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    High Voltage Readings

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    rudes

    Posts : 33
    Join date : 2012-03-07

    High Voltage Readings

    Post by rudes on Thu Jan 10, 2013 3:02 pm

    OK...I purchased a VTA st70 board awhile ago that uses 3 12AT7 tubes. Everything is new except for the iron. Just finished putting together. My line voltage is 124. On 5AR4 pin 8 I get 490v. I have a 6.8K resistor between sections 3 & 4 and 1 & 4 have been connected to each other on quad cap. My B+ is good @ 405v but all other voltages are 35-40v high. Is this normal?

    Other problem is that there is a noticeable hum or buzz from only one channel. Noise goes away when input jack is in place. Why does this occur and only one channel? Thanks in advance for any info!

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: High Voltage Readings

    Post by Bob Latino on Thu Jan 10, 2013 3:23 pm

    If your voltages are too high on your ST-70 with the VTA driver board, the simplest way to drop the voltages down to something near normal is to use a 5U4 rectifier in place of the 5AR4. A 5U4 will drop the B+ voltage off pin 8 of the rectifier tube another 30 volts or so. If you do use the 5U4, you may have to pull the 6.8K resistor on the quad cap and now use a lower value resistor to get about 405 volts or so (which is good) for the B+ line which goes to the driver board.

    Bob

    rudes

    Posts : 33
    Join date : 2012-03-07

    Re: High Voltage Readings

    Post by rudes on Thu Jan 10, 2013 3:37 pm

    Hey Bob,

    Is this a common problem? I had to use 5u4 with my 2 MK3 amps. I was not concerned about the power trans being over taxed but can the st70 tranny handle the extra amperage?

    Also what about the noise issue?

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: High Voltage Readings

    Post by Bob Latino on Thu Jan 10, 2013 3:49 pm

    The original Dynaco ST-70 was set up for 115 volt mains that was prevalent in the 1950's and 1960's. When run on modern 120 volt mains the amp is stressed a little but will usually play OK although the power transformer will run hotter than normal. When you get above 121 to 122 VAC on your lines, you are wise to use a 5U4 in an ST-70 to drop the high voltage a little ...

    A 5U4 does draw more current than a 5AR4 but you should be OK ... I have used a 5U4 rectifier in an original ST-70 many times over the years with no problems. Even Dynaco, in the original assembly manual (on page 12), mentioned about substituting a 5U4 for a 5AR4 in the ST-70.

    Bob

    rudes

    Posts : 33
    Join date : 2012-03-07

    Re: High Voltage Readings

    Post by rudes on Fri Jan 11, 2013 10:41 am

    Can someone please answer the question regarding the low hum/buzz noise comimg from only ONE speaker with the VTA board installed when there is no input jack in place?

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: High Voltage Readings

    Post by Bob Latino on Fri Jan 11, 2013 11:01 am

    Re; > "Can someone please answer the question regarding the low hum/buzz noise comimg from only ONE speaker with the VTA board installed when there is no input jack in place?"

    This is NORMAL... > Without the input jacks terminated with two RCA interconnects plugged in which are either ..

    A. Terminated on the other end by being plugged into a preamp
    B. Shorted between the center pin and outer shield on the other end ...

    the amp will pick up some hum/buzz from the amp's circuitry. Sometimes one channel will pick up more noise than the other channel ...

    Bob

    rudes

    Posts : 33
    Join date : 2012-03-07

    Re: High Voltage Readings

    Post by rudes on Sat Jan 12, 2013 10:01 am

    What amazes me is how other people run their vta amps with 5AR4 rectifiers, unless their line voltage is considerably lower than mine. I have even seen amps on ebay with no resistor on the quad cap. When they rebuild an amp do they even bother to take voltage readings? I understand it will work, but how long will the tubes and other components last?

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: High Voltage Readings

    Post by peterh on Sat Jan 12, 2013 10:17 am

    rudes wrote:What amazes me is how other people run their vta amps with 5AR4 rectifiers, unless their line voltage is considerably lower than mine. I have even seen amps on ebay with no resistor on the quad cap. When they rebuild an amp do they even bother to take voltage readings? I understand it will work, but how long will the tubes and other components last?
    We have 230V here and my voltages are pretty much "correct" using the official schematic.
    As regards to hum mine vta is dead silent even when turned on :-) I used a "star grounding" and awoided grounding at the tubesockets, instead i installed solder lugs connected to the "star ground"


    rudes

    Posts : 33
    Join date : 2012-03-07

    Re: High Voltage Readings

    Post by rudes on Sat Jan 12, 2013 11:07 am

    Running a different power trans. If running PA521 trans was designed for 240v operation therefore 230v will result with lower voltage. I also used "star grounding" and is dead quiet WITH inputs connected.

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: High Voltage Readings

    Post by peterh on Sat Jan 12, 2013 11:24 am

    rudes wrote:Running a different power trans. If running PA521 trans was designed for 240v operation therefore 230v will result with lower voltage. I also used "star grounding" and is dead quiet WITH inputs connected.
    If your amp has tendensys to hum it might have to do with the environment.
    I have grounded all equip ( the vta has a IEC-well), the preamp a fixed grounded cable, in addition
    the shelf has steelparts that are grounded.

    But the fact that an open input picks up hum but otherwize quiet, is no reason to start rebuilding.
    Just listen to the Music !

    rudes

    Posts : 33
    Join date : 2012-03-07

    Re: High Voltage Readings

    Post by rudes on Sat Jan 12, 2013 11:29 am

    I agree...not rebuilding. Was just curious as to why...thanks!

    tubes4hifi
    Admin

    Posts : 1286
    Join date : 2008-11-30

    Re: High Voltage Readings

    Post by tubes4hifi on Sat Jan 12, 2013 7:28 pm

    an open input is an antenna to hum and/or hum pickup, that's why so many suppliers sell overpriced shorted RCA jacks for unused inputs on preamps.
    Fortunately on the ST70 that input is only about 1" long of wire. Also dependent on the environment - stay away from florescent lights!

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