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Thread: vox ac30 cc2 blowing fuses and rectifiers sparking????

  1. #1
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    vox ac30 cc2 blowing fuses and rectifiers sparking????

    Hello all!

    My vox is having issues. Its blowing fuses. I have replaced the tubes twice this week thinking I just picked up faulty tubes. I noticed that whenever i put a new fuse in, the rectifier tube starts sparking then the fuse blows within a second. If I take the rectifier out the amp runs without blowing a fuse. Any ideas?????

    Oh! One more thing. With the amp turned off and with my diode check on my multi meeter there is a short across every output jack. ?????????

    Thanks in advance!

  2. #2
    Senior Member Enzo's Avatar
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    The rectifier tube is sparking? Then I'd replace the rectifier tube and see what happens.

    Try this. Leave in the rectifier tube, but remove the power tubes. Does the rectifier still spark?

    Certainly it is possible there is a shorted filter cap, or a short in the output transformer. But I'd bet on the rectifier tube first.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    The rectifier tube is sparking? Then I'd replace the rectifier tube and see what happens.

    Try this. Leave in the rectifier tube, but remove the power tubes. Does the rectifier still spark?

    Certainly it is possible there is a shorted filter cap, or a short in the output transformer. But I'd bet on the rectifier tube first.

    Sparking- illuminating bluish/purple colors. When it happens the lights in my room dim. Its the second rectifier tube I bought for it. the power amp tubes are the second set this week that I purchased for it as well.

    I tried pulling out the power tubes and the sparking still occurred.
    Should all the output jacks be shorted?

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    Quote Originally Posted by sdre View Post
    Sparking- illuminating bluish/purple colors. When it happens the lights in my room dim. Its the second rectifier tube I bought for it. the power amp tubes are the second set this week that I purchased for it as well.

    I tried pulling out the power tubes and the sparking still occurred.
    Should all the output jacks be shorted?
    Hi sdre,
    Are you sure your output jacks are shorted? OT's secondary windings have a pretty small DC resistance....so....do you have a dead short ( bad, indicating a possibly fried OT, though this is a rare event ) or a very low resistance ( normal )? Are you measuring with a jack plug or straight on the sockets? I' m asking this because some output sockets short the signal to GND when a jack plug is not inserted, to prevent the amp from working with the OT secondary open and the output tubes from arcing. Unlike SS amps, tube amps should never work without a load connected.

    BTW I wouldn't measure DC resistance with a diode tester, which does not measure resistance, but rather gives a diode's forward voltage reading.

    If your OT reads OK you can proceed as follows:

    If you replaced the power tubes AND the rectifier, the rectifier is still sparking, and your room's light dim when you turn on the amp ( indicating the mains voltage is dropping due to excessive current, so it looks like a dead short ) I would check for dead shorts in the rectifier/filter caps area and the +B rail, like a bad solder joint, a solder joint touching the chassis somewhere, a bent socket lug or shorted filter caps.

    Hope this helps

    Best regards

    Bob
    Last edited by Robert M. Martinelli; 01-16-2009 at 07:23 AM.

  5. #5
    Senior Member Enzo's Avatar
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    Your output is not shorted, the OT secondaries have a very low resistance. With the power tubes removed it wouldn;t matter at all if they were.

    If any rectifier fails in there then you have a short across the B+ somehow. The secondary winding of the output transformer is not the issue, we need to check the primary. The center tap goes to B+ and the ends of the winding go to the power tube plates. Are the power tube plates - or their socket pins - shorted to ground? Is the main filter cap shorted?

    The output transformer could have a short from the primary winding to the frame, or to the secondary windings. SInce the secondary has very low resistance and is grounded, it becomes the same thing.
    Education is what you're left with after you have forgotten what you have learned.

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    Oops, Enzo, looks like we just had a "simulpost" ( sorry if I dare to use this new word you created, hope you don't want royalties on that ).
    Cheers
    Bob

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    The short, which i'm testing with a diode test on the meter, is present no matter which output jack has a load plugged into it.

    Is there a way of testing the O.T without power? I do not have access to a variac.

    Im not familiar with vox's circuit, do any of you have a link to the schematics on a vox ac30 cc2? That would be amazing.

