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    ST-70 new build and bias too high in right channel

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    dsallean

    Posts : 3
    Join date : 2015-07-08

    ST-70 new build and bias too high in right channel

    Post by dsallean on Sun Jul 12, 2015 12:40 pm

    Hello,

    I have a new ST-70 that I built up with the VTA board.  Upon testing, all three 6SN7 tubes lit up.  So I removed them and installed the rectifier tube and the two new 6CA7 output tubes in the left channel and was able to bias them to .40 volts no problem.  I then removed those and put the two other new 6CA7 tubes in the right channel and when I started the amp up as the tubes came up the volts quickly rose over .60 volts so I shut it down quickly.  I thought it might be the tubes so I switched them out with the ones I used for the left channel and same problem.  So I removed all the tubes and powered it up and got -47 to -50 DC volts on pins 5 and 6 of all 4 tube sockets.  

    Some help would be appreciated as I am not an expert at this.  I will try to attach a picture of the internals to see if something is off and I just can't see it.  

    Dave in TX


    Bob Latino
    Admin

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

    Re: ST-70 new build and bias too high in right channel

    Post by Bob Latino on Sun Jul 12, 2015 1:12 pm

    Your wiring looks very good ... Did you have speakers connected when you did your bias test ? If not, attach some speakers and try it again .. Without a load on the output transformers (the speakers) some ST-70 type amps can be unstable. Sometimes, it is also a good idea to plug in two interconnects into the two input jacks and terminate them on the other end by plugging them into your preamp. Keep the preamp off ...

    If you have a capacitor tester, you may also want to check (with the amp OFF) to see if both coupling caps on the right channel test as good. You can test them in circuit.

    Bob

    dsallean

    Posts : 3
    Join date : 2015-07-08

    Re: ST-70 new build and bias too high in right channel

    Post by dsallean on Sun Jul 12, 2015 3:37 pm

    Bob,

    Thanks so much for the message.  I did have speakers connected, but not my preamp.  I didn't have a capacitor tester so I just checked over everything and reconnected/reinstalled the tubes and set up my preamp.  It was able to set up the bias on all four tubes and she is up an running.  Not sure what happened...  One tube seems to be glowing brighter than the others.  Not sure what's up with that.  The amp sounds great to me thus far.

    Dave


    audiobill

    Posts : 271
    Join date : 2014-03-13
    Location : Philadelphia

    Re: ST-70 new build and bias too high in right channel

    Post by audiobill on Sun Jul 12, 2015 4:10 pm

    I would also check tube sockets for a tight fit, and retighten if necessary.

    Loose pins can lead to runaway....


    dsallean

    Posts : 3
    Join date : 2015-07-08

    Re: ST-70 new build and bias too high in right channel

    Post by dsallean on Mon Jul 13, 2015 9:59 am

    Thanks audiobill. I did a recheck and retighten of all sockets prior to firing it up. Seems to be doing just fine now after about 2 hours of break-in.

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