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    Blowing rectifier tubes

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    krebob

    Posts : 14
    Join date : 2011-01-08

    Blowing rectifier tubes

    Post by krebob on Mon Dec 19, 2011 12:01 am

    Hi folks
    I've been blowing rectifier tubes like crazy in my st-70 since I put in the VTA mod w/ the power supply cap upgrade 6 months ago.
    At the same time I put in a new 80-40-30-20 quad cap with the 80 uf section oriented towards the back of the amp.
    I've also done the diode mod with in 4007's and that hasn't helped.
    Mostly there have been sparks at the bottom of the tubes after warm up.
    Two have glowed bright red, also after warm up.
    Voltages at pins 7&5, before the diodes are 380ac.
    And the voltage between 8&2 is 5.6 ac.
    Does anybody have any ideas?
    Seems like a lot of people think that modern gz34's are pretty unreliable.
    I did have a PM that lasted 5 months.
    Thanks.
    Bob

    tubes4hifi
    Admin

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    Re: Blowing rectifier tubes

    Post by tubes4hifi on Mon Dec 19, 2011 2:42 am

    are you running the correct amount of bias?

    JunkyJan

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    Join date : 2008-12-09
    Location : BC, Canada

    Re: Blowing rectifier tubes

    Post by JunkyJan on Mon Dec 19, 2011 6:18 am

    Hi Krebob

    I built two Mark III kits with the same VTA boards... I recall that I managed to "pop" one of the rectifier tubes by incorrect wiring between the quad cap (also an 80-40-30-20) and the VTA board (when building the 2nd amplifier - lost concentration I guess). Check the wiring VERY carefully, unfortunately I can't remember how / what I managed to mess up - but I DID mess up something really simple for sure, and it did take out the rectifier tube.

    FWIW, I work from home and my Mark IIIs are running almost constantly every day for going on two years now - probably 1000s of hours of operation - and I have never lost a rectifier tube again (I actually "cycle" between my Mark IIIs and my ST-70, just to spread the "load" a bit).


    -- JJ

    krebob

    Posts : 14
    Join date : 2011-01-08

    Re: Blowing rectifier tubes

    Post by krebob on Mon Dec 19, 2011 9:46 am

    Hello,
    Yes, I've double checked the bias and the wiring around the quad cap and rectifier tube.
    I will go through it all again.
    Also, I did just put in the triode mod.
    I'll go through that wiring again too.
    Thanks for your ideas.
    Bob

    Bob Latino
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    Re: Blowing rectifier tubes

    Post by Bob Latino on Mon Dec 19, 2011 1:12 pm

    It is also possible that you have a bad output tube or tubes OR bad output tube sockets. You did mention "sparks at the bottom of the tubes after warm up". If you have other output tubes for your amp, I would try them in the positions that "sparked". Sparking under tube sockets could also indicate bad tube socket pins that are loose and don't grip the tube pins with enough tension. This is especially true of pins 3 and 4 on each output tube socket which carry about 425 to 440 volts DC and would have a high voltage gradient from a connect to a non connect situation. You could also try squeezing (with the tubes NOT plugged in) the socket pins for the tube sockets that sparked ...

    Bob

    anbitet66

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    Location : Valley Stream, NY

    Re: Blowing rectifier tubes

    Post by anbitet66 on Mon Dec 19, 2011 2:26 pm

    Bob (krebob),

    Look at the sockets that sparked under the tubes for carbon tracks. You may need a strong light source to tell if there are any, but it is possible the socket(s) may have a conductive carbon track to the chassis. I found (caused Embarassed ) a carbon track on my dad's EL84SE stereo amp by accidentally feeding a signal without a speaker connected. That caused an arc to ground, and the arc caused a carbon track to form and it was severely loading the rectifier tube and power transformer.

    Fastest socket replacement I ever did in my life. He still doesn't know about it nearly 20 years later.

    Tony

    wedg714

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    Join date : 2011-11-23

    Re: Blowing rectifier tubes

    Post by wedg714 on Mon Dec 19, 2011 3:52 pm

    what bias are you using?

    krebob

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    Join date : 2011-01-08

    Re: Blowing rectifier tubes

    Post by krebob on Mon Dec 19, 2011 9:46 pm

    Hi Folks
    The sparks I'm getting are on the inside of the rectifier tubes, towards the bottom.
    It's happened to several of them.
    The output tubes are fairly new winged c's
    The bias I'm using is 0.40 vdc.
    Thanks,
    Bob

    krebob

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    Join date : 2011-01-08

    Re: Blowing rectifier tubes

    Post by krebob on Mon Dec 26, 2011 5:51 pm

    Hi Folks
    I'm still trying to understand why I'm going thru all these rectifier tubes.
    Any ideas would be greatly appreciated.
    Thanks,
    Bob

    Bob Latino
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    Re: Blowing rectifier tubes

    Post by Bob Latino on Mon Dec 26, 2011 6:14 pm

    Hi Bob,

    I know that you said you replaced the quad cap - BUT - there is a possibility that one section of the quad cap has gone bad. See if you can beg, borrow, steal or buy a cap tester or a new multitester that measures capacitance. Now - WITH THE AMP OFF - measure the capacitance of each section of the quad cap. I know that your cap is an 80, 40, 30, 20 cap but - the capacitance from one section will leak over to the other sections. With a VTA driver board and an 80, 40, 30, 20 cap ...