    Im not sure where the mains filter cap is located. Even if I where to know, i'm not sure I can even get to the legs of the cap. They are all so close to the board and it looks like i may have to do some de-solidering to get the board off the chassis.

    Enzo I checked for shorts on the power tube sockets and pins 3, 4 and 5 are shorted to ground on all of them. is this pointing to the mains filter cap being shorted? if so where is it?

    You have to forgive my terminology as I am new to tube amps.
    Thanks guys.

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    I strongly advise you to use the proper instrument for your measurements.

    A diode tester is NOT what you need for proper resistance measurements.

    You need to set your DMM to Ohms to measure resistance.

    The reason why you read a "short" ( which I believe to be a small resistance instead ) on pins 4 and 5 is that they're the heater pins, so, if your PT 6,3 VAC secondary has one side or the center tap to GND ( usually done to reduce hum ) you can think there's a "short" to GND there.

    Also, as already stated, the OT secondary has a very low DC resistance ( a few Ohms ), but it's ( normally ) not a dead short to GND.

    Pin 3 is cathode on EL84s - 6BQ5s ; if the AC30CC2 is cathode biased like the old ones ( these new lookalikes are quite different ) then you should read about 50 Ohms to GND with the tubes in, if it is a "fixed bias" then it's normal for the cathode to be connected to GND as the grid is made negative with respect to the cathode by external means ( a negative voltage from the PS section ).

    Whatever the amp, the filter cap(s) are located right "after" the +B rectifier section.

    Regards

    Bob

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    The schematic can be found here.
    Code:
    http://www.mzm.us/AC30CC_Schematics.pdf

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    Thanks Fragger,


    sdre,
    So...the amp has a switchable cathode bias, so, depending on the bias switch position, you should read about 80 Ohms or 50 Ohms between pin 3 ( EL84s ) and GND.

    C4 and C5 are the filter caps we were talking about. Be careful and make yourself sure to discharge them before working on the amp.

    Good luck

    Bob

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    Guys forgive me but when you say "B+" what are you referring to?

    I set the dvmm to ohms and 50ohms is what is reading across pin 3 (with the cathode switch to 50ohms)

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    Senior Member trevorus's Avatar
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    Mine did this once. It was a bad rectifier. I have a Ruby Tubes rectifier in there now, and haven't had problems since. I wondered when it happened to mine how common of a problem it would turn out to be.

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    But what are the odds that I have purchased two bad rectifier tubes?

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    Quote Originally Posted by voxrules! View Post
    Thanks Fragger,


    sdre,
    So...the amp has a switchable cathode bias, so, depending on the bias switch position, you should read about 80 Ohms or 50 Ohms between pin 3 ( EL84s ) and GND.

    C4 and C5 are the filter caps we were talking about. Be careful and make yourself sure to discharge them before working on the amp.

    Good luck

    Bob

    VOXRULES, With a dmm what is the best way to test the caps?

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    Senior Member trevorus's Avatar
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    Quote Originally Posted by sdre View Post
    But what are the odds that I have purchased two bad rectifier tubes?
    What brands were they?

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    sovtek

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    Senior Member trevorus's Avatar
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    Quote Originally Posted by sdre View Post
    sovtek
    That's the one that failed on me. I'd go with something different. Maybe this amp and that tube just don't get along.

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    sdre,
    "+B" ( or + HT ) referred to tube ( or valve ) amps refers to the high DC voltage rail supplying the tubes' ( or valves ) plates ( or anodes ). In your amp the +B is the positive terminal of C4-C5.

    In Push Pull amps it goes from the filter caps to the OT primary winding's center tap, with the two ends of the primary winding "feeding" the power tubes' plates.

    Push Pull designs need little filtering because whatever hum/ripple tends to be canceled by phase opposition on the OT; the preamp stages need better/additional filtering to keep the hum/ripple from being amplified together with the "useful" signal.

    As to measuring caps with a DMM, you could try with the DMM set to measure high resistance. Discharge the caps first, and MAKE YOURSELF SURE they' re discharged by measuring the DC voltage across them.

    Disconnect one terminal from the cap under test to keep the measurement from being spoiled by other components.

