ST-120 Issue

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.

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

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

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.

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

[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

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

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

https://2img.net/h/i1325.photobucket.com/albums/u632/RoscoeAJones/photo_zpsfa051e3e.jpg
https://s1325.photobucket.com/user/RoscoeAJones/media/photo_zpsd694713c.jpg.html?sort=3&o=0

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

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

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.  

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!!!!