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    ST-120 Rectifier Choices

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    davidnes
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    ST-120 Rectifier Choices

    Post by davidnes on Wed Sep 15, 2010 10:38 am

    Bob,

    I've had some problems with my ST-120 blowing up. Some time back you had me replace the cap-can, and that helped a lot. But a couple of weeks ago I did it again; I turned off the ST-120, then (mistakenly) turned it right back on. It blew the slo-blo fuse, it flashed the Ruby 5AR4 rectifier tube, and apparently took out a KT-88 (or two).

    I replaced these three items (fuse, rectifier, and KT88) and all has been fine, but I'm still nervous.

    I didn't know about the potential use of a Weber WZ68 on my ST-120. Would this help?

    I have ordered a Weber Copper-Cap GZ34, plus I bought a GZ33/CV5745 Mullard NOS tube off eBay that you mentioned somewhere, plus I also have "1 NEW IN THE BOX 1961 MULLARD GZ34 5AR4 TUBE" that I just won on eBay coming in.

    My ST-120 is on a big APC UPS, in order to avoid momentary power outages.

    Can you take a moment to describe rectifier tube choices on the ST-120, and their pros and cons, especially with regard to holding up to power cycling.

    Thanks!

    PS: I've also 'upgraded' by power cord to a 3-wire grounded cord, and attached the ground to the chassis. Was this a good or bad move?

    davidness

    Posts : 39
    Join date : 2010-09-15

    ST-120 Rectifier Choices

    Post by davidness on Wed Sep 15, 2010 11:14 am

    Duh. I now see the thread just a couple of posts lower "Choices in rectifier tubes for VTA?" that does have some of the discussion I was looking for. But I am still interested in the use of the WZ68 in the ST-120, so anything else that can be added to this discussion would be greatly appreciated.

    Thanks!

    Bob Latino
    Admin

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    Location : Massachusetts

    Re: ST-120 Rectifier Choices

    Post by Bob Latino on Wed Sep 15, 2010 11:15 am

    Hi David,

    You should never "short cycle" any ST-70 or ST-120 amp. If you shut the amp OFF then wait one minute before turning it ON again. This gives the capacitors a chance to discharge. Short cycling any Dynaco amp can cause the rectifier to "arc over". 90% of the time a short cycle will only take out the rectifier and fuse. It usually does not not harm any KT88 tube. In the latest kits there is a warning against short cycling the amp on the back of the parts list page. Any rectifier - tube or solid state can be damaged by short cycling the amp.

    I think the Weber WZ68 solid state rectifier is the best rectifier to use in the ST-120. It will flow more current (450 milliamps) than any GZ34 tube rectifier (250 milliamps). If you play the ST-120 amp loud a lot I recommend the Weber WZ68. At normal volumes any GZ34 is fine to use in the ST-120. You had mentioned the "Weber GZ34" in your post. The Weber WZ34 is fine in an ST-70 but you need a Weber WZ68 in the ST-120.

    You can use a 3 wire cord in the ST-120 and ground the third wire to the chassis. The only problem is that a 3 wire cord in conjunction with your other 2 and 3 wire components can SOMETIMES create a ground loop and some hum/buzz. If you get any hum by using a 3 wire cord with your ST-120 then just use a "cheater plug" (a 3 wire to 2 wire AC adapter) and the hum should go away.

    Bob

    davidness

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    Re: ST-120 Rectifier Choices

    Post by davidness on Wed Sep 15, 2010 11:44 am

    Bob,

    Thanks, as always. I'm ordering a Weber WZ68 or two right now.

    davidness

    Posts : 39
    Join date : 2010-09-15

    Re: ST-120 Rectifier Choices

    Post by davidness on Fri Sep 17, 2010 11:48 am

    Bob,

    I would say you're spot-on that most of the rectifier problems I've had were caused by my short cycling, and I really try to not do it. The last time was an accident... muscle memory.

