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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    Keep blowing Weber WZ68 rectifier on ST-120

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    nathico


    Posts : 7
    Join date : 2012-04-27

    Keep blowing Weber WZ68 rectifier on ST-120 Empty Keep blowing Weber WZ68 rectifier on ST-120

    Post by nathico Sat Oct 08, 2022 9:47 am

    My first WZ68 rectifier lasted 2011 to 2015, my second 2015 to 12/2022 and never went out, I just replace preventatively. I have now blow two in less than a year, first lasted about 7 months, and just blew the second after only a couple months (pretty sure it is the rectifier at least, took out the fuse again and don't have another back up right now). Last 2 took out the fuse. Both went after hours of music not at startup.

    Any ideas if something may be off I should check? Not much has changed in the last year, but I have been using KT120 power tubes since I first built the amp and did replace those at the same time as the rectifier in 12/2022 because my bias was not holding and figured was time to update (kept the old for backups), so I replaced the outputs and rectifier at the same time. So both the rectifiers that went this year were with the new power tubes (from wpc1896 on eBay).  But everything else seems fine, and the bias has been holding well ever since the change. I keep bias about 0.550. Maybe I just got some poor webers from a bad batch last time (both the ones that blew this year from the same order).
    tubes4hifi
    tubes4hifi
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    Keep blowing Weber WZ68 rectifier on ST-120 Empty Re: Keep blowing Weber WZ68 rectifier on ST-120

    Post by tubes4hifi Sat Oct 08, 2022 10:38 am

    as long as your bias is holding steady you should be fine, the only other thing to go bad is the quad cap, which can go bad when the rectifier/Weber blows out
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    nathico


    Posts : 7
    Join date : 2012-04-27

    Keep blowing Weber WZ68 rectifier on ST-120 Empty Re: Keep blowing Weber WZ68 rectifier on ST-120

    Post by nathico Sat Oct 08, 2022 11:20 am

    tubes4hifi wrote:as long as your bias is holding steady you should be fine, the only other thing to go bad is the quad cap, which can go bad when the rectifier/Weber blows out

    Thanks, so you think the short life span of the last 2 webers may just be bad luck and not a sign of something else being off?
    Bob Latino
    Bob Latino
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    Keep blowing Weber WZ68 rectifier on ST-120 Empty Re: Keep blowing Weber WZ68 rectifier on ST-120

    Post by Bob Latino Sat Oct 08, 2022 1:15 pm

    1. In 2016 we lowered the VTA ST-120 bias setting from .55 VDC per each output tube to .50 VDC and put a notice on the forum here. The link is below. Try lowering the bias setting to .50 VDC I have listened to my own VTA ST-120 amp here myself and also with others listening here with me. We could not tell any difference in the sound quality of the amp at either bias setting. .50 VDC bias will go easier on today's modern tubes made in China, Russia and the Slovak Republic. The newer tubes are not as durable as tubes made in the '60's and '70s which were made in the USA, Great Britain and Germany. The older tubes had better quality control of what they let out of the factory. The metal inside the older tubes were also made out of higher quality alloys.

    VTA ST-70, ST-120 and M-125 info

    2. Check your incoming AC line voltage ... Anything at 122 VAC or above can cause short rectifier life. If need be, use a variac to bring the line voltage to your amp to about 117 or 118 VAC. The 5 amp unit at the link below will work fine with any VTA ST-120 or a VTA ST-70. The 20 amp unit will run TWO VTA M-125 amps.

    Variacs at Circuit Specialists

    Bob
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    nathico


    Posts : 7
    Join date : 2012-04-27

    Keep blowing Weber WZ68 rectifier on ST-120 Empty Re: Keep blowing Weber WZ68 rectifier on ST-120

    Post by nathico Sat Oct 08, 2022 1:20 pm

    Thank you, I do remember seeing that advice to drop bias but I think at some point forgot...I will do just that. And my voltage is high and varies from 120 to 126. I will order up one of the VACs....lot cheaper than replacing the Weber every few months!!!

    Thank you!

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    Keep blowing Weber WZ68 rectifier on ST-120 Empty Re: Keep blowing Weber WZ68 rectifier on ST-120

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