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    Dim chrome top 12au7 in VTA 70 on right channel

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    tubenutr

    Posts : 61
    Join date : 2015-01-25
    Age : 59
    Location : Northern Ontario

    Dim chrome top 12au7 in VTA 70 on right channel

    Post by tubenutr on Thu Jul 09, 2015 9:51 pm

    This is driving me crazing. When volume is turned up, right channel starts with a popping sound.
    If turned down popping goes away.
    I have checked all solder joints, wire routing, etc.
    All my tubes were checked before my tester thru a hissy fit.
    Anyone???????????????? HELP
    If tube amps didn't sound so good I'd throw in the towel and
    go back to gritty SS.

    Maintarget

    Posts : 208
    Join date : 2013-02-10

    Re: Dim chrome top 12au7 in VTA 70 on right channel

    Post by Maintarget on Fri Jul 10, 2015 12:05 am

    Switch tube to left channel does problem follow?
    Clean and adjust/tension tube sockets (especially if you are a tube roller)
    Visually check All solder joints then check'em again using magnifying glass
    "USING EXTREME CAUTION LETHAL VOLTAGE" wiggle all related wiring while powered up using an insulated tool or DRY wood stick
    Re flow solder joints

    All specific information on these topics can be found on this awesome web site, just use the search feature...........
    Good Luck and let us know what you find.


    Last edited by Maintarget on Fri Jul 10, 2015 12:06 am; edited 1 time in total (Reason for editing : speeling)

    Kentley

    Posts : 300
    Join date : 2015-03-06
    Age : 64
    Location : Worcester, MA

    Re: Dim chrome top 12au7 in VTA 70 on right channel

    Post by Kentley on Fri Jul 10, 2015 12:16 am

    Maintarget wrote:Switch tube to left channel does problem follow?
    Clean and adjust/tension tube sockets (especially if you are a tube roller)
    Visually check All solder joints then check'em again using magnifying glass
    "USING EXTREME CAUTION LETHAL VOLTAGE" wiggle all related wiring while powered up using an insulated tool or DRY wood stick
    Re flow solder joints

    All specific information on these topics can be found on this awesome web site, just use the search feature...........
    Good Luck and let us know what you find.


    Absolutely concise methodology, sir.

    BTW, Dim Chrome Top is a treasured delicacy in the mountainous regions of China.

    Bob Latino
    Admin

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

    Re: Dim chrome top 12au7 in VTA 70 on right channel

    Post by Bob Latino on Fri Jul 10, 2015 8:40 am

    mlabrash wrote:
    All my tubes were checked before my tester thru a hissy fit.

    A tube tester can usually tell you whether a tube is BAD if the emissions are low and/or it has shorts. A tube that tests GOOD on a tube tester is no absolute guarantee that the tube is in fact good. The tube could have some hidden issue that does not show up unless the tube is pushed hard at high volume levels. The only absolute way to check out whether a tube is truly good or bad is by substitution of the questionable tube with another tube that is "known to be good".

    On the VTA ST-70 and ST-120 the CENTER driver tube affects both channels while the left driver tube affects just the left channel and the right driver tube affects just the right channel. If you have a noise/buzz/hum etc. that is on both channels, then suspect the main power supply - the rectifier tube, quad cap or any other component that may be common to both channels. If the hum/buzz/noise is on ONE channel then one triode section of the center driver tube could be bad or any component on that side of the amp from the input jack to output transformer could be the cause.

    Always try to eliminate tube problems FIRST by substitution. Always keep an extra rectifier, driver tube and output tube around to substitute for any suspect tube. If tube substitution does not eliminate the problem, then do a voltage check on the amp (if the amp will stay on without blowing the fuse). If the voltage check shows normal voltages, then shut the amp OFF and do some resistance and capacitance checks. If you intend to be your own "tube amp mechanic", you should have both a resistance and voltage meter AND a capacitance meter. Some multimeters do have a capacitance tester built in. Most DIY people have a multimeter but many do not have a capacitance meter. If you don't have a capacitance meter, you should get one.