    As I said, I do have a big APC UPS on the ST-120 to avoid externally-caused power interruptions. But that begs the question; In the 60's through the 90's, when UPS's were rare, did this condition cause many, many failures during storms or other events causing short-term power interruptions?

    And, do amps like the McIntosh MC2102 do something in their circuit design to avoid this? Is there something more I can do?

    j4570

    Posts : 141
    Join date : 2010-08-30

    Re: ST-120 Rectifier Choices

    Post by j4570 on Sat Sep 18, 2010 8:01 am

    A cool upgradge would be an add on board that waited like a minute before applying power to the amp (of course that would add to startup delay).

    Like this one (but it's for 220V):

    http://www.electronics-project-design.com/DelayTimer.html

    They are used in Home A/C's to protect compressors from restart after power failure.

    Jason

    Bob Latino
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    Re: ST-120 Rectifier Choices

    Post by Bob Latino on Sat Sep 18, 2010 9:43 am

    davidness wrote:Bob,

    I would say you're spot-on that most of the rectifier problems I've had were caused by my short cycling, and I really try to not do it. The last time was an accident... muscle memory.

    As I said, I do have a big APC UPS on the ST-120 to avoid externally-caused power interruptions. But that begs the question; In the 60's through the 90's, when UPS's were rare, did this condition cause many, many failures during storms or other events causing short-term power interruptions?

    And, do amps like the McIntosh MC2102 do something in their circuit design to avoid this? Is there something more I can do?


    David,

    Dynaco tube amps do not have sophisticated delay circuits like many modern tube amplifiers. If you shut a Dynaco tube amp OFF after it has been on for a period of time you should leave it OFF for about a minute just to make sure that all the capacitors have discharged before you turn it back on again. Failure to do so can take out the rectifier tube and/or the fuse. This is called "short cycling" and is not good for any Dynaco tube amp.

    I know of a case where someone was having problems with short interruptions of the AC power in his area. The power would go out for a second or two and then come back on. Sometimes it took out the rectifier tube on his Dynaco tube amp. He cured this by plugging the amp into some type of AC terminal strip or power conditioner that stayed OFF (for all inputs) if power was interrupted. He would have to manually reset the AC. I don't know what the name of the item was or where he got it ? On my Monster Cable power conditioner on my downstairs system SOME of the AC inputs will STAY OFF if the power is interrupted while others will come back on as soon the power comes back on. I have the two ST-120's on that system, however, plugged directly into wall outlets and I won't play the amps during a storm when the possibility of a power outage exists.

    Bob

    j4570

    Posts : 141
    Join date : 2010-08-30

    Re: ST-120 Rectifier Choices

    Post by j4570 on Sat Sep 18, 2010 10:11 am

    Bob,

    That's a good point. I think a "non-latching" GFCI will do that. Most of the ones on our walls (if you have a new enough house, or have added them) are latching types, meaning they don't trip during a power interruption.

    However, I do believe there are power strips out there that will do just this, though it's probably not easy to find one, as most people would get annoyed by it. Who's wants to root around behind the TV console after a power outage to reset the GFCI?

    That's a good solution for the power interuuptions. Might be good to buy one to run tube amps from. Then just to remember NOT to turn it back on within a minute.

    I'm going to look on the wonderous google and ebay and see what turns up.

    Jason

    GP49

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    Re: ST-120 Rectifier Choices

    Post by GP49 on Sat Sep 18, 2010 1:58 pm

    Many separate power strips that have a GFCI (they are sold for use with power tools on outdoor construction sites) have a GFCI of the nonlatching type.

    j4570

    Posts : 141
    Join date : 2010-08-30

    Re: ST-120 Rectifier Choices

    Post by j4570 on Sat Sep 18, 2010 4:38 pm

    GP49,

    do you have any one you know of?

    I did a quick google and the Stanley and Tripplite for construction sites looked good (except for the bright yellow) but in their descriptions never mentioned non-latching, or tripped with power failure.

