Fender Princeton Reverb Output Transformer

[CJ]

this was purchased as a direct replacement for a DIY princeton reverb amp

it goes pri-sec-pri

1500 turns of #36 
95 turns of heavy stuff,
1500 turns of #34

turns ratio is 32:1, Z ratio is 8K to 8 ohms,

done on a square stack of EI 625, 29M6 steel, laced 1 x 1, no shields,

215 calculated henries pri, and we hit 18.4 k gauss at 26 hertz, not bad, this is a guitar amp, which is mostly mid-range, Fender might even tune the speaker cab with the transformer,

dcr is balanced by using 2 wire sizes for the pri,  so 6v6gt tubes run balanced and core does not freak out from dc cause by imbalance, 147 ohms each for 294 total,

sec is about 0.5 ohms






it scoped out real fine, 5 hz to at least 60 k hz pretty flat,

so we decided to investigate... ;D

 

[CJ]

no fancy insulation, just a few turns of poly tape.

hand wound pri at 30 turns per layer, which leaves air between turns which is how they get the hi end response,

since you have cross-overs and lack of insl between the pri-sec, then reliability is not as good as with no cross-overs and some hi voltage insl between pri and sec as one side of sec usuall gets grounded at the speaker jack,

more pics,

 

[CJ]

first layer of a pri>

 

[CJ]

sec>

 

[CJ]

lead hookup>

 

[CJ]

notice the lead breakout,
the pri runs underneath the heavy sec, not the best way to wire,
it is better to drill holes in the bobbin or use a bobbin with slots,

this method runs the thin pri wire by the heavy speaker winding,
and this unit was wired out of phase just like the Vibro Champ output i took apart from the same company, 

brn goes to the top 6V6, there is a 2.2 k feedback from the sec,

so this will feed back if wired normally, maybe i should give Antique a ring?  ???

so things we can improve on:

dcr is 300 ohms,
the KT88 output i just made is much bigger but has a pri dcr of 60 ohms,
lower dcr and you lower distortion,

insulation is bare minimal, we can use nomex in between pri/sec/pri,

use a 1 inch stack-this means less turns for the same henries,
less turns means lower dcr, and we have room for bigger wire which means even lower dcr, however, our MTL(mean turn length) will go up 20 percent.

lay the wire flat, this will make the xfmr more reliable which means no costly court battles, and we can use bigger wire again, since the turns per inch will go up.
leakage C might go up due to closer wires, but we are using less turns so it should cancel this out.

use 7 sections, 4P and 3S, for better balance and less leakage,

hook up the black lead to the finish, not the start!

use some Ni lams to add perm and thus henries at the expense of a more saturated sound, oh darn, 

 

[sahib]

Well, CJ. I am a fan.

 

[CJ]

we squashed some #30 and #26 (bigger wire) onto a bigger bobbin with fewer turns,

result is a drop from 295  to 90 ohms pri DCR

sec R did not change much,

and we added 37 percent screen taps for ultra linear operation,

so we can now put a switch on the amp for switching from ultra liner to regular operation,

i have not heard of an ultra linear guitar amp, i am sure somebody has spliced on an ultra linear transformer onto a Fender somewhere,

but usually 6L6 or 6550 or some other big power tube, not the 6V6 so it should be interesting,

we have about 1800 trns instead of 3000, so some nickel lams will be added.

this operates at the exact same flux level, 18 k gauss at 26 hertz at 12 VAR, has the same inductance,l and freq roll off as the stock xfmr,

but we did lower DCR by a factor of 3 which means lower distortion and less heat,

and we have a 4 pri 3 sec structure instead of P-S-P as in the stock xfmr.

plus the screen taps and a 16 ohm winding for an additional phone jack for speakers,

Q goes from 115 to 375, which is where we like it,

 

[desun]

Very interesting CJ! I've always fancied winding my own transformers for a guitar amp... perhaps I will take inspiration to do so!

Some of the Hiwatt amps used the ultralinear taps on the OT, but I agree that it is rarely seen-- I have never played such an amp (apologies for the large image-- it is not mine):

 

[idylldon]

I think the Fender 75 from the early 80s used an ultralinear OT.  Kind of a rare beast and, frankly, it didn't sound all the wonderful. 

Cheers,
--
Don

 

[gfr]

The silverface Fender 75, the 135.

http://www.prowessamplifiers.com/schematics/fender/Bassman_135.html
http://www.prowessamplifiers.com/schematics/fender/75w_Lead_Amp_with_Reverb.html
http://www.prowessamplifiers.com/schematics/fender/Twin_Reverb_sf_135.html
http://www.prowessamplifiers.com/schematics/fender/Ultralinear_Proreverb.html

Most people don't like the way they sound, but there are other changes in the circuit compared to a Blackface, I'm not sure the UL transformer is the only one to blame

Altough I've heard that some country players do like these amps.

The Marshall 200W

http://www.drtube.com/schematics/marshall/200w.gif

This sounds good IMHO but it's insanely LOUD.

Some Dr. Z amps like the Route 66 have a UL output (but no NFB loop) and sound great.

http://www.flickr.com/photos/49108115@N04/4501477895/

 

[walter]

Back in the day... I modified several early silverface twins to blackface, some got new transformers. Our resident guitar virtuoso thought the ultra linear output sounded better.

 

[CJ]

wow, thanks for the replys!

i did not realize that there would be all those UL amps,

Route 66, that looks like a cool amp, Vox front end with the pentode, but 6V6 instead of EL84,

plate load for 6V6 and 6BQ5 are both at 8000 for push pull,  so the same output could be used for both,

notice in the tube catalogs that distortion for a class A single tube is about 10 percent, and for push pull it drops to 3-4 percent due to cancelled harmonics in the xfmr
RCA manal has the distortion specs,

good to see the Marshall pig mp with 6550's ,
this DIY 6550 output xfmr has the taps so that should be fun,

 

[fazer]

How much to own one of these transformers.  PM me.  Not sure about the UL tap but its just a DPDT to go each way.

 

[CJ]

let me test this proto-type out in the amp, if it rocks we can make a few more i am sure,

i stuck 11 Ni lams in the middle of the stack, each Ni lam is the equivalent of 10 regular silicon grain lams, so we have about 45 regular lams and 11 Ni = 110 steel lams,
so we have 150 lams worth of inductance for levels below 5 k gauss,
the Ni will saturate above 6 kg but then the regular core takes over,

so we have the regular core stack for hi flux situations (amp on 11) and we have low volume inductance with the Ni lams,

this thing runs flat from 5 hz to 80 k hz, then starts a steady rise to 275 k hz as leackage C takes over, then a second minor bump at 675 k hz, the other xfmr simply rolls off after 80 k, so we might have a different sound on the high end,

that is the 6550 UL below, we need a bake and dip on these,

 

[CJ]

Old thread,  new print,