    Bob

    tubenutr

    Posts : 61
    Join date : 2015-01-25
    Age : 59
    Location : Northern Ontario

    Dim chrome top 12au7 in VTA 70 on right channel

    Post by tubenutr on Fri Jul 10, 2015 3:04 pm

    Well, I'm not in China yet, but I would like some direction as to how to correct this problem:

    Tube is dull, at about midnight on the VC it start popping. Swap tubes, tube from left goes
    dim and popping starts again at midnight or high noon position.
    Bad cap or resistor (its futile you know). Checked all solder joints with a touch of a hot
    iron, made sure all wiring was it should be.

    In closing I would like to say "If its not Scottish, its crap"

    May the farce be wit u.

    Mike


    tubenutr

    Posts : 61
    Join date : 2015-01-25
    Age : 59
    Location : Northern Ontario

    Dim chrome top 12au7 in VTA 70 on right channel

    Post by tubenutr on Fri Jul 10, 2015 3:11 pm

    Bob,

    Is there any reasonably priced capacitance decent quality testers available that test
    high voltage caps?
    I'm a retired person (Section Cool and other considerations always get in the way, food, shelter,
    power bill, wife (SHH).

    Bob Latino
    Admin

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

    Re: Dim chrome top 12au7 in VTA 70 on right channel

    Post by Bob Latino on Fri Jul 10, 2015 3:25 pm

    mlabrash wrote:Bob,

    Is there any reasonably priced capacitance decent quality testers available that test
    high voltage caps?
    I'm a retired person (Section Cool and other considerations always get in the way, food, shelter,
    power bill, wife (SHH).

    Hi Mike,

    You can get them on Ebay for less than $20 shipped. You are in Canada so shipping would probably be a little more .. Check the link below ..

    Capacitor testers for sale on Ebay

    Bob

    tubenutr

    Posts : 61
    Join date : 2015-01-25
    Age : 59
    Location : Northern Ontario

    Testers

    Post by tubenutr on Fri Jul 10, 2015 4:14 pm

    Thanks Bob.

    Are sufficient for high voltage caps?

    Mike

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dim chrome top 12au7 in VTA 70 on right channel

    Post by Bob Latino on Fri Jul 10, 2015 4:19 pm

    Hi Mike,

    Since you test the caps with the amp OFF, the voltage rating of the caps doesn't really matter .. The caps will only look at the 9 volts from the battery inside the meter ...

    Bob


    tubenutr

    Posts : 61
    Join date : 2015-01-25
    Age : 59
    Location : Northern Ontario

    VTA 70 issues

    Post by tubenutr on Fri Jul 10, 2015 4:50 pm

    Bob,

    As always thanks.
    What kinda bothering me is when I re-oriented the quad cap, the popping is solely on the right
    channel.

    Could this be a bad cap on the right tube socket 6 or something deeper?

    My final test was with new tubes, not NOS that verified by the vendor.

    Every tube that when on the right side was dimmer than the rest. Went as far
    as swapping out the EL34's for brand new matched and no change.

    If possible can you provide me with areas to check, so as to cut down my time listening to a SS amp?

    Thanks muchly again.

    Mike

    Bob Latino
    Admin

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

    Re: Dim chrome top 12au7 in VTA 70 on right channel

    Post by Bob Latino on Fri Jul 10, 2015 4:58 pm

    Mike,

    Re: >"If all the tubes on the RIGHT side are dimmer than the left side then there is the possibility that the 6.3 volt AC line (the two brown wires that go to pins 2 and 7 on V7) are not putting out the full 6.3 volts AC. Check the AC voltage across pins 2 and 7 on V7.

    Re:> "Could this be a bad cap on the right tube socket 6 or something deeper?" It could be a bad main coupling capacitor on the driver board to V6? You can test the main coupling caps in circuit (with the amp OFF) ..

    See if you can post on the forum here a photo of the inside wiring of the amp in question. We can take a look at it and maybe find something in the wiring that could cause the issue you mentioned?  See the Sticky post above on how to post a photo on the forum here ..

    Bob

    tubenutr

    Posts : 61
    Join date : 2015-01-25
    Age : 59
    Location : Northern Ontario

    VTA 70

    Post by tubenutr on Fri Jul 10, 2015 6:27 pm

    Not all the tubes are dim, just the 12au7 on the driver board, which has been changed out, swapped, etc.

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