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    Star Grounding Scheme Observations and Questions

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    tomlang

    Posts : 34
    Join date : 2009-08-12

    Star Grounding Scheme Observations and Questions

    Post by tomlang on Thu Oct 29, 2009 12:05 pm

    I am presently restoring a ST70 and while at it, installing a VTA driver board. In so doing, I have some observations about how the board itself is grounded. First off, the board does have a dedicated single wire leading to the chassis star ground. However, the board itself is held to the chassis with metal screws that contact the printed circuit board ground bus that goes around the perimeter of the board itself. Doesn't this negate the star-ground effect? Shouldn't the board be insulated electrically from the chassis? Or am I missing something and doing this wrong?

    And what about the 10 ohm cathode resistors for biasing? Those are attached to the chassis at each tube and not carried back to the star ground point either.

    Please comment on this, thanks.

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Star Grounding Scheme Observations and Questions

    Post by Bob Latino on Thu Oct 29, 2009 6:28 pm

    Hi Tom,

    You are correct in your observations about the grounding scheme when using a VTA driver board with the ST-70 amp. The board is grounded with one wire to the two main grounding lugs AND also grounded with the four metal screws which connect the board to the chassis. At least on the ST-70 it works fine and causes no ground loops and does not introduce any noise into the amp. The four 10 ohm resistors do ground directly to the chassis at four different points but on the stock Dynaco amp the two 15.6 ohm resistors were also connected directly to grounding tabs on two tube sockets. These octal tube sockets were grounded to the chassis by the two 4-40 mounting screws and nuts. The total number of grounds on an ST-70 qualifies to what is known as a "modified star ground" in which most of the amp (the three transformers) are star grounded to the two main grounding lugs next to the quad cap but other parts of the circuit are "locally grounded". An ST-70, if properly built, with either a stock driver board or the VTA driver board is very quiet. With most ST-70's (with the input jacks terminated with interconnects connected to your preamp and the preamp OFF and) with speakers of average sensitivity you should have to literally place your ear ON THE GRILLE CLOTH of your speakers to hear (maybe) the slightest amount of hum and noise. At your listening position you should here nothing. If you can hear hum and noise on an ST-70 at your listening position there is a problem in the amp ...

    Bob

    kost

    Posts : 33
    Join date : 2009-05-12
    Age : 52
    Location : athens

    hum

    Post by kost on Wed Nov 04, 2009 9:15 am

    Hi

    I have a VTA st70 that sound super but i hear an audible hum
    from my speakers.
    Any possible reasons?

    kost

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Star Grounding Scheme Observations and Questions

    Post by Bob Latino on Wed Nov 04, 2009 11:38 am

    kost wrote:Hi

    I have a VTA st70 that sound super but i hear an audible hum
    from my speakers.
    Any possible reasons?

    kost

    Kost,

    To test for hum on an ST-70 disconnect all associated equipment from the ST-70 and plug in a set of RCA interconnects into the two input jacks and then short between the center pin and the outer shield of the interconnects on the OTHER END of the two interconnects. If the amp still hums > the hum is coming from the amp. If it does NOT hum then the hum is related to the associated audio gear connected to the ST-70.

    If the amp still hums check below for a possible solution ...

    1. Did you switch to a 3 wire AC power cable on this amp? If so try lifting or disconnecting the ground wire.
    2. If any associated equipment or power strip connected to the ST-70 has a 3 wire cable try lifting the ground on that piece of equipment.
    3. Is there an internet or TV cable line connected to anything on the same AC circuit in your home? This happened to me and caused hum at the other home I lived in.
    4. Try replacing the CENTER 12AT7 on the driver board. In the VTA circuit this tube is much more sensitive to causing hum than the other two 12AT7 tubes.

    As an aside > On an ST-70 you will always hear a little hum if your two RCA input jacks are NOT terminated.

    Bob

    kost

    Posts : 33
    Join date : 2009-05-12
    Age : 52
    Location : athens

    Re: Star Grounding Scheme Observations and Questions

    Post by kost on Sun Nov 15, 2009 3:15 am

    Hi Bob

    Thank you for your reply.
    I ve checked everything you mention but the hum still
    exists.The only parts left after the modification
    are the sockets,the resistors under the EL34s,the old
    power transformer,the 0.22 ceramic cap and the choke.
    Any suspects between the above?

    regards
    kost

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Star Grounding Scheme Observations and Questions

    Post by Bob Latino on Sun Nov 15, 2009 11:04 am

    Hi Kost,

    Chokes rarely go bad. Measure across the two quad cap terminals that the choke is connected to and see if you get about 50 to 60 ohms resistance. If you do then the choke is probably OK.

    You can replace the dual .022 cap with TWO individual .022 @ 100 volts or higher caps and the four 1000 ohm resistors between pins 5 and 6 but I don't think that either of these is the issue. You can also try cleaning the tube socket pin holders with any electrical contact cleaner (Deoxit is very good). You can also try another rectifier tube in there. Sometimes a bad rectifier tube can cause hum - even a new rectifier tube.

    If all of the above do not solve the problem then my feeling is that your power transformer may be bad. In fact if you have replaced the quad cap (which is the usual cause of hum in an ST-70) then the power transformer is the only "suspect" left. The output transformers on the ST-70 were of very good quality but the POWER transformers were undersized for the current required to properly run that amp. The original power transformers only had a stack lamination of 1.5 to 1.6 inches. Less iron in there means more heat. The wire size used in the windings was smaller than it should be. All the modern upgraded power transformers for the ST-70 have stack laminations of 2.1 to about 2.5 inches and they all use heavier gauge wire in the windings. They all run cooler and have greater current transfer capabilities. If you are going to play an ST-70 on a daily basis as your main amp a few hours a day you should replace the POWER transformer.

    Sorry to rant here but I see Dynaco amp "rebuilders" on Audiogon sell "rebuilt" Dynaco ST-70 amps that LOOK beautiful. They use, however, the original transformer set. It is fine IMHO to reuse the OUTPUT transformers but they should not reuse the ST-70's POWER transformer because they are selling the amp to people who will use this new LOOKING amp on a daily basis. A power transformer is the "engine" of your amp. Do you really want an engine from the 1960's powering the automobile that you will use on a daily basis to get back and forth to work ? I don't think so ...

    Bob

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