    The 40 section should measure 70 to 100 uF
    The 80 section should measure 160 to 200 uF
    The 20 section should also measure about 160 to 200 uF
    The 30 section should measure about 40 to 50 uF

    When you measure > the capacitance will take 10 to 15 seconds to stabilize as that cap section charges up from the battery in your meter. If the capacitance is either considerably lower than normal OR will not stabilize and the value keeps bouncing around - then that quad cap is bad.

    Never assume that just because a tube OR a quad cap is new or near new that it "has to be good". One good test of a bad output tube is to power up (on an ST-70/ST-120) JUST ONE CHANNEL. If one channel won't trip the fuse and the other does trip the fuse then usually one of the two output tubes on the channel that trips the fuse is bad. You can also look at it this way - IF one channel will power up and play without tripping the fuse then - you have exonerated the rectifier and the quad cap as the cause of the problem. If EITHER channel (separately) will trip the fuse then probably the quad cap is bad.

    Bob

    krebob

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    Join date : 2011-01-08

    Re: Blowing rectifier tubes

    Post by krebob on Mon Dec 26, 2011 6:41 pm

    Thanks for your reply Bob
    My DMM does have a capacitance test on it.
    I took the cap out of the unit and each section measures very close to what it's rated.
    I haven't been blowing fuses, just the rectifier tube.
    Is it still possible that I have a bad output tube?
    Unfortunately, I don't have any gz34's in stock that don't spark right now.
    Thanks,
    Bob

    Bob Latino
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    Re: Blowing rectifier tubes

    Post by Bob Latino on Mon Dec 26, 2011 6:47 pm

    Bob,

    Get a NEW rectifier tube and try powering up just ONE channel. Pull the output tubes from the other channel. When you turn the amp on keep your eyes on the rectifier tube and your hand on the power switch. The key time is maybe 15 seconds or so after turn on. At the slightest sparking on the bottom of the rectifier tube - > TURN THE POWER SWITCH TO OFF. Next try the same thing with the other channel alone with the power tubes pulled from the channel that you just tried.

    Bob

    burnedfingers

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    Re: Blowing rectifier tubes

    Post by burnedfingers on Tue Dec 27, 2011 11:27 am

    There are a few 5AR4's to stay away from. The cheap Chinese 5AR4's will always go up in a display of sparks. Another is the JJ tube as these don't last long either. a suitable replacement is the Sovtek 5AR4 rectifier. There is absolutely no need to put out your hard earned bucks on a Mullard or GZ37 from across the pond.

    Have to ask this....which cap section follows the 5AR4? About 40-45mfd is all that one can safely use after the 5AR4. Larger first cap than that usually ends up bad.

    krebob

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    Join date : 2011-01-08

    Re: Blowing rectifier tubes

    Post by krebob on Tue Dec 27, 2011 11:44 am

    Hello
    So far I've had 2 JJ's, 2 sovetek's go up in sparks.
    I had a pm that lasted for 5 months before it started glowing bright red.
    I do have the 40uf section of the quad cap following the 5ar4.
    I do appreciate the question because I have made plenty of those kinds of mistakes.
    I should have a TAD 5ar4 in the mail today.
    Thanks for your reply.
    Bob

    burnedfingers

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    Re: Blowing rectifier tubes

    Post by burnedfingers on Tue Dec 27, 2011 11:52 am

    How old is your cap? Is it a NOS? Where did you purchase it? Brand?

    krebob

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    Join date : 2011-01-08

    Re: Blowing rectifier tubes

    Post by krebob on Tue Dec 27, 2011 12:06 pm

    It's 6 or 7 months old from Dynakit parts.
    My DMM has a capacitance test and shows it to be okay.
    Is there any way to test it under full voltage?

    Thanks,
    Bob

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: Blowing rectifier tubes

    Post by j beede on Tue Dec 27, 2011 12:24 pm

    I have had good success with cheap Shuguang and Sovtek rectifiers. I don't think the GZ34/5AR4 are your problem. "Pre-rectifying" with silicon diodes should be fine, but I would avoid bypassing your 5AR4 with diodes until you root cause the failures. Are you still seeing glowing red plates in the output pentodes? You may need to darken the room to see this. By any chance would anyone be cycling your amp on/off without waiting a minute or two between turn-on events?

    burnedfingers

    Posts : 31
    Join date : 2009-05-06

    Re: Blowing rectifier tubes

    Post by burnedfingers on Tue Dec 27, 2011 12:49 pm

    If you have had good luck with the Shuguang tubes you are in the minority because most do NOT survive past 6 months tops. The Sovtek on the other hand have had a good reliability record and the tube is known on several other forums for reliability. Note* I actually liked the sound of that rectifier up until the sparks flew.

