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    ST-120 Issue

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    Maintarget

    Posts : 208
    Join date : 2013-02-10

    ST-120 Issue

    Post by Maintarget on Sun Dec 01, 2013 6:35 pm

    I have an ST-120 with 125 hours on it other than a self inflected start up issue that took out the kit supplied Rectifier tube it has been working great up until yesterday when the rectifier tube flashed and the music died, I was near the amp and shut it down quickly.
    I removed the rectifier tube (JJ GZ-34 with 125 hours) and examined carefully didn't see anything burnt or bad, I replaced the 5 amp slow blow fuse removed all but the three 12BH7 tubes and powered the amp up, all three lit up so I went to the next step and installed the rectifier tube and two KT-88 (I had marked tube location so the two tubes went back to the exact left front & rear positions) I hooked up the speakers and turned the bias pots full CCW then powered up the amp, all tubes lit up until the time delay relay clicked and then the rectifier tube flashed again.
    I removed all tubes, 5 amp slow blow fuse and examined all Capacitors & wiring, nothing smelled or looked burnt, at this time I did not check the values of the capacitors.
    I replaced the 5 amp slow blow fuse, three 12BH7 tubes and powered up the amp, all three lit up fine and stayed on for over two minutes, next I installed a new spare JJ GZ-34 rectifier tube and installed the two right side KT-88 in the left positions and powered up with the same results, rectifier flashed after the time delay completed.
    I did check my outlet and it measures 120.7 V
    I have very basic electrical skills and am seeking help on what to check next?
    I don't have anymore known Good rectifier tubes so I need to order and am wondering about going with a Weber WZ-68 but cant seem to locate where to buy one, is there anything I can check/verify with out the rectifier tube at this point?
    I do want to mention that I choose to post on the forum rather than going directly to Bob not because I felt like I had to but rather to share with others like myself that want to learn while enjoying some awesome sounding amps.
    Any help is greatly appreciated.

    arledgsc

    Posts : 330
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: ST-120 Issue

    Post by arledgsc on Sun Dec 01, 2013 7:08 pm

    Sorry to hear about your rectifier woes.  You did a good job of trying to isolate any heavy loading of the high voltage circuit.   Not much it can be other than the capacitors.   But three rectifiers seems like something is fishy.  

    But the GZ34s are worked hard in this amp and right up to the limit so only the very strong will survive for any length of time.  My Sovtek blew up on first power up and it has been Copper Caps ever since.  In the first link below you can find the ORDER button directly below the table of Copper Cap offerings.  Or use the 2nd link...

    Weber Copper Cap Page
    Copper Cap Order Page

    Bob Latino
    Admin

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

    Re: ST-120 Issue

    Post by Bob Latino on Sun Dec 01, 2013 8:21 pm

    Of all the GZ34/5AR4 rectifiers available today, I have found that the JJ is probably the worst from the standpoint of durability. Even the generic Chinese Shuguang made 5AR4/GZ34 is better. Do to recent improvements in the Russian Sovtek GZ34/5AR4 I have learned about though some tube industry sources, I now use the Sovtek GZ34/5AR4 in all the tube sets shipped with a tube rectifier.

    Were the tubes that you used in your amp at the start NEW ? I don't believe that you got them from me NEW because I only used a few JJ GZ34's maybe 3 or 4 years ago until I found out that they usually died an early death.

    A rectifier "flashover" usually indicates that the rectifier is bad - BUT - if other new rectifiers also flashover then the new rectifier is working into a partial of full short somewhere on the high voltage system. Some things to check ...

    1. A problem in the wiring ( bad solder connection) of the high voltage system including the quad cap, SCM, ESL, Russian PIO caps, the four caps on the driver board OR a failure of one or more of these parts
    2. A short in a tube - either driver tubes or output tubes. Try some new output tubes first ..

    You could also switch over to the Weber WZ68 solid state rectifier, two of which have worked well in my M-125's for about 3 years now. Maybe order TWO WZ68's and keep one as a spare. If a WZ68 blows in there then look for a problem in the high voltage system as outlined above.

    Bob

    anbitet66

    Posts : 131
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: ST-120 Issue

    Post by anbitet66 on Sun Dec 01, 2013 8:55 pm

    Maintarget,

    I assume since you measured the line voltage you have a multimeter.  Can you measure the resistance of pin 8 of the GZ34 socket?  That will tell you if the rectifier is trying to force a high voltage into a short or near short.  A reading low reading is definately not good.  The manual shows voltages at the tube pins, but right now a resistance reading will tell you if there is a short taking out the rectifier.  Tony

