Help! V2, 12AU7 blow?

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Help! V2, 12AU7 blow?

Postby gab07 » Sun Nov 14, 2010 8:19 pm

Hello!
I just finish today my ST-35 amplifier!
It work very nice for a couple of hours, then I lost the left channel!!! So I recheck everything and find this voltage on V2:
Pin#1 271v
Pin#2 50v
Pin#3 65v
Pin#6 227v
Pin#8 120v

So, for me, 12AU7 is broken! I am lucky, I have an other tube at home!
I replace the tube and averything come to normal! I have every good voltage.
So, after a couple hour again, I ear a very strong distortion on right channel... I put off the amp... Recheck the voltage, I have bad voltage value on 12AU7..

Also, I notice when the amp was working good, the 12AU7 bright more than the 12AX7, it is normal?

Where is my amp problem?
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Postby DeathRex » Sun Nov 14, 2010 9:07 pm

Pin 3 and 8 should be the same, about 120VDC. Pin 1 and 6 should be the same about 205VDC. Pin 2 should be about 115 and 4 volts under pin 3. Pin 7 should also be about 115 and 4 volts under pin 8.

So pin 2 is low, pin 3 is low, pin 1 is high. Might need to check 12AX7 also.
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Postby TomMcNally » Sun Nov 14, 2010 9:36 pm

You should check the resistor values of R-9 and R-10 and make sure they
are close to each other, and correct (27K) and also R-11 and R-12 which should be 33K and close to each other.
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Postby gab07 » Mon Nov 15, 2010 8:17 am

R-9 and R-10 are good. R-11 and R-12 are good to!

I'm sure if I put an other new 12AU7 in the amp, it will work very well for a couple of hour!

Can it be the heat of the resistor that change the value?
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Postby DeathRex » Mon Nov 15, 2010 9:04 am

Put in a good 12AU7, let it warm up for 1-2 minutes, get the voltages for the pins of the 12AU7 and 12AX7, and turn it off. Then post what you got.

Too much current should be what killed the 12AU7. Make sure the plate is not glowing.
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Postby gab07 » Mon Nov 15, 2010 9:57 pm

First of all, I recheck everything in the amp and found a bad soldier on C22 between (C2-R12-P8 of V2) Can this was the problem?

I put an other tube and here is the value!

-------- V1 ------- V2
P1---- 114------- 220
P2---- 0---------- 114
P3---- 0.818---- 118.5
P4---- 3.125---- 3.140
P5---- 3.125---- 3.140
P6---- 111------ 222.2
P7---- 0--------- 111.2
P8---- 0.832----- 116.2
P9---- 3.125----- 3.140

220v and 222v on V2 is to much? I have a beautiful 320v coming out of the power supply!
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Postby EWBrown » Tue Nov 16, 2010 2:20 pm

It looks like you found the problem! Most problems with DIY built equipment will be caused by soldering problems firswtly, then incorrect component values. The test voltages shown in the instructions are a guideline, and the actual voltages can vary higher or lower due to the resistor tolerances and those of the tubes themselves.

The 12AU7 phase splitters should each consume between 3 and 4 mA, 3.5 mA is average, so the voltage drops across the 27K and 33K can be used to determine the actual current. The 12AX7 stages each consume less than 1 mA, typically around 0.7 mA +/- 0.1 mA.

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