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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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    10A or 20A Bucker Transformer for M125's?

    LeGrace
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    • Post n°1

    10A or 20A Bucker Transformer for M125's? Empty 10A or 20A Bucker Transformer for M125's?

    Post by LeGrace Sun Feb 04, 2018 1:16 pm

    To move my mains (typ 122v) closer to the comfort zone a bucker seems like the most economical way to proceed. I see folks using 6A rated ones quite often, but then the M125's are packing more horsepower then most amps, so at first I was contemplating a 10A rated model. But do I really need to assemble one for each amp? To save on effort couldn't I simply go with a single 20A rated transformer, like a Hammond 266V6B?

    Any helpful advice appreciated.
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    audiobill
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    • Post n°2

    10A or 20A Bucker Transformer for M125's? Empty Re: 10A or 20A Bucker Transformer for M125's?

    Post by audiobill Sun Feb 04, 2018 1:19 pm

    6 amps is plenty for each, 10 amps is plenty for two.
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    Bob Latino
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    • Post n°3

    10A or 20A Bucker Transformer for M125's? Empty Re: 10A or 20A Bucker Transformer for M125's?

    Post by Bob Latino Sun Feb 04, 2018 2:43 pm

    audiobill wrote:6 amps is plenty for each, 10 amps is plenty for two.

    Bill is correct here .. What I would do if it were my pair of M-125's running on 122+ AC line voltage would be to get the 20 amp variac at Circuit Specialists at the link below. They offer free shipping to the USA but Legrace is in Canada so the shipping would be extra. This model also has TWO AC outputs - one for each M-125.

    20 amp variac at Circuit Specialists in AZ

    They also sell a 5 amp model that would be fine for a single ST-70 or ST-120.

    5 amp rated variac

    Set your variac to give about 118 VAC output with whatever AC output is coming from your wall outlet.

    Bob
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    Dahlberg
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    • Post n°4

    10A or 20A Bucker Transformer for M125's? Empty Re: 10A or 20A Bucker Transformer for M125's?

    Post by Dahlberg Sun Feb 04, 2018 5:56 pm

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    LeGrace
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    • Post n°5

    10A or 20A Bucker Transformer for M125's? Empty Re: 10A or 20A Bucker Transformer for M125's?

    Post by LeGrace Mon Feb 05, 2018 11:18 am

    Thanks for all the help, not to mention saving me some $$!
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    • Post n°6

    10A or 20A Bucker Transformer for M125's? Empty Re: 10A or 20A Bucker Transformer for M125's?

    Post by gktamps Mon Feb 05, 2018 5:58 pm

    Although it sounds like you will be buying a Variac instead, just to add an observation to this discussion; I made a 8A bucking transformer for my pair of M-125s running KT120s, and it got pretty warm in operation. I replaced it with a 15A.
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    • Post n°7

    10A or 20A Bucker Transformer for M125's? Empty Re: 10A or 20A Bucker Transformer for M125's?

    Post by BNR_1 Tue Feb 06, 2018 12:39 am

    I know for vintage gear people drops their line voltage to around 115 to 117v. But for these modern made amps, I thought the PT was designed for 120 to 124 volts.
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    • Post n°8

    10A or 20A Bucker Transformer for M125's? Empty Re: 10A or 20A Bucker Transformer for M125's?

    Post by Guest Tue Feb 06, 2018 11:51 am

    I use a 15 amp rated (bucker) transformer for the entire system. It’s a 5V CT filament tranny that I have wired to drop 2.5V and 5V. The power can be a little erratic around here; some days it’s 122,so I drop 5V, other days it’s 120, so I drop 2.5V. It’s controlled by a simple “ON-ON” toggle switch. A 3-way light switch will work too.
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    LeGrace
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    • Post n°9

    10A or 20A Bucker Transformer for M125's? Empty Re: 10A or 20A Bucker Transformer for M125's?

    Post by LeGrace Tue Feb 06, 2018 4:43 pm

    I see a 16A rated Hammond that is only a few dollars more then the 10A.  Whereas the 20A units cost like double.
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    • Post n°10

    10A or 20A Bucker Transformer for M125's? Empty Re: 10A or 20A Bucker Transformer for M125's?

    Post by cci1492 Wed Feb 14, 2018 12:29 pm

    Bob Latino wrote:
    audiobill wrote:6 amps is plenty for each, 10 amps is plenty for two.

    Bill is correct here .. What I would do if it were my pair of M-125's running on 122+ AC line voltage would be to get the 20 amp variac at Circuit Specialists at the link below. They offer free shipping to the USA but Legrace is in Canada so the shipping would be extra. This model also has TWO AC outputs - one for each M-125.

    20 amp variac at Circuit Specialists in AZ

    They also sell a 5 amp model that would be fine for a single ST-70 or ST-120.

    5 amp rated variac

    Set your variac to give about 118 VAC output with whatever AC output is coming from your wall outlet.

    Bob

    Today only! I used PROMO CODE "VDAY" and got it for 15% OFF:

    Shipping: Free
    Total: $84.96

    Happy Valentines Day!
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    10A or 20A Bucker Transformer for M125's? Empty Re: 10A or 20A Bucker Transformer for M125's?

