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    el34 st70 voltages

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    corporalspiffy

    Posts : 15
    Join date : 2015-04-23

    el34 st70 voltages

    Post by corporalspiffy on Thu May 14, 2015 2:05 pm

    are these voltages acceptable?

    1. 1.05 v
    2.-7. 6.8v AC
    3. 416v
    4. 419v
    5. -32
    6. -32
    8. 1 v

    2. should be 6.3
    3. should be 435-445
    4. should be 440-450
    5. should be -35
    6. should be -35

    is something out of balance?

    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: el34 st70 voltages

    Post by Bob Latino on Fri May 15, 2015 8:54 am

    First of all do not worry about the slight voltage difference that you see on pins 1, 8, 5 and 6. These voltages are controlled by the bias pots. If you turn the bias pots one way or the other, these voltages will change.

    The voltages that you get across pins 2 an 7 (6.8 VAC which should be about 6.3 VAC) are probably normal if you are using an original Dynaco ST-70 that was designed for voltages that they had in the 1960's of about 115 - 117 VAC and you are now operating the amp on 120 VAC.

    The voltages that don't seem right are the B+ DC voltages on pins 3 and 4. Your 416 and 419 VDC readings are low. This is usually indicative of either a leaky quad cap or a weak rectifier. If you have an original Dynaco ST-70 with the original quad cap - just replace the quad cap. Electrolytic caps do not last forever and do deteriorate with age. A weak rectifier tube is less likely than a weak quad cap. Does your amp have a slight hum on both channels whose hum level does not change with the volume control of your preamp? This is another indication that the quad cap is bad.

    Bob

    corporalspiffy

    Posts : 15
    Join date : 2015-04-23

    Re: el34 st70 voltages

    Post by corporalspiffy on Fri May 15, 2015 12:45 pm

    YES THERE IS A SLIGHT HUM, I REALLY HAVE TO PRESS MY EAR TO THE SPEAKER TO HEAR IT THOUGH.

    SHOULD I MAKE SOME MODIFICATION TO BRING THE HEATER VOLTAGE DOWN .5 VOLT? IF SO WHAT DO YOU RECOMMEND?

    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: el34 st70 voltages

    Post by Bob Latino on Fri May 15, 2015 4:45 pm

    Just ignore the slightly over voltage 6.8 VAC. This is common on older original Dynaco ST-70 amps and causes no harm to the amp...

    If your hum level is so low that you must press your ear to the speaker to hear it, then that is about a normal hum level for these amps ..

    Bob

    corporalspiffy

    Posts : 15
    Join date : 2015-04-23

    Re: el34 st70 voltages

    Post by corporalspiffy on Sun May 17, 2015 7:09 am

    A SOLID STATE RECTIFIER BRINGS THE VOLTAGES UP BY 10 VOLTS SO I GUESS IT'S A WEAK GZ34 THAT ACCOUNTS FOR THE LOW VOLTAGES?

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