784-128 vs 784-139 Transformers
Posted: 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
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784-128 vs 784-139 Transformers
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Posted: 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.
Posted: 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.........
Posted: 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
Larry
Posted: 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
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
Posted: Wed Nov 07, 2007 11:07 am
Oh yesRoe wrote:brian's 128 has 3,5k (el34s) and a burried 100v tap.
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
Posted: 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.
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.
Posted: Wed Nov 07, 2007 3:43 pm
Anybody got these for sale? Originals not reissues.
784-103 OT and a 1202-55 PT
784-103 OT and a 1202-55 PT
Posted: Thu Nov 08, 2007 5:49 am
Good luck Alex, I have been looking for a while. It will probably require the promise of $$$ to prise either of those away from someone908ssp wrote:Anybody got these for sale? Originals not reissues.
784-103 OT and a 1202-55 PT
Posted: 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.