: If you're considering buying the kit without the chassis; and/or
    modifying the layout as baddog suggests (http://dynacotubeaudio.forumotion.com/t978-futuristic-st-120)
    couldn't you space the components out a bit more to help resolve the heat problem?
    The footprint I see for the ST-120 is about 13"x9" (right?);
    much smaller than my current amp at 17 3/8" x 15".

    Your thoughts?

    To Tom,

    The VTA ST-120 may be purchased without the chassis ... Email me if you ever want to purchase an ST-120 kit without the chassis. A few people have used their own chassis and the ST-120 components and built a "custom" VTA ST-120. The ST-120's chassis is 13 X 9 1/2 inches. The output tubes are spaced 2 1/2 inches apart on the VTA ST-70/ST-120 which is about average for most older vintage tube amps. Modern tube amps do have a somewhat larger spacing - maybe 2 3/4 to 3 inches apart on center.

    The biggest problem we now face is the somewhat poorer quality of tubes available today from Russia, China and the Slovak Republic (JJ tubes) in comparison to the quality of tubes made in the '50's and '60's in the USA, Great Britain and Germany. Tubes made today are just not as "robust" (a kinder term for lower quality) as tubes made 40 to 50 years ago. That is why NOS tubes from those "golden years of tubes" go for such a high price on Ebay. Years ago in the mid '60's, as a kid, I built an ST-70 from a kit. I sold it about 10 years later with the ORIGINAL Mullard GZ34 rectifier which had maybe 10,000 hours on it. I had replaced one pair of the original (Mullard) EL34 output tubes. The other pair was never replaced. You won't get 10,000 hours on any GZ34 made today ... You'll be lucky if you get 1000 hours on any of today's GZ34's.

    One way around this issue is to use a USED USA, British or German 5U4 in place of the GZ34 in your ST-70/ST-120. There is no free lunch however ... The B+ will drop and the amp will lose a little power BUT - unless you are the type who really pushes the amp hard with inefficient speakers, you may never even notice the loss of power ? 5U4's are relatively cheap and available on the used market (Ebay). The reason for their availability is that many of the original tube color TV's of the 1960's used a 5U4 rectifier. Even with many hours still on them, these 5U4's are pretty durable and will probably last longer than a new GZ34.

    Bob

    Heaviside

    Posts : 6
    Join date : 2011-10-06

    Re: My Recent ST120 Tube Failure

    Post by Heaviside on Mon Oct 10, 2011 10:09 pm

    Sal wrote:
    ArlanB wrote:Dan,

    I have gone through 4 sets of KT88's over the last three years

    ArlanB

    Hey, that means you have 9 good tubes from the first 3 sets, if they test good, you would have 2 quads to use... :-)

    Sal

    It's an interesting point, isn't it? My arcing tube tests good for shorts and transconductance on my Hickok 600A. So what's causing them to fail? If it's arcing, doesn't that imply an air leak? My Hickok is supposed to be able to test for gas, but it's not in the best shape at the moment. So, the mechanism is somewhat mysterious for me at this stage. Is air leaking in, or is something being vaporized inside the tube?

    I have three non-arcing KT88's. Are they ok to use, or do I risk burning things up again? At $50 a pop, it would be nice to know.

    Best,

    Dan


    Heaviside

    Posts : 6
    Join date : 2011-10-06

    Re: My Recent ST120 Tube Failure

    Post by Heaviside on Mon Oct 10, 2011 10:23 pm

    Tom wrote:Meh. Maybe "heat problem" was a bad choice of words? Mea culpa!!


    Not at all. I just didn't want anyone to think I was dissing Bob's great product, especially as a newbie to this forum.

    Best,

    Dan

    Tom

    Posts : 166
    Join date : 2011-04-04

    Re: My Recent ST120 Tube Failure

    Post by Tom on Tue Oct 11, 2011 8:49 am

    Thanks Bob!

    Note to self, keep an eye out for mother lode of NOS KT88/6550/EL34's at garage sales etc...

    Very Happy Tom

    GP49

    Posts : 735
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: My Recent ST120 Tube Failure

    Post by GP49 on Tue Oct 11, 2011 11:11 am

    [quote="Heaviside"][quote="Sal"]
    ArlanB wrote:
    It's an interesting point, isn't it? My arcing tube tests good for shorts and transconductance on my Hickok 600A. So what's causing them to fail? If it's arcing, doesn't that imply an air leak?

    No. Arcing can be caused by shifted or off-tolerance internal parts in the tube. If you were to take apart an old tube, you might be surprised as to how precisely the wires of the grids are wound, and how precise their spacings from other parts in the tube must be. Those precise spacings can also alter as the tube heats up; operating temperatures are hundreds of degrees higher than the temperatures at which the tube is built.

    I would "credit" the longevity issues of present-day tubes to cheaper, less pure materials and lesser quality control. Both COULD be implemented but at much higher cost; the Chinese and Eastern Europeans who now make all tubes do not make a habit of superior quality control at the materials and labor costs it would entail.

    Mike G.

    Posts : 5
    Join date : 2010-10-07
    Location : Connecticut & Florida

    MATM13

    Post by Mike G. on Tue Oct 11, 2011 12:10 pm

    I read Bob's remark where you can substitute a 5U4 in place of the GZ34. Is a 5U4GB the same replacement? Also, I have a NOS RCA 5V4. How about this tube? Will any of these tubes affect the quality?

    Regards...

    Mike G...

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: My Recent ST120 Tube Failure

    Post by Bob Latino on Tue Oct 11, 2011 12:26 pm

    A 5U4 is NOT a direct replacement for a GZ34 but will work in the VTA ST-70 or the VTA ST-120. A 5V4 will work also. Both have higher voltage drops than a 5AR4 and you will exprience, as I mentioned, a slight B+ voltage drop and a slight loss of power. The VTA ST-70 has a 4 amp rectifier line and the ST-120 has a 5 amp rectifier line. Virtually any of the 5 volt rectifiers listed below will PROBABLY work with the VTA amp kits although I have not tried all that are listed below. I would be careful in using alternative rectifiers in a STOCK ST-70 with an original PA-060 power transformer. I believe that the 5 volt filament line was a 3 amp line on the original power transformers. Some of those listed below (like a 5V3) have a current draw over 3 amps.

    Bob



    Sal

    Posts : 221
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: My Recent ST120 Tube Failure

    Post by Sal on Tue Oct 11, 2011 1:29 pm

    Great chart Bob, I printed it for future reference.

    Sal

    tubes4hifi
    Admin

    Posts : 1291
    Join date : 2008-11-30

    Re: My Recent ST120 Tube Failure

    Post by tubes4hifi on Tue Oct 11, 2011 2:16 pm

    I think maybe the original 60s Dyna ST70 transformers were only rated for 2amps at 5v, so as Bob mentioned, I'd avoid using a 3A rectifier tube, but then I'd avoid using a 50 year old power transformer also.
    And as Bob mentioned, he'll sell you the kit without a chassis. This chassis below is a cheap 17"x10" Hammond box, with a machined top plate of aluminum, around $80, plenty of room to spread those tubes out . . . .
    This was built using some old H-K transformers, and SS rectifiers (with a separate B+ switch), but the concept is the same


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