    Maintarget

    Posts : 208
    Join date : 2013-02-10

    Re: ST-120 Issue

    Post by Maintarget on Mon Dec 02, 2013 12:54 am

    I had the Weber part description page but not the order page, Thanks arledgsc!
    Bob you are correct the ST-120 kit supplied rectifier tube was made in china and didn't survive the initial start up/bias procedure (I'm guessing from my ignorance)
    I found a replacement JJ GZ-34 at a local guitar shop and have been enjoying the amp up until yesterday.  
    I checked pin 8 as suggested by antbitet66 and got a reading of 0.596-0.601 ohms so I'm guessing there is no ground.
    I didn't realize that a driver tube would have an effect on the rectifier tube so Based on the input from all my plan is to replace one of the driver tubes (12BH7) that I had rolled in (Even though it lit up), the other two were known good and been used with no issues, inspect wiring & solder joints while waiting for two GZ-68 as Bob has suggested and then try again.
    I'm guessing all four output tubes didn't go bad at once.
    Thank you all for your help, I will update soon.

    anbitet66

    Posts : 131
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: ST-120 Issue

    Post by anbitet66 on Mon Dec 02, 2013 5:19 am

    Maintarget,

    The reading you give suggests there is a short.  This is  why the rectifiers are flashing as soon as the delay board kicks in.  The next step would be to isolate the short.  This involves disconnecting components one at  a time to find which is shorted.  I'd start with the main filter caps; the ones before and after the  choke but  it  seems from the very low  resistance to be the first section immediately follwoing the  rectifier tube.  Check that first.

    Tony

    Bob Latino
    Admin

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

    Re: ST-120 Issue

    Post by Bob Latino on Mon Dec 02, 2013 1:10 pm

    Tony (Maintarget) has given good advice. A resistance test (with the amp OFF) on all sections of the quad cap should give a resistance of at least 10K and really over 1 meg ohm is more like the proper resistance. With the AMP OFF > Set the resistance meter to 20 meg and keep the RED probe on the 40 section of the quad cap which is connected to pin #8 of the rectifier tube socket. Place the BLACK probe on the chassis. You should get a rising resistance (as the cap charges from the 9 volt battery in your meter). It should top out over a meg on all four sections of the quad cap. If one section gives a low reading then the cap is probably bad. If you are getting .596 to .601 ohms then one or more sections of the quad cap are bad OR the SCM and ESL caps are bad.

    This is where a capacitor tester would be handy to really let you know what is happening. You would test the quad cap in circuit with the power OFF. You should get.

    SQUARE section that faces the to the amp's LEFT channel > 150 to 180 uF
    HALF-CIRCLE section  - faces the back of the amp > 250 to 300 uF
    CIRCLE section - faces to the right side of the amp > 250 to 300 uF
    TRIANGLE section - faces the front of the amp - 40 to 80 uF

    If one section doesn't come up to par then you will have to unsolder all the connections and remove the quad cap. Then test each section individually OUT of the amp. In this case the four sections should measure about their value 80, 40, 30, 20.

    If the quad cap proves to have a bad section, I can send you another one at no cost because your amp dates from March of this year (2013) and the parts are still under warranty. You would have to send the bad quad cap back to me.

    See if you can determine if the quad cap, SCM or ESL caps are bad ?

    Bob

    Maintarget

    Posts : 208
    Join date : 2013-02-10

    Re: ST-120 Issue

    Post by Maintarget on Tue Dec 03, 2013 12:56 am

    [b]Bob, Tony,

    I have a Fluke 87 meter and checked capacitance and got the following readings
    Square     308 uF
    Half circle 307 uF
    Circle       308 uF
    Triangle     58 uF

    ESL         308 uF

    SCM        412 uF
                 413 uF

    I don't understand capacitance enough to know if the higher reading on the quad cap square is an indication of trouble or not, based on this is this enough evidence to warrant pulling the quad cap out for further testing or not?
    I'm not so sure about the resistance test, I decided to check pin #8 again and got a reading of 1.13 M one time and then another at .654 K so I'm wondering if my poking & tugging on wires may have exposed a bad solder joint, I have examined all solder connections using a magnifying glass and could see nothing that looked bad.
    Thanks again for your help.
    Roscoe

    Bob Latino
    Admin

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

    Re: ST-120 Issue

    Post by Bob Latino on Tue Dec 03, 2013 4:23 am

    Hi Roscoe,

    Your readings are OK - except that the SQUARE (40 uf section) is measuring a little high. Question > Do both chokes have one lead on the 40 section and one lead on the 80 section?

    With the amp OFF > Check the resistance from the 40 section to the 80 section. You should get about 20 ohms to 30 ohms.

    Those caps are OK. If one section was bad, then it would drag down the readings on the other sections

    Try reheating the all the solder connections on the quad cap, SCM and ESL cap ... be careful that the solder doesn't leak down and short to the chassis below.

    See if you can post a photo or two up here. Maybe a full sized one of the interior wiring and a closer look at the quad cap area.

    Bob

    j beede

    Posts : 317
    Join date : 2011-02-07
    Location : California

    Re: ST-120 Issue

    Post by j beede on Tue Dec 03, 2013 2:16 pm

    Couple things...

