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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    octal vta problem

    kost
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    kost


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    • Post n°1

    octal vta problem Empty octal vta problem

    Post by kost Thu Sep 11, 2014 7:02 pm

    hi

    I have a problem with my vta octal st70 and i wonder if
    Bob could help.
    I have a low B plus voltage,330 ac and the 2k2 3watt resistor between the capacitors
    to output transformers and b plus runs very hot.
    I tried some measurments and i noticed that pins 4 and 6 of the rectifier
    have 338 ac, lower than normal as far as i know.(pins 2 and 8 have 415 dc)
    Can you advise please?

    thank you in advance
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    Bob Latino
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    • Post n°2

    octal vta problem Empty Re: octal vta problem

    Post by Bob Latino Thu Sep 11, 2014 7:37 pm

    Hi Kost,

    An original or a VTA ST-70 should have about 360 VAC from pin 4 and pin 6 to chassis ground. Probably the transformer you are using is expecting 240 VAC IN and in Greece you have 220 - 230 VAC @ 50 Hz. This is the reason for your low voltage. You may have to get a 240 volt variac and run the amp off a variac set to 236 - 240 VAC. I would adjust the variac until pins 4 and 6 on the rectifier tube socket are about 360 VAC. Pins 2 and 8 should read about 435 - 440 VDC so your DC voltage, off pins 2 and 8 which is 415, is also low ..

    Bob
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    kost
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    • Post n°3

    octal vta problem Empty Re: octal vta problem

    Post by kost Thu Sep 11, 2014 8:03 pm

    Hi Bob
    Thank you for the fast responce.
    This is the reason why the B plus is also low and the resistor runing too hot?
    I have the new power transformer (220volt) from dynakit parts.
    If i go back to the old one and use a step down transformer, will solve the problem?

    THANK YOU
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    Bob Latino
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    • Post n°4

    octal vta problem Empty Re: octal vta problem

    Post by Bob Latino Thu Sep 11, 2014 8:24 pm

    Kost,

    If the older transformer still works and will give you near 360 VAC off pins 4 and 6 of the rectifier tube socket to chassis ground, that would most likely solve your low voltage problem.

    Bob
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    DynakitParts
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    • Post n°5

    octal vta problem Empty Dynakit PA-521 PT

    Post by DynakitParts Thu Sep 11, 2014 8:39 pm

    Hi Kost,
    Our Dynakit PA-521 Power Transformer is wound for a 240 vac primary....If your power source is less than 240 vac all of your secondary voltages will be lower than expected.

    In most instances, this is not an issue. We do not have a 220 vac wound power transformer for the ST-70.

    Kevin
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    kost
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    • Post n°6

    octal vta problem Empty Re: octal vta problem

    Post by kost Fri Sep 12, 2014 12:22 am

    Hi Kevin and Bob

    Do you think that the HOT resistor mentioned is another problem?
    resistor

    Thank you.
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    DynakitParts
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    • Post n°7

    octal vta problem Empty Hot Resistor

    Post by DynakitParts Fri Sep 12, 2014 9:09 pm

    Kost,
    It is not uncommon for a resistor to get Hot....I would be more concerned if this resistor failed.

    Kevin @ Dynakit
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    Guest
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    • Post n°8

    octal vta problem Empty Re: octal vta problem

    Post by Guest Sat Sep 13, 2014 8:59 am

    .


    Last edited by PeterCapo on Tue Dec 01, 2020 10:11 pm; edited 1 time in total
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    Bob Latino
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    • Post n°9

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    Post by Bob Latino Sat Sep 13, 2014 9:49 am

    Peter, Kost,

    The resistor in question is a 2200 ohm 3 watt resistor that is used to drop the B+ voltage to the driver board. You would need a 46 VDC or so voltage drop across that resistor to even create a power of 1 watt being used. I do not believe that the voltage drop is even as high as 46 volts. You could probably use a 2 watt resistor in that postion.

    The resistor will get pretty warm after time but the wattage rating (3 watts) gives plenty of headroom above what the driver board will ever use ..

    Bob
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    • Post n°10

    octal vta problem Empty Re: octal vta problem

    Post by Guest Sat Sep 13, 2014 11:28 am

    .


    Last edited by PeterCapo on Tue Dec 01, 2020 10:11 pm; edited 2 times in total (Reason for editing : Correct wording.)
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    Bob Latino
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    • Post n°11

    octal vta problem Empty Re: octal vta problem

    Post by Bob Latino Sat Sep 13, 2014 12:10 pm

    Hi Peter,

    My only thoughts on this are that in the few hundred VTA ST-70 amp kits out there, to my knowledge, no one has ever reported that this resistor failed.

    Bob
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    GP49
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    • Post n°12

    octal vta problem Empty Re: octal vta problem

    Post by GP49 Sat Sep 13, 2014 1:27 pm

    Something to remember about "hot resistors" is that how much heat is independent of the resistor's wattage rating. A resistor dissipating one watt may seem hot to those accustomed to cool-running transistor gear but as long as it is rated to easily handle that, it's okay. Some will replace a 2-watt resistor dissipating one watt with an equal-ohms five-watt resistor, thinking it will run cooler. It won't, unless it is heatsinked. It will dissipate the exact same one watt...and the same amount of heat.
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    Roy Mottram
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    • Post n°13

    octal vta problem Empty Re: octal vta problem

    Post by Roy Mottram Sun Sep 14, 2014 2:24 am

    I emailed "kost" directly last week about this, the 2.2K resistor with the octal driver board should drop 55v and have 25ma of current, which is just under 1.4w,
    thus a 3w resistor was spec'ed and as Bob has mentioned, to my knowledge none of these have ever failed, and as GP  mentioned, heat is relative and subjective,
    and also a bigger resistor will still dissipate 1.4w
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