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    M125 blown fuse help

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    mikew

    Posts : 2
    Join date : 2013-07-06

    M125 blown fuse help

    Post by mikew on Mon Sep 09, 2013 6:49 pm

    Let me start with stating, I’m below the novice level. With that being said, I did build two M125s without any problems. They fired up day one and have been in service for over a year. This speaks to Bob’s amps not my abilities.

    Amps had been on for a few hours. When I was changing albums (no music playing) heard a pop, and one of the amps blew the fuse.
    With power tubes and rectifier (WZ68) out, the unit powers up without blowing the fuse, with WZ68 in fuse blows. I put in the  WZ68 from the other M125, and power tubes, it did not blow the fuse. So I concluded, WZ68 was bad, and ordered two new ones.

    My concern is what may have caused the rectifier to die in the first place. I installed the good WZ68 from the second amp and started to adjust the bias setting. I noticed anytime my hand was close to the tubes in the driver board there was an increase in voltage on one channel and a decrease on the other. This would happen without touching anything, just getting close.  A visual inside and out didn’t  reveal anything suspicious. What can cause this?

    I don’t want to install the new WZ68 only to mess something else up.

    Any help on what to check would be greatly appreciated.

    Thanks


    Last edited by mikew on Mon Sep 09, 2013 7:15 pm; edited 1 time in total (Reason for editing : wrong name)

    Bob Latino
    Admin

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

    Re: M125 blown fuse help

    Post by Bob Latino on Mon Sep 09, 2013 9:41 pm

    Hi Mike,

    Sometimes a slight power surge happens and you are not even aware of the surge. A voltage spike can cause the rectifier to operate on voltages higher than normal for a second or two which can take out a rectifier. Another possiblity is some "transient" issue inside a power tube which can cause a partial short, increased current flow which can take out the rectifier. Another possibility is the surge resistors inside the Weber WZ68. I believe there are TWO wirewound resistors in there whose job is to slow down the initial application of high voltage to the amp. If one or both fail, the Weber cannot supply high voltage anymore. The Weber gets fairly hot just like a tube, Excessive heat over a long period of time can cause a part to fail.

    When you deal with tube gear you have to realize that there is more "maintainence" with tubes. Tubes (or the Weber SS rectifiers) don't last forever and periodically have to be replaced.

    If the amp in which the Weber failed will play fine with the other Weber, it is probably just a failure of the Weber and your amp is OK.

    As an aside > On any Dynaco based tube amp the rectifier is the item most likely to fail. This tube (or SS Weber) looks at 350 + volts on four pins. I recommend that you always keep a spare rectifier handy ...

    Bob

    mikew

    Posts : 2
    Join date : 2013-07-06

    Re: M125 blown fuse help

    Post by mikew on Tue Sep 10, 2013 11:29 am

    Thanks Bob,

    I understand that the rectifier and tubes have a life cycle, and some may go, and others may stay with you for longer than you would think. I have no problem with a $22 part being replaced after 14 months, and it’s a simple plug and play.

    My concern is the electrical field that is around the VTA driver board. If my hand is anywhere close (not touching) to this area the bias settings jump up on one channel and down on the other. I have not noticed this happening before the WZ68 went. The other M125 doesn’t do this. I wanted to know if there was something I should be checking before I plug in the new rectifier and screw something up.

    I have had your amps for over a year and still very happy with them. Troy, is building a SP14 for me because of the quality of these amps. I will post a review of both once I test drive the sp14.

    Thanks, Mike

    Bob Latino
    Admin

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

    Re: M125 blown fuse help

    Post by Bob Latino on Tue Sep 10, 2013 11:48 am

    Hi Mike,

    The thing with your hand near the driver board may be an issue with one of the driver tubes on that amp. You could try swapping the driver tubes between amps and see if the same thing happens with your hand near the driver board in that amp? It could be some type of electrical field from your hand somehow changing the electron flow inside one of the tubes ? That is just a guess though .. I can't really be sure ..

    Bob

    Analog Man

    Posts : 56
    Join date : 2013-09-01
    Age : 54
    Location : St. Louis, MO - On The Hill

    Re: M125 blown fuse help

    Post by Analog Man on Mon Sep 23, 2013 7:53 pm

    If your running the Tung - Sol KT - 120 tubes, Check the bias voltage on these after the fuse blew. I discovered a shorted KT - 120 after only 6 hours use.
    I don't know if this is your problem but it is worth checking out.

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