    Set the DMM to measure high resistances ( MOhms ) and put the leads on the cap you're measuring.
    You should see the resistance start from zero and then slowly rising to infinite, if this does not happen and the reading stops showing a certain resistance, then the cap is leaky.

    Obviously, the best thing to do would be to get a capacitance meter, because the DMM method tells you nothing about the caps' real capacitance and ESR, but in this case, since we're looking for a leaky cap, that's not of our concern so the DMM method should probably suffice.

    Though unlikely, it's always possible you got yourself two bad rectifier tubes, as unfortunately tubes' quality is not as good as it used to be in the "old days". If all the filter caps test OK you either have a bad rectifier or a short to GND somewhere else. As already said, check for shorts, bent lugs, solder drops etc.

    Hope this helps

    Best regards

    Bob

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    You guys thank you so much for your time! I went ahead an tested the caps..... they where fine. so i bit my tongue and bought a new tube of a different brand. Works perfect! Now for future reference are their any accurate tube testers somewhat affordable so I don't run into this again???

    I appreciate your time and patience.

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    One more thing. For testing caps in tube amplifiers, are there any recommendations for a capacitance meter?

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    Hi sdre,
    glad you fixed it, glad we've been able to help and glad you've probably learned something new, which is always a good thing....

    As to choosing a good/reliable tube tester I can't help you, as I use a self-built one, but I'm sure you'll get a lot of good suggestions from our fellows solder fume breathers ( thanks Enzo! )

    As to the capacitance meter, i use a Lutron 9073 ( LCR Meter ) and it's good value for the money....I use it also to measure pickups with pretty good results; anyway, if you only need to state if a cap is leaky or not the DMM method I have explained you in one of my previous posts will suffice.
    Best regards
    Bob

  22. #22
    Senior Member Enzo's Avatar
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    I don't use a tube tester. One can tell me a tube is definitely bad, but it cannot tell me a tube is definitely good, and that is what I need to know. If I want to know if a 12AX7 is bad, then I stick a new one in the socket and find out. That is a ton faster than testing the old tube. If the new tube makes the difference, then thre you are. if the new tube makes no difference, then stick the old one back in.

    And not so be a stick in the mud, but I don;t use a cap tester either. In my repair experience, I don't find caps off value much. If they test off value, it is most likely because they are real leaky or dried out. ANd leaks often don't show up at hand held tester voltages. A cap might be fine at 2v, or 10v, or even 100v. But leak like a sieve at 400v. I gotta know what it will do under the conditions of a real tube amp circuit. So unless the tester puts 400v across the cap to do a leakage test, it doesn;t help me.

    I find it simple enough to find a cap is leaky by using my voltmeter in circuit. And just like the tube tester, swapping parts is a lot faster. If I think a cap is bad, I replace it. At a dollar a minute labor, how long should I spend removing a cap from a circuit and setting up the test gear all to see if a dollar part is bad, then returning it?
    Education is what you're left with after you have forgotten what you have learned.

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    Similar problem

    I've got a similar problem wth my AC30-CC. No lights, no power, no nothing. I've replaced the rectifer tube(it had a sovtek, so I've replaced it with the same) thinking that might be problem. But it has done nothing, the fuses seem ok, and all connections seen fine too. the only thing I've noticed is that one of the caps(22uF on the board the retifier tube is connected to) has a darker connection than the others. could it be a falty new valve or more likely the cap, or perhps somthing else.

    Thanks alot in advance

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    Fixed

    I fixed the problem, I mistook the HT fuse for the main fuse which was hiding in the input connector, wierd place to put it I thought, but once I changed that, she was all good.

    So in the end, just the rectifier tube and main fuse, so stoked...

    Ps. I know this thread is from ages ago, but it may help someone else.

  25. #25
    Supporting Member Alex R's Avatar
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    Yep that little black drawer under the mains input socket is only easy to spot after you've opened for the first time and found the fuse in there. Generally they have room for a spare too, which is handy, though I know from experience that it is possible to fit a fuse in the space for the spare and find the amp still don't work, and then take it to a repairer - and then to be lucky enough to find that the repairer (me) was having a good month and didn't charge the bench fee for moving the fuse.

    Rectifier tubes do fail all on their own often enough, and you can see sparks in there sometimes.

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