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    Using AVariac

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    Daveinthedesert

    Posts : 8
    Join date : 2011-02-15

    Using AVariac

    Post by Daveinthedesert on Sun Nov 19, 2017 12:23 pm

    Hey my first post..have mercy on me. Have a pair of Mk3's that I recently inherited. Looks like they have been modded somewhat. Keep going thru rectifier tubes way too fast. I have pretty hot mains voltage of 125VAC and I know that Bob suggests about 40ma per tube (6550's). So I will be getting the 20a Circuit Specialties Variac. Here's my question. What is the correct way to use it? Use it as the main "on" "off" control..plug the Mk3's into it, leave the Mk3's on/off switches on all the time and just turn on the amps with the Variac, or turn everything on separately? Also start the Variac at about 50 wait a bit then take to 118VAC? Any thoughts would be helpful.
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    Bob Latino
    Admin

    Posts : 2707
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Using AVariac

    Post by Bob Latino on Sun Nov 19, 2017 1:18 pm

    Daveinthedesert wrote:Hey my first post..have mercy on me. Have a pair of Mk3's that I recently inherited. Looks like they have been modded somewhat. Keep going thru rectifier tubes way too fast. I have pretty hot mains voltage of 125VAC and I know that Bob suggests about 40ma per tube (6550's). So I will be getting the 20a Circuit Specialties Variac. Here's my question. What is the correct way to use it?  Use it as the main "on" "off" control..plug the Mk3's into it, leave the Mk3's on/off switches on all the time and just turn on the amps with the Variac, or turn everything on separately? Also start the Variac at about 50 wait a  bit then take to 118VAC? Any thoughts would be helpful.

    Hi Dave,

     With 125 VAC coming in, set the variac for an output of 118 VAC. Starting a variac out at 50 VAC and bringing it up slowly is only for old gear that hasn't see any voltage in maybe 30+ years. This will help in reforming (maybe) old electrolytic caps that are on the ragged edge of failure. On new gear just leave the variac set at an output of about 118 VAC. 6550's in any of the VTA amps should be biased at 50 milliamps per output tube. They will work fine at a bias setting of 40 ma but IMHO, they should be run at a little higher idle current.

    Bob
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    PeterCapo

    Posts : 633
    Join date : 2008-12-05

    Re: Using AVariac

    Post by PeterCapo on Sun Nov 19, 2017 3:58 pm

    Daveinthedesert wrote:Hey my first post..have mercy on me. Have a pair of Mk3's that I recently inherited. Looks like they have been modded somewhat. Keep going thru rectifier tubes way too fast. I have pretty hot mains voltage of 125VAC and I know that Bob suggests about 40ma per tube (6550's). So I will be getting the 20a Circuit Specialties Variac. Here's my question. What is the correct way to use it?  Use it as the main "on" "off" control..plug the Mk3's into it, leave the Mk3's on/off switches on all the time and just turn on the amps with the Variac, or turn everything on separately? Also start the Variac at about 50 wait a  bit then take to 118VAC? Any thoughts would be helpful.

    125 VAC mains is 1 VAC above the maximum originally specified by Dynaco for their Mark III.  While it should be addressed, it might not necessarily be the reason why you are going through rectifier tubes.  Is it possible your AC mains goes higher than 125 VAC at times?  Or, again, it might possibly be something else.

    I'd suggest identifying the configuration of your Mark IIIs - posting some images would help.

    Putting an appropriate NTC thermistor in series with the power transformer primary winding would compensate for the AC mains being a bit too high, without the use of a variac.

    If you are going to make use of a variac, the scale on the variac's face will probably not be reliable.  The variac's output must be set with something like a handheld multimeter.

    What you really need to do is to use the variac to set up the proper test condition (117 VAC across the power transformer primary) and then take readings at the various voltage check points given in the original Dynaco manual and see what you find.  Note that if you have later production Mark IIIs, the reference voltage across the power transformer primary might be 120 VAC instead of 117 VAC.

    If the amps have been in regular use to the present time, just set the output of the variac with an external meter and leave it there. If you continue to run the amps off the variac, you do not have to gradually turn up the variac every time you want to use the amps. But, if your AC mains frequently fluctuates higher than it was when you first set the variac output, then check the variac output every time you power up.

    For safety, I’d use all switches - the variac and the amps - all the time.
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    peterh

    Posts : 1068
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Using AVariac

    Post by peterh on Sun Nov 19, 2017 4:20 pm

    Daveinthedesert wrote:Hey my first post..have mercy on me. Have a pair of Mk3's that I recently inherited. Looks like they have been modded somewhat. Keep going thru rectifier tubes way too fast. I have pretty hot mains voltage of 125VAC and I know that Bob suggests about 40ma per tube (6550's). So I will be getting the 20a Circuit Specialties Variac. Here's my question. What is the correct way to use it?  Use it as the main "on" "off" control..plug the Mk3's into it, leave the Mk3's on/off switches on all the time and just turn on the amps with the Variac, or turn everything on separately? Also start the Variac at about 50 wait a  bit then take to 118VAC? Any thoughts would be helpful.
    If there is problems with rectifiers i recommend to verify that the amp is built
    according to the dynaco manual. In particular, the main B+cap should use 30uF
    closest to the rectifier.Much larger then this will risk the rectifier.

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    PeterCapo

    Posts : 633
    Join date : 2008-12-05

    Re: Using AVariac

    Post by PeterCapo on Sun Nov 19, 2017 4:56 pm

    Agree with this possibility that peterh points out.  This is one possible example of why it is imperative to assess exactly what you have, that is, the configuration of the amplifier, first.

    Learning safe work practices around tube gear and having the use of a decent multimeter will get you going in the right direction.

    wildiowa

    Posts : 203
    Join date : 2012-03-19

    Re: Using AVariac

    Post by wildiowa on Sun Nov 19, 2017 9:12 pm

    My line AC is usually always 125 and often hits 126. After several "incidents" I put my amp on a Variac, set at 118, and no problems since. Use a better meter than the ones that are included on the Variac. I fortunately have my amps near one of those wall outlets they used to wire with a wall switch, so all I have to do is hit the switch and the amp is on. Try and rig up a master switch for the Variac maybe even a regular wall switch in an electrical box of some kind you can probably do something to dress it up a bit if it looks too funky.

    Daveinthedesert

    Posts : 8
    Join date : 2011-02-15

    Re: Using AVariac

    Post by Daveinthedesert on Mon Nov 20, 2017 9:36 am

    Thanks to everyone for all the valuable information. Luckily, for me our good friend Bob Latino referred me to a Dynaco tech that lives right here in Tucson


    Last edited by Daveinthedesert on Mon Nov 20, 2017 9:43 am; edited 1 time in total (Reason for editing : Added information)

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