    Post by LeGrace Thu Feb 15, 2018 2:50 pm

    So got the bucker put together, using a Hammond 167U6 transformer. 16A model that I suppose could be described as overkill, but difference cost wise from a 10A was so minor I figured what the heck.

    Voltage reading is now 117.8 versus 122v. I cant help but wonder if I had done this sooner would it have prevented some of my prior tube failures? Will never know, but I see it as good insurance for the future.
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    Bob Latino
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    • Post n°12

    10A or 20A Bucker Transformer for M125's? Empty Re: 10A or 20A Bucker Transformer for M125's?

    Post by Bob Latino Thu Feb 15, 2018 3:08 pm

    On the back of the parts list on every VTA amp kit is a note to check your incoming AC line voltage. There is also the suggestion to use a variac if your normal incoming line voltage is 122 VAC or above.

    Bob
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    LeGrace
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    • Post n°13

    10A or 20A Bucker Transformer for M125's? Empty Re: 10A or 20A Bucker Transformer for M125's?

    Post by LeGrace Thu Feb 15, 2018 5:47 pm

    When I built my M125's I had no clue about the importance of line voltage. I did not know I was falling into the red zone, just wondered why am I blowing tubes left and right. Lesson learned, no more living on the edge!
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    • Post n°14

    10A or 20A Bucker Transformer for M125's? Empty Re: 10A or 20A Bucker Transformer for M125's?

    Post by buchela Thu Feb 15, 2018 6:24 pm

    What are the possible consequences to run M-125 monoblocks on 122V ?.
    What is the ideal voltage for these amps ?


    Last edited by buchela on Thu Feb 15, 2018 6:26 pm; edited 1 time in total (Reason for editing : added second paragraph)
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    • Post n°15

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    Post by cci1492 Wed Apr 11, 2018 7:01 pm

    Update, the 20 amp variac I purchased almost 2 months to the day from Circuit Specialists today started intermittently tripping the breaker. Now trips the breaker all the time. With no load and turned all the way down still breaker trips (15A and 20A dedicated breaker). Left a couple of messages for CS...nothing (and I buy stuff from them....scope, light, other crap I can't recall right now..etc).


    Plugged it into my Staco variac and it can be turned on slowly. So I stuck one of the thermistors (995-SL12-12102) we use on the Webers WZ68 and that works. Now I can turn that sucker on and off all day without tripping the breaker (and no longer see between 2A and 9A upon turning it on). The only problem is that thermistor starts to smoke at about 6A output. If you never hear back from me I'll be living under a bridge because I burned my house down!
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    • Post n°16

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    Post by audiobill Thu Apr 12, 2018 7:49 am

    Used US made variacs can be found regularly on Craigslist. My staco 15A was $100, like new, and $750 list price.
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    • Post n°17

    10A or 20A Bucker Transformer for M125's? Empty Re: 10A or 20A Bucker Transformer for M125's?

    Post by cci1492 Thu Apr 12, 2018 8:11 am

    That's exactly how I purchased my Staco years ago but on ebay. It had a missing dial, found the dial on ebay new and cheap. I think I'm going to go the bucker TA way using the Hammond 167U6 transformer LeGrace wrote about. Anyone have the circuit for this one using the 167U6?
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    • Post n°18

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    Post by corndog71 Thu Apr 12, 2018 9:24 am

    I just built one using a Hammond 167T6 6.3V 12A transformer for my M125's. The little plug in voltage meter was under $8 from amazon.

    10A or 20A Bucker Transformer for M125's? 41408682211_e5c4deeb43_c
    Bucking transformer v2
    by Rob C, on Flickr
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    • Post n°19

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    Post by cci1492 Thu Apr 12, 2018 9:42 am

    That's what I'm looking for, do you have a wiring diagram for it?
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    • Post n°20

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    Post by corndog71 Thu Apr 12, 2018 12:31 pm

    cci1492 wrote:That's what I'm looking for, do you have a wiring diagram for it?

    You will need to figure out which secondary lead goes to hot for the voltage drop.   I would label the leads after testing to aide in final assembly.  The 5A circuit breaker was for a previous bucker I built.  I used a 12A CB for this one.  

    10A or 20A Bucker Transformer for M125's? 41412320791_1a6ffe10d7_c
    Bucking transformer schematic
    by Rob C, on Flickr
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    • Post n°21

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    Post by cci1492 Thu Apr 12, 2018 12:58 pm

    Cool, thank you!
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    Post by cci1492 Thu Apr 12, 2018 1:33 pm

    So it's 6.3VAC that you're using? My line VAC in is never lower that 122.3, would a 5VAC work too?
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    • Post n°23

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    Post by corndog71 Thu Apr 12, 2018 4:27 pm

    cci1492 wrote:So it's 6.3VAC that you're using? My line VAC in is never lower that 122.3, would a 5VAC work too?

    Yes.
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    Post by cci1492 Thu Apr 12, 2018 6:42 pm

    OK, thanks for the schematic, I'm leaning towards the 5V to get closer to the 118 Bob recommends. The 6.3 will get me in the 115VAC number.
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    • Post n°25

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    Post by LeGrace Fri Apr 13, 2018 8:13 am

    Used a 6.3v to build my bucker.  At 122.4 line in voltage my output is 118.2v.  Seems to be doing the trick, 7 weeks in now without a red plate event, hallelujah!  

    5v will not get you down to 118, as it's not a linear reduction.
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