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Master volume Twin Reverb blowing fuses

Discussion in 'Amp Tech Center' started by jondanger, Sep 24, 2016.

  1. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    So I couldn't resist a MV Twin Reverb that was on Craigslist for $200. It had blown the fuse, so I took one with me along with a light bulb limiter. It powered up with a new fuse and made a sound, so I grabbed it.

    I get it home and dig into it, and when I plug it in without the limiter, the fuse goes. Here are some voltage in various states:

    With all the power tubes pulled, the fuse stays, and the limiter is barely glowing. The limiter is slightly brighter with the standby in the operate position. The fuse does not pop when plugged in without the limiter and all power tubes pulled.

    Pin 2 of power tubes: 2.38vac
    Pin 3 of power tubes: 342v
    Pin 7 of power tubes: 3.08vac

    With the power tubes in, the fuse pops when amp is plugged in without the limiter. With the limiter, the fuse stays, but the limiter glows brighter than it did with the power tubes pulled. Still not full brightness though. Marked difference in brightness with standby in the operate position. I'm getting sounds out of it like this, but they're not pretty. The bias probe says I'm drawing 12.3ma on the power tubes.

    Pin 2 of power tubes: 1.67vac
    Pin 3 of power tubes: 171v
    Pin 7 of power tubes: 1.68vac

    The amp is generally clean, and doesn't look like it has been abused or wet. One filter cap has been replaced, the rest are original. Tranny dates are '75.

    Any ideas? Am I right that if the fuse doesn't blow with the power tubes pulled, then I'm likely to not have a blown PT? I unfortunately don't have a new quad of 6L6s handy.
     
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Try adding one power tube and power it up with the limiter.

    Try one tube at a time in one socket and see if the limiter goes brighter with one of the tubes--indicating a bad tube.

    If it doesn't try a tube in the other three sockets, one at a time. If it goes bright you found a bad socket--check all the components connected to that socket, especially the screen resistor.
     
  3. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    Thanks Rob - I went through that sequence and didn't find a tube or a socket that stands out in terms of brightness. One thing that I noticed though was that if I left the amp on with either one or all the tubes in, after about 30-40 seconds, the brightness increases significantly.

    All the electrolytics on the board are original white Mallorys. I'm wondering if the caps in the bias circuit aren't doing their job anymore. It's going to need to be done anyway I know, but I'd rather know that there wasn't a larger issue before I start sinking parts dollars into this thing.
     
  4. Thin69

    Thin69 Friend of Leo's

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    At $200 you're not going to go wrong. Since all but one filter caps are original I'd go ahead replace all of them with F&T's and do the bias cap also. Caps do short.

    Your plate voltage is way low even without a tube inserted.
     
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  5. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    Thin69 - yeah I figured at $200 there's not a whole lot to lose. I guess I'll order filters and bias supply caps and see where it goes from there.

    Does the low plate voltage suggest a shorted cap somewhere to you?
     
  6. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The low plate voltage means the power transformer is being loaded down by a higher than normal current that's blowing the fuse. It might not be a complete short but something is passing a crap load of current to ground which a bad filter cap can do.
     
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  7. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    Got it - thanks Rob, just the explanation I was looking for.
     
  8. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    Before I place this order, does anything about the voltages or behavior of the amp with the tubes in vs out scream "blown power tubes"? If I can keep these JAN Phillips in there, that would be cool - although I know the 6L6WGB is not the same as a 6L6GC. I've liked every JAN Phillips 12A*7 that I've used, suspect these might be good sounding tubes.

    [​IMG]
     
  9. Thin69

    Thin69 Friend of Leo's

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    It's not definitive proof but the caps certainly can pull down your plate voltage. With the tubes installed I've got 443 volts (power tube plates) on my MV 72 SFTR and it will should be a little higher with the tubes out. Your amp should be about the same. I think the filter & bias caps are a logical starting point as they need to be changed out anyhow.

    For your reference here is Geofex's power transformer troubleshooting web page. It's fairly simple and straightforward if you would like to test you PT while you're waiting for caps. Just be extremely careful with the high voltage portion of the test.

    http://www.geofex.com/ampdbug/pwrtrans.htm
     
    Last edited: Sep 25, 2016
  10. Thin69

    Thin69 Friend of Leo's

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    You might have a bad tube but your plate voltage is very low without the tubes so I would suggest fixing the low plate voltage condition first. With the Standby off B+ should be ~478 volts, w/Standby on should be ~450 volts. I haven't used 6L6WGB myself but lots of folks like them.
     
    Last edited: Sep 25, 2016
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  11. Thin69

    Thin69 Friend of Leo's

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  12. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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  13. elmoscafeo

    elmoscafeo Tele-Afflicted

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    In addition to plate voltage being low, combined with a huge drop in plate voltage when the tubes are plugged in...your heater voltage is way too low also. You should be up at 6.3Vac. Check your voltage at your pilot light. That should be a direct connection from the secondary side of your PT.
     
  14. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    Do I check between the two sides of the pilot light, or between one of the sides and ground?

    Yeah, I noticed the heater voltage was low, and was wondering what the reason for that might be. Worst case scenario a short between the heater and high voltage secondaries? I'm a lot more novice than a lot of y'all on here, but I'm trying to learn by osmosis.
     
  15. Thin69

    Thin69 Friend of Leo's

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    I'd check between the two side then also each side to ground. Each side to ground should be identical on that amp.
     
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  16. elmoscafeo

    elmoscafeo Tele-Afflicted

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    Pull all of your tubes so you have no heater influence. Check each side of the light to ground, then side to side. You should have very little voltage side to side, and approximately 6.3 Vac each side to ground
     
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  17. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    I'm not around the amp right now, but I'll get those voltages when I get home. Thanks guys.
     
  18. elmoscafeo

    elmoscafeo Tele-Afflicted

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    Since you have a limiter, I've attached the following link. Hopefully, you can get this nailed down in about an hour. Personally, I'm leaning towards a bad filter cap, though the low heater voltage has me concerned also. 2 problems?


    https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm

    Plug the amp in to the light-bulb limiter, turn the amp on in 'standby' mode (if available).

    Switch standby to 'play'. If the lamp lights brightly this indicates a short somewhere in the amp.

    Remove power tubes, one at a time. If the light stays bright, that tube can be put back into the amp. If the light dims, the tube you just removed is shorted. If none of the tubes appears to be shorted, go to the next step.

    Remove all power tubes together. If the light dims, the problem is probably in the bias supply. If the light remains bright, the problem is (probably) a bad rectifier, shorted power transformer, or shorted filter cap - see next step.

    Now switch the amp to standby mode. If the light goes out, the problem is in the filter caps. If the light keeps shining bright, the problem is is the transformer or the rectifier - see next step.

    Remove the rectifier (or disconnect the diodes if it's a hard-wired solid-state rectifier). If the light dims, the rectifier is bad. If the light stays bright, the problem is probably in the power transformer.

     
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  19. elmoscafeo

    elmoscafeo Tele-Afflicted

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    Be VERY, VERY careful!

    Since you have a limiter, you probably have some knowledge, but the area that you will be playing in has lethal voltage. One hand in amplifier, One hand in pocket.
     
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  20. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    Yeah, I am pretty careful about that. I've brought a SF Vibrolux that had similar issues back to life with help from TDPRI, but the Twin is a different beast.
     
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