784-128 vs 784-139 Transformers

parkhead · Post by **parkhead** » Wed Nov 07, 2007 12:44 am

What is the difference between these transformers most of the specs I see show the same data for both

p

Roe · Post by **Roe** » Wed Nov 07, 2007 4:41 am

the 128 doesn't work with 4 ohms. it's supposed to sound slightly smoother than the 139.

Flames1950 · Post by **Flames1950** » Wed Nov 07, 2007 7:22 am

I think the 128 still had the 100-volt tap instead of the 4-ohm tap. Brian Wallace's version of the 128 probably has a four-ohm tap and buries the 100-volt tap, so the tone is the same with no confusion -- IF I'm not mistaken. I'm sure Brian will pipe in if he spots this thread.........

novosibir · Post by **novosibir** » Wed Nov 07, 2007 8:15 am

The 128 is designed for KT66's and does have 6.6K primaries, while the 139 is designed for EL34's with a primary of 3.4K

Larry

Roe · Post by **Roe** » Wed Nov 07, 2007 9:52 am

brian's 128 has 3,5k (el34s) and a burried 100v tap.

from his page:

JTM50 Style Output Transformer
784-128

This transformer is a copy of the one used in JTM50 amps from
1967. It is hand wound on a paper form and uses paper
insulation between the windings just like the old ones.

Specifications on the 128 transformer:
Primary Impedance: 3.5K (EL34)

Secondary Taps: 4, 8, 16 Ohms, buried 100V tap

Output Rating: 50W Conservative

novosibir · Post by **novosibir** » Wed Nov 07, 2007 11:07 am

Roe wrote:brian's 128 has 3,5k (el34s) and a burried 100v tap.

Oh yes

I pulled it out of my head too quick w/o verifying it about some writings.

The 784-103 is the one for KT66's

The 784-128 is the later one for EL34's, but still w/ the 100V tap

The 784-139 (current version) is the same as the previous, but w/o 100V tap and 4/8/16 ohms secondary

sorry for the confusion!

Larry

LordAli · Post by **LordAli** » Wed Nov 07, 2007 2:14 pm

Because
784 - 128 is for JTM50 and
784 - 139 is for JMP50

JTM amps got 100V taps unlike JMP amps. But, I'm not expert on marshall amp history. I only focus on circuit design.

908ssp · Post by **908ssp** » Wed Nov 07, 2007 3:43 pm

Anybody got these for sale? Originals not reissues.
784-103 OT and a 1202-55 PT

neikeel · Post by **neikeel** » Thu Nov 08, 2007 5:49 am

908ssp wrote:Anybody got these for sale? Originals not reissues.
784-103 OT and a 1202-55 PT

Good luck Alex, I have been looking for a while. It will probably require the promise of $$$ to prise either of those away from someone

908ssp · Post by **908ssp** » Thu Nov 08, 2007 10:09 am

I'll pay good money to get this amp restored. I would also consider blown transformers. I'll settle for rewinds too.