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    Looking for quick help with VTA st-120

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    Bennyhaha812

    Posts : 1
    Join date : 2016-12-06

    Looking for quick help with VTA st-120

    Post by Bennyhaha812 on Sun Nov 05, 2017 9:23 pm

    I build an amp a while ago and it's been awesome but I need feedback as to what might have happened before I troubleshoot. I was listening to music this evening and the sound had a pop and silence and the back right power tube 6550 turned blue. I shut it down immediately and posted here. Ideas?
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    j beede

    Posts : 412
    Join date : 2011-02-07
    Location : California

    Re: Looking for quick help with VTA st-120

    Post by j beede on Sun Nov 05, 2017 10:54 pm

    Here's a start...

    I gather the fuse is not blown. Pull the rectifier and power on the amp. Do you see all filaments lit? If yes, power down the amp, replace the rectifier and move on to step 1) below. If you see a dark tube come back here.

    1) Use a Sharpie to add identifiers to the 6550s: I Like to use LF, LR, RF, RR
    2) Pull out RF and RR then see if the left channel still plays properly.
    3) If yes, shut the amp down and replace LF and LR with RF and RR. If no, report back with how the left channel behaves.
    4) Does the left channel work with RF and RR installed? If yes re-tension the right channel sockets. If no, you probably have a bad 6550.

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    Bob Latino
    Admin

    Posts : 2579
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Looking for quick help with VTA st-120

    Post by Bob Latino on Mon Nov 06, 2017 12:39 pm

    Bennyhaha812 wrote:I build an amp a while ago and it's been awesome but I need feedback as to what might have happened before I troubleshoot.  I was listening to music this evening and the sound had a pop and silence and the back right power tube 6550 turned blue.  I shut it down immediately and posted here.  Ideas?

    With the amp OFF measure the RESISTANCE to chassis ground at all 4 bias measuring points. They should all measure about 10 ohms, Normally you measure DC voltage here withe amp ON but you will be measuring resistance instead. You may have blown the 10 ohm bias resistor for the socket. If the resistance on that tube socket reads ZERO or if it reads some high resistance, you may just have to replace that 10 ohm 2 watt resistor. If that resistor is blown, then it is probably a sign that the tube on that socket is bad. Replace that output tube.

    Bob

    scottlowe76

    Posts : 15
    Join date : 2017-06-15

    Re: Looking for quick help with VTA st-120

    Post by scottlowe76 on Sun Nov 12, 2017 3:25 pm

    hi bob . i just got done cleaning and tightening my tube sockets . then I just checked the resistance on bias measuring points on my st 120 they measured 11.2 11.4 11.7 and 11.5 are they all bad ?
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    peterh

    Posts : 874
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Looking for quick help with VTA st-120

    Post by peterh on Sun Nov 12, 2017 4:02 pm

    scottlowe76 wrote:hi bob . i just got done cleaning and tightening my tube sockets . then I just checked the resistance on bias measuring points on my st 120 they measured 11.2 11.4  11.7 and 11.5 are they all bad ?
    Given measure accuracy i would say that they are all good.

    scottlowe76

    Posts : 15
    Join date : 2017-06-15

    Re: Looking for quick help with VTA st-120

    Post by scottlowe76 on Sun Nov 12, 2017 5:14 pm

    thank you peter .
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    Bob Latino
    Admin

    Posts : 2579
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Looking for quick help with VTA st-120

    Post by Bob Latino on Sun Nov 12, 2017 7:24 pm

    scottlowe76 wrote:hi bob . i just got done cleaning and tightening my tube sockets . then I just checked the resistance on bias measuring points on my st 120 they measured 11.2 11.4  11.7 and 11.5 are they all bad ?

    Peter is correct .. The fact that your meter measures a little high is of no consequence. All those resistors are good ..

    Bob

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