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    Separate filament transformer to power a 5U4GB.

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    Yvon Massicotte

    Posts : 13
    Join date : 2009-07-28
    Age : 74
    Location : Montréal, QC, Canada

    Separate filament transformer to power a 5U4GB.

    Post by Yvon Massicotte on Wed May 01, 2013 12:20 am

    In a previous post about voltage values found on a VTA board (see topic: "DC Voltages look suspicious"), Bob Latino suggested using a 5U4 rectifier tube instead of the GZ34 because the power transformers in old Dynaco amps from the 60s are designed for 110 VAC mains voltage input instead of the now common 120 VAC.

    He added that the 450 VDC that I had measured on the output transformers' center taps was a little high and added: "higher than normal voltages usually translate into shorter tube life".

    This amp has the original and precious Dynaco/Mullard EL-34s and this made me realize that I better do something about this higher than normal B+. I thus bought a 5U4GB.

    This tube, however draws too much current (1.1 A) for the 5V winding of my Dynaco power transformer. Someone in a post said the overdraw shouldn't damage the latest power transformers. But mine becomes so hot after an hour of operation that I decided to buy a separate filament transformer to power the 5U4.

    With the VTA board, the fixed bias circuit is now on the board itself and this frees enough space under the left channel output transformer to put a filament transformer there. It needs to be of a size similar to the choke found under the right channel output transformer.

    Fortunately I found exactly what I needed: a Hammomd model 166-L5 power transformer with a 5 VAC, 2 A secondary. And what is even more interesting, it has EXACTLY the same same dimensions as the Dynaco choke. It fits perfectly inside the chassis! Obviously, transformer laminations and cores come in standard sizes.

    After installing and wiring the filament transformer and re-adjusting the bias and AC balance, I measured the voltages and now the primaries' center taps show a 410 V which should be fine and allow the longest practical life for my output tubes.

    I'm including a picture showing the modification. Notice on the rectifier socket 2 1N4007 diodes to block inverse voltages and protect the 5U4 as some have suggested.

    Surprisingly, the Dynaco power transformer is now distinctly less hot.

    [img][/img]

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Separate filament transformer to power a 5U4GB.

    Post by tubes4hifi on Wed May 01, 2013 2:06 pm

    great work and great post!

    Yvon Massicotte

    Posts : 13
    Join date : 2009-07-28
    Age : 74
    Location : Montréal, QC, Canada

    What about the 2K2 resistor between sections 3 and 4 of the quad cap ?

    Post by Yvon Massicotte on Wed May 01, 2013 4:08 pm

    Roy,

    Thank you for the compliment. Very Happy

    I'm wondering though if the change has not had an unexpected side effect. The voltage going to the VTA board is now 360V instead of the previous 380V. Wouldn't it be better to change the 2K2 resistor between sections 3 and 4 of the Quad cap to 1K3 to raise to voltage going to the board back to 380V ?

    I would appreciate receiving your advice on this.

    Yours truly,

    Yvon

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Separate filament transformer to power a 5U4GB.

    Post by tubes4hifi on Wed May 01, 2013 9:56 pm

    yes, perfect, thank God someone knows how to use Ohm's Law ! Very Happy

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Separate filament transformer to power a 5U4GB.

    Post by GP49 on Thu May 02, 2013 12:46 pm

    OHM'S LAW???? affraid

    Heck, most of the people on the more popular audio boards can't even read a SCHEMATIC.

    They're the ones who think they're technically competent because they "recap" their Japanese receivers from
    the 1970s...but then they ask why the FM tuner drifts, after they put new capacitors into the front end. pale

    "Temperature coefficient? We don't need no stinkin' temperature coefficient." Rolling Eyes

    peterh

    Posts : 641
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Separate filament transformer to power a 5U4GB.

    Post by peterh on Thu May 02, 2013 2:05 pm

    GP49 wrote:OHM'S LAW???? affraid

    Heck, most of the people on the more popular audio boards can't even read a SCHEMATIC.

    They're the ones who think they're technically competent because they "recap" their Japanese receivers from
    the 1970s...but then they ask why the FM tuner drifts, after they put new capacitors into the front end. pale

    "Temperature coefficient? We don't need no stinkin' temperature coefficient." Rolling Eyes

    I recently read (*) another murphys third "law";
    "if it works at once it has an even number of faults that cancels each other"

    *)Swedish ampguru
    Alex Tubewonder, Louder&More, Zagray!

    Zimmer64

    Posts : 112
    Join date : 2013-01-29
    Age : 52
    Location : Switzerland

    Re: Separate filament transformer to power a 5U4GB.

    Post by Zimmer64 on Thu May 02, 2013 2:26 pm

    Ohm's law: http://youtube.com/watch?v=0W83SOfSJoQ

    Came across this recently, as I am trying to learn to understand better what I am building Very Happy

    Best

    Michael

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