Fender Blues DeVille Pwr XFMR + Amp Rebuild

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CJ

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got plenty of audio stuff in the catalog but we need more pwr trans data to bolster up that catalog,

p/n  A-041752      EIA 606-513 

606 = Shumacher, same company that has been winding or Fender since  they put in Disneyland,

 

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leads all coming out the same side, wtf? that is is dangerous way to build a coil, got the line juice next to the HV juice next to the LV juice and the in-between juice,

we are going to split that up and put the mains on one side and the sec winds on the other,

and we ar going to drop the 6L6 B+ volts from 471 which is too high and shortens tube life and sounds harsh and is too loud for a 4-10" speaker amp,

looking to rewind at 400 to 420, this also allows for 500 volt filter caps so you do not have to put two 350 Vs in series, saves space and $$$.

heater VAC was 6.9 volts, so we are going to remove a turn to get us down to 6.3.  schematic has 5.52  VAC listed for reasons unknown.

core is laced 1 x 1 and we still only have 1.34 H pri ind which means 26 ma excitation current so we might juice that up a bit,

does have a Faraday shield,



 

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the coil has a shim for first layer cooling,

wound with the layered paper approach, crazy for a modern power transformer, no sonic advantage, less creepage protection with the lack of a nylon bobbin, even if you do wind with healthy margins, sometimes they cut the coils apart of center and you get arc over.

what we will do is change to a center tap full wave setup, this cuts down on heat as only one secondary half is conducting at a time. This is a healthy core for a 40 watt amp so no need to compensate for  the 20.7% power increase due to interrupted current  flowing in the secondary winding die to the half wave rect circuit over the bridge,



 

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get our blueprint template set up with some bits and pieces and we are ready to commence to a hackin and a hewin at these laminations with the heat gun and Swiss army knife,

 

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turn up the heat!

big mama to the rescue, normal iron would not break the copper seam,

heat gun on auto pilot, that core is about 4 lbs big,
 

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taking apart a big Lap 1 laced transformer is three times the work but we made it,  cold rolled steel, bends easy so you have to keep it heated pretty well so we can re-use the lams,

got the meat off the coil too,

 

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found a mandrel that was close and shimmed it with cardboard strips from an Anita Bryant album cover no great loss,  ;D

gave it a hair cut and we are ready to roll,
















i mean unroll,  :D


 

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splice cover removed,  knew it was gonna be busy in there,  but they did a nice job of keeping everything apart,

when you have a large coil, you can run two tiers of leads,  like playing cards on  Harley Davidson gas tank,

 

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nice chunk of polyester under the leads,

whats this, a cross over to the brn/yel CT?

that's gonna mean a bi fi wind,

how do i know?  am i that experienced?

no,.....we already took the coil apart!  ;D
 

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bi filar wound primary,  thick wire, low DCR,  typical of a pwr xfmr primary,

we will use Nomex between layers to get like 3 KV of isolation between layers ,  (3 mil)

 

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lay the winds out with big loops so we can easily measure the wire length which gives us the Mean Length Turn, and thus the DCR per 1000 ft and thus makes the wire gauge guessing game a little easier,
 

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we have a shield between the primary and the rest of the coil,

this is common and serves to keep any gremlins from the power lines out of the secondary winding and thus, the amplifier circuit,

we might have a few tricks we can use when re-winding to avoid this extra hassle,  :p
 

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nice!  a tri-fi heater wind,  less stress on the inner winding's as you do not have to pull as hard on the wire to get it to lay flat like you would with some #17 monster wire,

13 turns,  if we want to go from 6.8 VAC heater voltage to 6.3,

6.3/6.8=0.926

0.926% x 13 = .............................................12.04 turns!  Perfect!

could also use PRR method and tweak primary turns if we hit a half turn or 3/4 turn adjustment, and then re-calculating the HV secondary.

 

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every time we open up a transformer we see something different,

this time it is a maple dowel and a press board  tunnel for the primary start lead.

they wanted that black mains wire on the other side of the coil and there were an even number of layers, so it has to cross over.  why they ran it under the shield i do not know.

 

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next winding down under is the low voltage center tapped secondary.

this is also wound bi-fi.  why?

they wanted equal voltage from each half which means keep the DCR exactly even and keep the turns the same distance from the core for equal coupling.

probably not necessary as voltage regulation is done with zener's which are anything but exact.

the first wind is the HV secondary, just a simple single strand coming in at 683 turns @ about  72 turns per layer.





 

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so we need a voltage reduction of 470/400 = 85%

we have 683 turns or the 470 wind, so .85 x 683 = 581 turns.

since we are going full wave, we need to double that so 1162 turns.

will we be able to fit the extra turns? yes, because with full wave, we can use a gauge smaller as ther is only a 50& duty cycle for the each half of the secondary due to the rectifier circuit, each rectifier only conducts for the positive half cycle.

we will also tap off a bias supply so we do not have to kludge it onto that low voltage wind like Fender did. We will go old school like the blackface amps.

this was a nice coil, bi-fi and tri-fi wound, plenty big wire for good regulation, could have used grain orient for better induction and a little less stray flux but competition is tough now days.

the print fills up a bit more, will add the rest tomorrow>
 

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Another good job in the works by the looks ,
Just wondering ,would that same core with the new Hv arrangement support an extra layer for a rectifier heater winding ,or would that require a complete recalculation of everything from the ground up ?
470 volts ht does seem a little higher than what Ive seen before in those amps,was the guy running stock sovtek 6L6/5881 or US made old stock  tubes? I hear what your saying about that series though ,the output stage is almost too clean and next to impossible to distort, if you do manage to distort it, it tends to sound a little wheezy.
 
Hi

Nice work CJ and detailed process as usual !
At least you won't have new entry in the shock log with this (for the moment)  :-X
8)

Best
Zam
 
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