Metropoulos Forum

Hello! I am brand new to this forum. I've lurked off and on for years, but I'm finally diving in and sourcing parts for a JTM50 build. I can read a schematic, follow a wiring diagram, and I'm pretty good with a soldering iron but I know very little about transformers. I can't find it now, but I read somewhere that a 784-139 output transformer is for a solid state rectifier and the 784-128 is for a tube rectifier. Also, a 1202-133 PT would provide 500+ voltage to the power tube plates and the 1202-118 PT would supply 430-450 volts. I've checked the Marstran site and found the chart for Marshall specs... according to the chart, 50 watt JTM/JMPs typically have either the 1202-133/784-139 or the 1202-118/784-128 combination. Later on I saw a 1202-118/784-139 combination was the norm. Is there any reason I couldn't use a 1202-133/784-128 together? Unfortunately, I haven't found a manufacturer that lists a repro 784-128 in their inventory though I have emailed a couple. I know this is a totally newb question, but I've always heard the only stupid questions are the ones you don't ask. That's also when I tend to make mistakes. Forgive me if I shouldn't post links here, but this is the chart I was referring to...

http://marstran.com/Historic%20Data%20Base.htm

Also, according to the chart I noticed that contrary to the information I had found previously, solid state and tube rectifiers have both been used with either output transformer, which brings me to another question... Wouldn't the power transformer determine the rectifier type due to existence/absence of a lead for the tube rectifier? Told you I don't know much about transformers.
One more thing. What value choke should I use? The chart doesn't list choke values but does mention c1999 a few times which I'm thinking is the 8 Henry choke. Marstran has 3 and 20 Henry chokes listed. I'm not sure what would be most appropriate for a JTM50. If I know little about transformers, I know next to nothing about chokes.

I guess that's all for now. Thanks.

http://forum.metroamp.com/viewtopic.php?f=7&t=41831

Best combination (IMO) is a 784-128, 1202-118 and 354-114 choke and GZ34.

The correct choke is 3H (C1999 or 354-114) that is what I would use

This is the best combination for modern EL34s. Most decent -118 clones have the 5vac tap for a GZ34

AFAIK, no-one makes a -133 PT, Marstran does a -128 OT but the differences between a -139 and -128 are very subtle, I have used both in BF clones to good effect.

Welcome to the forum! Choosing the correct PT will depend on whether you intend to use tube rectification or solid state and also, what you would like the B+ to end up at in the finished build. Marshall started with a very high B+ in the early days (500v) and slowly came down over the years as the quality of power tubes declined. If you wanted to build an accurate clone of a particular model year then I would suggest sticking with the proper PT. If not, you have the option of choosing which type of rectification you would like to use and the level of B+ voltage. Forum member "Neikeel" knows the JTM circuit well as do several of the other members here. I'm sure one of them will be along to offer some suggestions.

Thanks for the help, guys. Meant to post sooner. Ordered transformers from Brian Wallace last week. I'll start a build thread once all my parts arrive and I start assembly. I'm sure I'll have a million more questions once I get going.