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    My VTA ST-70 Just Died

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    thegage

    Posts : 7
    Join date : 2011-10-04

    My VTA ST-70 Just Died

    Post by thegage on Mon Sep 24, 2012 10:20 pm

    I built one of Bob's great ST-70 kits about a year and a half ago. Since then it's run flawlessly, dead quiet and producing wonderful sound. Tonight it just popped the fuse, and keeps blowing replacements. I was listening at low volume, and nothing's changed in the system for weeks. Where should I start to look for trouble?

    For reference, I'm running a Weber WZ68 rectifier, CV4024 inputs, and 63PSE power tubes. I also have the upgrade caps.

    John K.

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: My VTA ST-70 Just Died

    Post by Bob Latino on Mon Sep 24, 2012 11:31 pm

    Hi John,

    1. The most common cause of a blown fuse and no sound from any ST-70 is a bad rectifier. Most likely the Weber WZ68 solid state rectifier died. Try another Weber WZ68 rectifier or a 5AR4/GZ34 tube rectifier and (important) rebias the amp. A bad rectifier is THE most common cause of no music and blown fuses.

    2. Other less likely causes > A bad section on the quad cap or a bad output tube ...

    A much less likely cause is a bad power transformer winding. To check for a bad power transformer > REMOVE the rectifier and turn the amp ON. If the amp stays on and doesn't blow the fuse without the rectifier in there then the power transformer is GOOD and either #1 or #2 above is probably the problem.

    Bob

    thegage

    Posts : 7
    Join date : 2011-10-04

    Re: My VTA ST-70 Just Died

    Post by thegage on Tue Sep 25, 2012 12:02 am

    Bob,

    Thanks for the fast advice. The culprit is the Weber Copper Cap. Surprising as it's only a couple of months old. I didn't think they were that problematic.

    John K.

    GP49

    Posts : 718
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: My VTA ST-70 Just Died

    Post by GP49 on Tue Sep 25, 2012 4:55 pm

    It occurs to me that there are a lot more failures of the Weber Copper Cap rectifier substitutes than there
    should be in a modern silicon solid state rectifier.

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: My VTA ST-70 Just Died

    Post by Bob Latino on Tue Sep 25, 2012 5:57 pm

    Once or twice a year I get an Email from someone that tells me "... I go through 3 or 4 rectifiers a year" or "your amp eats rectifiers .." etc etc. In almost ever case these issues can be traced back to ..

    1. Higher than normal line voltage. When the line voltage goes UP then the power transformer secondary goes up which causes the B+ DC voltage to go way up. I have someone in Texas who told me that his line voltage averages 126.7 volts AC. The B+ on his VTA ST-120 was about 550 VDC when it should have been just about 500 VDC. Modern GZ34 tube rectifiers or the Weber solid state rectifiers won't last long dealing with these higher than normal AC line voltages.

    2. Voltage spikes. Everybody assumes that his line voltage is 120 VAC all day every day. If it is, then you are lucky. Measure your line voltage at various times during the day. I've done it here. Mine runs between 119.0 and 120.6 VAC. I have never seen a bad voltage spike here but you may have them where you live. Forum member "Baddog1946" lives (or works) in Costa Rica and mentioned to me by Email that he had voltage spikes at times over 130 VAC. The higher line voltage SPIKES kept taking out his Weber solid state rectifiers. Get a line voltage monitor and check your line voltage at various times during the day. If it goes much above 122 VAC this will shorten rectifier life.

    3. Quick ON/OFF/ON (short cycling) of the incoming AC line voltage. This can easily take out a rectifier due to the backward flow of voltage from the quad cap into the rectifier. There is a "tube rectifier diode mod" in the sticky's above that you can check out if this happens to you. I have a customer in Florida that mentioned to me that he experiences more than normal complete "dropouts" of his line voltage for sometimes less than a second.

    I respect GP49's statement about more than normal failures with the Weber solid state rectifiers but at least in my case that has not happened here. In my ST-120 I have had the same Weber WZ68 rectifier for over 2 years now. I don't play this amp every day but I know it has over 1000 hours (at least) on it. The same Weber rectifier is still working fine.

    If you have issues like #1 and #2 above, I recommend using a variac for your amp and set the output at about 117 VAC with 120 VAC coming in. Plug your amp into the output of the variac. If you have an issue with dropouts like #3 either do the diode mod or get a VTA TDR (Time Delay Relay) board for your amp. If the power goes out even for a split second, the relay will reset and you won't get any high voltage for 17 seconds after the AC returns.

    Bob

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