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    WZ-68 death by overheatng


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    WZ-68 death by overheatng

    Post by baddog1946 on Sat Dec 05, 2015 4:38 pm

    I have 2 ST-120 amps and a pair of M-125's all were using WZ-68 rectification. After suffering continual  failures I want to share some observations on why the WZ-68 may have s many reliability issues:

    My observation is that they fail usually after about 3000 hours with regularity.

    In many of the previous comments in the forum on this recurrent topic there are both a lot of complaints and testimonials regarding the WZ-68. When they work I love the things   but   have had at least 8 failures.

    I want to mention here the fact that I also use  them almost constantly during the day and evenings. I swap out amps regularly in the case of the ST-120's but still they get an average of 4-6 hours each every day if the time is averaged out.

    I have replaced the diodes with a more robust version (IN-5408,3amp 1000V) as well as the sag reisistors I even drilled big holes in the caps on a couple for better cooling but none of this has solved the fried thermistors. It always seems that after roughly 3000 hours at operating temp the thermstors disintegrate.

    It makes some sense as current inrush limiters operate much like a resistor and heat is a by product of resistance.
    I am not sure what more can be done to mod them to be more robust n their current configration.

    Replacing the thermistor with a higher value but in the same physical size (so it still fits in there) may not be much help ether if overheating is actually the issue. I now think it is the main culprit in many WZ-68 failures.
    Whether direct overheating of the thermistor itself or the combined heat of the whole cap.
    It could also be too close to the power tranny which also produces considerable radiant heat adding to the problem.

    When I look at the WZ-68 circuitry it seems to have good specs but the compact format and the copper shield still retain a lot of heat. Even the ones I drilled out the cap on are still too hot to touch.

    I'm thinking of making one on a perf board that can sit out in the open to test this theory.
    Of course it would under some kind of cover to avoid accidentally touching it and perhaps death by electrocution.

    Meanwhile I have finally bit the bullet and ordered 2 brand new Mullard GZ-37's to keep at least two of my systems operating.

    I know I live in a place where the electrical service is very poor (Costa Rica) but I have latch breakers on all of them and a variac to control voltage on the one in use but the thermistor failures keep occurring regularly. With the doubled protection I think a bad grid is not the problem. More likely the problem is  the thermistor deteriorates from heat over time.

    Bob mentions his have been good for years without a failure but he also mentions he does not log a lot of continuous long hours either.

    I have a system playing music or home theater at least 6-8 hours a day which is a quite a lot but it may explain why these things fail on me regularly.

    Meanwhile I will go back to a traditional quality tube rectifier (GZ-37 Mullard) while I run the perf board model I mentioned to test its longevity. If in time it proves the thermistor proves to be reliable that would confirm my overheating theory.

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    Re: WZ-68 death by overheatng

    Post by deepee99 on Sun Dec 06, 2015 1:36 am

    Hi, Baddog,
    I won't reiterate my own experiences with the Webers except to say they parallel yours. I have a pair of Holger's Tubes Nirvana "Big Bopper" mods to my original kit-built M-125s which were going through WZ-68s on a pretty regular basis.

    (He has since gone to solid-state rectification and there are arguments for and against) In the mods he did on mine he added an extra hole so that each Big Bopper M-125 runs a pair of parallel rectifiers of the 5AR4/GZ-34 variety.
    I had been running my single-holers with some very expensive Mullard GZ-33s but they are almost impossible to find nowadays. I currently am trying out a quartet of the new Gold Lion 5AR4s that Jim McShane sells, and so far, so good. If they crack up in a year or so I've got a stash of antique Mullard GZ-34s to fall back on, and they're still available, though not cheap.
    I haven't tried this, but if you're going to "breadboard" a Weber outboard anyway, why not wire it in parallel with the one on the chassis, see what happens. That might de-stress the WZ-68s and reduce the heat.


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    Re: WZ-68 death by overheatng

    Post by mazeeff on Sun Dec 06, 2015 6:32 am

    I have been running the Weber WS1 (no sag resistor) in my VTA-70 for the last two months, with no issues. I run my amp 8-10 hours a day, and the WS1 gets only warm to the touch. I understand the negatives of just running with diodes, but it seems to work for me.


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    Re: WZ-68 death by overheatng

    Post by baddog1946 on Sun Dec 06, 2015 9:20 am

    A good source of Mullard NOS tubes

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    Re: WZ-68 death by overheatng

    Post by arledgsc on Sun Dec 06, 2015 12:29 pm

    Interesting post.  I have been using the same WZ68 for about 3 yrs in my ST-120.  I do maintain gentle air flow across the WZ68 but still runs very hot.  So far no issues but the day is coming the WZ68 will die.  I use a TDR (time delay relay) as well.  And operate the ST-120 daily and probably put 20 hrs. a week on the amp.

    I would like to get rid the WZ68 and go with straight diode rectification.   But inrush current on start up is tremendous and has to be controlled for reliability.  I have been mulling over a 2-stage TDR circuit where stage 1 kicks at 20 sec. with power resistors in series to gradually charge up the filter caps.  And then at about 40 sec. stage 2 engages to bypass the inrush current control resistors for straight diode rectification to the prior charged filter caps.

    Just a pipe dream at this point but does anyone have an idea what ST-120 B+ voltage (120V ac line) would be with straight diode rectification?   Have to watch the working voltage of the filter caps.


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    Re: WZ-68 death by overheatng

    Post by turbotoy on Sun Dec 06, 2015 5:26 pm

    A potential elegant solution to the inrush current problem, space permitting, is:

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    Re: WZ-68 death by overheatng

    Post by sKiZo on Sun Dec 06, 2015 5:33 pm

    3K hours before failure, at a cost of $20 per pop?

    Sounds reasonable to me ... try substituting a Chinese rectifier and see how long it lasts ...

    (I still like my GZ37's here)

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    Re: WZ-68 death by overheatng

    Post by Tubes4ever on Sun Dec 06, 2015 11:40 pm

    Install a 5v3A. It has a voltage drop similar to a GZ37 and will probably be the last rectifier you need for a long time. Certainly longer than 3000 hours.....and it glows like a rectifier should!

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    Re: WZ-68 death by overheatng

    Post by davek65 on Mon Dec 07, 2015 11:46 am

    Reconstruct yours differently like i did over 1 year ago and you wont have any problems .
    Mine not the one you would want to maybe display but mine is hidden from sight and works
    like a charm . It's the resistors that need more cooling that cause the failures . Smile

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    Re: WZ-68 death by overheatng

    Post by corndog71 on Mon Dec 07, 2015 1:50 pm

    I built my ST120 with UF4007 diodes after losing many a rectifier tube with previous dynaco amps. The diodes haven't failed me yet and saved me a lot of money. cheers

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