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    Dynaco St 70 blowing fuses Help

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    merickrc

    Posts : 3
    Join date : 2009-03-25

    Dynaco St 70 blowing fuses Help

    Post by merickrc on Fri Nov 16, 2012 6:03 pm

    I have a dynaco 70 that I built about 3 years ago today when I turned it on it blew the fuse so I put in a new rectifier tube and it worked for a few min with a hum on the right Chanel so I checked the bias and the reading on the right channel was really low and wouldn't change at all took the tubes from the left Side out and put them in the right side and I was able to get the bias set up but once I put the other two tubes back in it blew the fuse again does that mean the two tubes are bad or is ther something else I can check please help

    nraghunadh

    Posts : 6
    Join date : 2012-10-05

    Re: Dynaco St 70 blowing fuses Help

    Post by nraghunadh on Sat Nov 17, 2012 1:57 pm

    merickrc wrote:I have a dynaco 70 that I built about 3 years ago today when I turned it on it blew the fuse so I put in a new rectifier tube and it worked for a few min with a hum on the right Chanel so I checked the bias and the reading on the right channel was really low and wouldn't change at all took the tubes from the left Side out and put them in the right side and I was able to get the bias set up but once I put the other two tubes back in it blew the fuse again does that mean the two tubes are bad or is ther something else I can check please help

    Please check the quad cap, I think the problem with 2nd position(D) 30 mfd section.
    Thanks
    Raghu

    Bob Latino
    Admin

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

    Re: Dynaco St 70 blowing fuses Help

    Post by Bob Latino on Sat Nov 17, 2012 2:50 pm

    If the hum/noise comes from just the RIGHT channel then the quad cap is probably OK since the quad cap affects BOTH channels. If a new rectifier tube did not help, I would try changing out the two right side output tubes. One or both output tubes could have a partial short.

    Bob

    merickrc

    Posts : 3
    Join date : 2009-03-25

    Re: Dynaco St 70 blowing fuses Help

    Post by merickrc on Sat Nov 17, 2012 7:25 pm

    I replaced all the tubes and know the front right tube has started to glow supper red and I shut it down and turned it back on then it blew the fuse thanks for your help

    Bob Latino
    Admin

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

    Re: Dynaco St 70 blowing fuses Help

    Post by Bob Latino on Mon Nov 19, 2012 2:03 pm

    merickrc wrote:I replaced all the tubes and know the front right tube has started to glow supper red and I shut it down and turned it back on then it blew the fuse thanks for your help

    If the right front tube glows red (red plates) then it probably has an internal short. What I would do is replace BOTH right channel output tubes and then use the tube that does NOT red plate as a spare.

    Bob

    dynacojoe

    Posts : 8
    Join date : 2012-08-22
    Age : 59

    Re: Dynaco St 70 blowing fuses Help

    Post by dynacojoe on Wed Nov 21, 2012 3:54 pm

    Bob Latino wrote:
    merickrc wrote:I replaced all the tubes and know the front right tube has started to glow supper red and I shut it down and turned it back on then it blew the fuse thanks for your help

    If the right front tube glows red (red plates) then it probably has an internal short. What I would do is replace BOTH right channel output tubes and then use the tube that does NOT red plate as a spare.

    Bob

    I read and and reread your notes merickrc. You have a control grid bias issue. Red plating is a sure sign of a bias issue. I am thinking it is time to make certain that the biasing circuits are stable and the great part is that it can be done without further risk to your tubes. Here goes: Remove the rectifier tube. I say this assuming your ST-70 has tube rectification. With a dvm connected to pin 5 of that right front tube and your body parts safely clear of high voltages still present prior to the rectifier, power on and you should see a negative voltage some where between -27 to -41 volts. If it is in this range you should be able to set it to -32 volts, which is pretty close to what these amps use for idle bias. It is well to keep in mind that it is this -32 volts (or very close to it) that makes possible the idle current that in turn produces the 1.56 volts cathode voltage we watch for when setting bias. I am assuming your amp uses 15.6 ohm Biaset resistors. Now try the same thing on the left front tube pin 5. Now measure the back pin 5s and they should read virtually the same as their front counter parts. If some or all of these voltages will not cooperate just remove the el34s and try again. By now you will have much a better handle on what is going on. Let me now how it goes.

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Dynaco St 70 blowing fuses Help

    Post by tubes4hifi on Fri Nov 23, 2012 2:51 pm

    very likely a bad connection on pin 5 or 6 of the output tube

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