    Maybe your local guitar amp shop will sell you a 47uF cap or two to keep near your DVM. Swapping these in for a suspect quad cap section might assist in your debug.

    I have been very satisfied with Sovtek 5AR4 from Guitar Center in my MkIII. $12-13.

    The GZ-34 "flash" that you see is a clear indicator of a problem. If you go with a solid state rectifier I guess you will "let the smoke out" if there is a problem? Sounds expensive. Maybe the Weber is protected against short circuit loads?

    I have adopted the series 100W incandescent lamp approach during debug. If you have a variac, use it. If not consider putting a light bulb in series with your mains input. This is a crude check but might spare your rectifier during debug.

    1) All tubes in. No light=check fuse
    2) All tubes in. Initial bright lamp that gradually dims to low light=normal.
    3) All tubes in. Immediate steady bright lamp (until fuse blows)=could be filament supply.
    4) Rectifier out. Steady bright lamp=check filament supply and loads.
    5) All tubes out. Immediate steady bright lamp (until fuse blows)=Check filament supply.
    6) All tubes in. Initial bright lamp, then dims, filaments glowing, then lamp goes bright (until fuse or rectifier blows)=bad rectifier or HV short

    Maintarget

    Posts : 208
    Join date : 2013-02-10

    Re: ST-120 Issue

    Post by Maintarget on Wed Dec 04, 2013 1:22 am

    I re flowed all solder connections on quad cap and V1 pin #8 I them measured resistance from pin #8 to ground and had an initial reading of 1.0M ohm and then it steadily declined from there to about .700k ohm
    I really though I was on to something when I measured the capacitance again on the ESL and discovered that it was the same reading as the circle on the quad cap 20 section and the other end was going to ground at base of socket V7 so I pulled the ESL to get an accurate measurement and came up with 1.03 uF I then found the correct value on the schematic on the last page of the assembly manual, so all is good there.
    I sent a couple of photos at this point I'm not sure what next.
    I did verify the three rectifier tubes (one original and two JJs) were shorted with a tube tester that I inherited from my dad before he passed a couple years ago, funny he is still helping me out!
    Thanks again for your help.
    Roscoe

    http://i1325.photobucket.com/albums/u632/RoscoeAJones/photo_zpsfa051e3e.jpg
    http://s1325.photobucket.com/user/RoscoeAJones/media/photo_zpsd694713c.jpg.html?sort=3&o=0


    Last edited by Maintarget on Wed Dec 04, 2013 1:23 am; edited 1 time in total (Reason for editing : value error)

    Maintarget

    Posts : 208
    Join date : 2013-02-10

    Re: ST-120 Issue

    Post by Maintarget on Wed Dec 04, 2013 10:09 am

    j Beede I would like to try your suggestion but not sure where to find a 100W incandescent bulb anymore will a 60w work?
    Or can I go green and use a CFL, just kidding............
    Thanks for your help

    Maintarget

    Posts : 208
    Join date : 2013-02-10

    Happy days are here again!

    Post by Maintarget on Mon Dec 09, 2013 12:03 am

    Thanks to everyone's help I was able determined that my rectifier tube flash issues are in fact directly related to the poor quality of the JJ GZ-34 and more importantly I learned some trouble shooting skills.
    I went through one JJ in about 125 hours and a replacement did not survive the initial start up so I followed the recommended trouble shooting steps, re-flowed all solder joints in the circuit, decided to add the rectifier diode upgrade while i was under the hood and waited for a SovTek GZ-34 to arrive, went through the start up procedure and am back in business!!   
    To anybody out there trolling the web site reading posts setting on the fence about joining VTA Dyanco family all I can say is if you want to enjoy quality sound don't wait any longer!
    Thanks

    pigface

    Posts : 70
    Join date : 2011-04-10
    Location : pittsburgh

    Re: ST-120 Issue

    Post by pigface on Mon Dec 09, 2013 8:32 pm

    I've been useing a Mullard GZ33  in my ST120 since I built it . It's been in there for 2 1/2 + years so far with no problems yet . There are a few  NOS ones on ebay at the moment  . Not cheap but not real expensive either  .
    At $20.00 - $40.00 ?  for a new  re-make , and it seems inferior  GZ34 , even if you pay twice as much for a Good  NOS Mullard GZ33 and it lasts  three times as long  you're still ahead  a few dollars.    
    They are tall tubes and you probably couldn't fit a cover over one , that is  if you use a cover . The big bottle looks cool too.  Smile

    Maintarget

    Posts : 208
    Join date : 2013-02-10

    Re: ST-120 Issue

    Post by Maintarget on Mon Dec 09, 2013 9:05 pm

    You make a good point pigface about the cost of a quality tube there may be a NOS MUllard in my future Merry Christmas to me!!!!

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