    A good post has been made here... what is your bias point 40mA per output tube? or higher? Individual tube bias? Roys board? A bit of personal experience here... had one of Roys boards in the past and the individual 50K bias pots are tricky to adjust and generally do not stay put regardless of if its new tubes or old. The 50K pots just make bias adjustment finiky in my opinion but are still better than the stock boards.

    The power supply mod (added diodes) is meant to take the strain off the 5Ar4 because they do the rectifying but the 5Ar4 still allows for that nice slow easy B+ ramp up. If it were me I would lower the bias to 40mA per output tube. Then systematically check the driver board stages for proper voltage and operation. Check and record all voltages and put them up against the normal voltages for this particular driver board.

    krebob

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    Join date : 2011-01-08

    Re: Blowing rectifier tubes

    Post by krebob on Tue Dec 27, 2011 9:01 pm

    Hello, and thanks for your interest,
    I haven't seen any glowing plates on the output tubes. Only on a couple of the 5ar4's.
    I do have one of Roy's VTA boards with individual bias pots set at .40 VDC as per Roy's instructions.
    The pots are sensitive, but I haven't had any problem with them.
    I should get new rectifier tubes on Thursday.
    Is there any other tests I can do without a tube?

    Also, can someone tell me what exactly is happening when the 5ar4 starts sparking?
    Is there too much current going through the tube?

    Thanks again,
    Bob

    j beede

    Posts : 328
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    Location : California

    Re: Blowing rectifier tubes

    Post by j beede on Tue Dec 27, 2011 9:40 pm

    One thing I did when I was having quad cap problems was I bought a single polarized 10-20uF, 600V electrolytic cap from a local guitar amp shop. I then used that cap as a surrogate for the each segment in the quad cap--one at a time--to see which segment was bad. Like you, my quad cap measured okay at the low voltage used by the multimeter. Of course you might have multiple bad segments.

    burnedfingers

    Posts : 31
    Join date : 2009-05-06

    Re: Blowing rectifier tubes

    Post by burnedfingers on Wed Dec 28, 2011 7:18 am

    quote
    Also, can someone tell me what exactly is happening when the 5ar4 starts sparking?
    Is there too much current going through the tube?


    Yes, the 5AR4 starts to spark or ark because of too much current. Usually this condition is caused by either a bad section of the Quad cap or a first section that is larger than the recommended value of 40mfd. This is exactly why I am a fan of the add on power supply boards that are available as an aftermarket thought. Once a tube does this the condition only gets worse in my experience. With respect to the bias setting pots... my experience again... even with new matched tubes the bias setting drifts a little. Have had the same experience with 50K pots in my Dynaco's and in my Quicksilver mono block amps. The problem seems to go away with 10K pots. On the positive side the 50K pots allow for a broader array of tubes. My dynaco's will allow the usage of EL34's,6BG6GA's 6L6GC's, and using a switch to modify the circuit I can use KT88's and 6550's.

    burnedfingers

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    Join date : 2009-05-06

    Re: Blowing rectifier tubes

    Post by burnedfingers on Wed Dec 28, 2011 7:28 am

    Quote:
    t's 6 or 7 months old from Dynakit parts.
    My DMM has a capacitance test and shows it to be okay.
    Is there any way to test it under full voltage?



    If it were mine I would construct a test circuit that woul allow me to power up the cap from a rectified variac and monitor the current draw on the cap. If the cap were to draw too much then the cap is defective. This is the problem with purchasing caps like the Quad cap off a suppliers shelf. You simply don't know how old it is. I personally have had several of the "new" quad caps fail in the past and whipe out a good rectifier. While not a fan of the Mullard or other expensive rectifiers I do like something along the lines of a Sylvania or Ge rectifier with some age on it.

    Your meter will show an obvious shorted condition but not necessarily a cap that is drawing too much on the get go.

    krebob

    Posts : 14
    Join date : 2011-01-08

    Re: Blowing rectifier tubes

    Post by krebob on Mon Jan 09, 2012 10:06 pm

    Hi Folks
    So I did figure out why I was going through all those rectifier tubes.
    I had just put in the pentode- triode switches and used the grounded lug on the terminal strip for pin 1.
    I must have looked at it a couple dozen times before I "saw" it.
    Thanks to all of you for your ideas and support.
    I'm learning a lot these days, though I'm often learning the hard way.
    I'll be coming back soon with my latest fiasco.
    Be Well,
    Bob

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