    Jason

    PeterCapo

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    Join date : 2008-12-05

    Re: ST-120 Rectifier Choices

    Post by PeterCapo on Sat Sep 25, 2010 3:09 pm

    I use one of these, and it trips when the power goes out:

    http://www.trci.net/products/shock-shield/power-strip

    Might want to double check with the manufacturer, though, because I can't say for sure how it will behave if the power fluctuates only momentarily.

    j4570

    Posts : 141
    Join date : 2010-08-30

    Re: ST-120 Rectifier Choices

    Post by j4570 on Sat Sep 25, 2010 6:30 pm

    It looks like it's what I was looking for. Amazon has the plug in one, but without the strip, about $20 for the white, only $17 for yellow. Probably go with yellow.

    I might buy two, one for my old radio.

    Website says it cuts off and require manual reset. 25 ms reaction time.


    Thanks.

    Falcon4646

    Posts : 9
    Join date : 2010-02-21

    Possible Short Cycle Solution?

    Post by Falcon4646 on Sun Oct 10, 2010 11:14 pm

    Outside of using a really good power filter unit that are made primarily for digital home theatre systems that include outlets that stay off after a power interruption, I've heard of people using magnetic power switches to prevent short cycling once a power interruption has occurred.

    See these website addresses:

    http://www.rockfordsystems.com/online/dsc/disconnsingle.cfm

    http://www.xmission.com/~jry/ww/shop/magnetic-switches.html
    http://www.xmission.com/~jry/ww/shop/mag-sw-parts_digi-key.html




    WntrMute2

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    Join date : 2010-11-21

    Re: ST-120 Rectifier Choices

    Post by WntrMute2 on Fri Jan 21, 2011 11:18 am

    Any more suggestions about power strips that trip and stay tripped with power fluctuations?

    j4570

    Posts : 141
    Join date : 2010-08-30

    Re: ST-120 Rectifier Choices

    Post by j4570 on Fri Jan 21, 2011 11:33 am

    I bought my plug in unit from Amazon:

    http://www.amazon.com/TRC-90265-6-012-Shockshield-Portable-Protection/dp/B000XU5MEG/ref=sr_1_fkmr0_1?ie=UTF8&qid=1295623793&sr=8-1-fkmr0

    in addition, I noticed that you can get it at my local Home Depot now in FL. for about $15. from Amazon, you can get a white color one, which my Home Depot didn't have.

    I like these so much, I put one in the garage for my overhead power reel, as it was plugged in with the Garage Door opener, and not protected by GFCI. Now it's protected, and I can move it if I need it somewhere else.

    Reid

    Posts : 9
    Join date : 2009-02-05

    Re: ST-120 Rectifier Choices

    Post by Reid on Sat Jan 22, 2011 1:19 pm

    You might consider this. I use it and when the power goes out even for a second it trips and stays off.

    http://www.belkin.com/IWCatProductPage.process?Product_Id=517511

    It is made by Belkin and called a "Conserve Switch"

    I then use the remote switch to turn everything back on when I want. No cycling to the power.

    sKiZo

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    Re: ST-120 Rectifier Choices

    Post by sKiZo on Fri Apr 12, 2013 2:09 am

    Hope yall ain't getting annoyed with me digging up all these oldie but not moldy threads. I've been digging, doing some research on the care and feeding of my new baby, a VTA ST-120.

    Wasn't aware of the short cycling issue. Just pulled the trigger on a

    TRC 26020L-4-001 Shockshield Inline GFCI with 14-Inch Cord

    To regurgitate ... the important thing is to get something that's "non-latching". This means you have to manually reset after any power outage or major blip. In this case, that's a good thing.

    Can't help but notice some of these tube thingies are kinda expensive. Seems like relatively cheap insurance against some of the bad stuff lurking in the wires ... (boo!)

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