68 12,000 with 2" high impedance OT

[emmjaydubya]

Hi guys, thought I would start a new thread down in the 12,000 basement since it's been kinda quiet in here! Been collecting parts for a while and just now getting started in the last couple of weeks on and off. This is my 2nd amp build (my JTM45 was the first): http://forum.metroamp.com/viewtopic.php?f=15&t=35820
I'm certainly not reinventing the wheel here, in fact I'm ripping alot of you guys off in most respects, haha! But I guess everyone has their own take on a 12,000 and besides, we never get tired of looking at amp pics, right?

I'm not going strictly for VH, I want a sound that only a plexi will get, but I am curious how close it will be right off the bat. I wouldn't consider myself a VH player but messing around with it from time to time is pretty fun! Here's some of the specs, subject to change of course:

- Metro 12,000 chassis I got shipped from neikeel in England, right before George started making them available again, ha!
- pretty much DF spec except I'm going with 64uf in the P.I. instead of the 100uf to start with
- 2" Marstran 132 2.5k high-impedance OT, this is one from the run Brian did a while back
- M****n PT, using the lower 465v B+ tap and eventually a Variac to bring it down around 400v
- Brian H boards
- Mustards, Pihers and (mostly) Iskras, oh my!!
- Larry grounding
- EH 6CA7s

This will probably take a little while, but hopefully I'll have it up and running in a few months or so.

This is the same rotating amp cradle I made that I used on my first build (JTM 45), but just changed the bottom piece out for the Superlead:



Went round and round about the power jack and came to the conclusion that the IEC was the best choice. I used a vintage Bulgin on my JTM45 build and it has proven to be a giant pain the ass when it comes to gigging, if you're strictly playing at home then it's fine. I love how they look but I drag my amps eveywhere and Bulgin plugs just aren't reliable enough for me:



Got a cool looking 70's Bakelite one off of ebay. Well, as cool as an IEC can be:



Next mounted the trannys, looks crushing!!





Used the vintage-style Classictone trans brackets for the OT, had to drill two holes to accomodate the huge 2" 132 and another hole for the primaries:





Finished the heater wiring, decided to use 18 ga wire for the power tube sockets and 22 ga for the pre-amps. Did some reading on this thread started by vanhalen5150:http://forum.metroamp.com/viewtopic.php?f=44&t=35338
Some of the amperage numbers posted by vh junkie were interesting, so I got a little crazy and used two different gauges! Maybe a little over the top?





18 ga isn't as easy to work with in a tight space as you would think, ha:

[emmjaydubya]

Next I decided to do something different with the PT primaries. Most builds I see, the 120v wire is used and the other primaries are just zip-tied up and pushed in the corner. Works fine, but I was inspired by a vintage plexi pic that Strat78 posted up that has a cool tagboard for all of the primaries (circled in red):



So I got Brian H to make me a custom voltage tagboard that I moved out of the way, to the right of the PT for easier P.I. cap fiddling:



I heat shrinked the taps I wasn't using and left the 120v open for a future wire:

[vanhalen5150]

YEE-HAW! Nice job so far.

[vanhalen5150]

I'm getting back to my 12000 this weekend. I also tried the 18 gauge I got from Valvestorm, but I found it just too hard to work with in the tight space. Looking at yours I realize I'm just trying to get too many turns in the power tube section. What taps are on your PT? Is there a lower tap rather than 465v?

That strip on the inner side wall looks like a great idea. I may steal that.

[emmjaydubya]

The M****n T2562 has a 500v tap and a 465v tap. The EH 6CA7s are apparently sensitive to high voltages (?) so the 500v tap is kinda worthless I suppose. I wish the other tap was a little lower than 465v but messing with the Variac will be fun.

I've seen guys twist their heater wires super tight and sometimes it's a bit much. All the original plexi pics I've seen show em kinda loose so I figure there's a happy medium. And you're right, 18 ga is not easy to work with up against the back wall, but I looked at some of George's builds and his stuff always looks just right. I made sure they all had the same number of twists and figured that was good enough.

And yeah, steal that strip idea, 'cause I did.

[BrownSound1]

Looks great! I hope my chassis comes in soon so I can start having a bit of fun with you fellas.

On the heater wire, if you guys are concerned 22 awg is too small, and 18 awg is too cumbersome, then why not meet half way and get 20 awg? Honestly, 22awg is fine as it has been used for years. Those amperage carrying ratings are conservative, and a few milliamps over would not be an issue.

[emmjaydubya]

Yeah, I agree, 18 ga is unnecessary but I thought it would look cool so why not. Nothing wrong with overbuilding, mil-spec style. Plus I thought it would be a little different.

[BrownSound1]

Yeah, I agree, 18 ga is unnecessary but I thought it would look cool so why not. Nothing wrong with overbuilding, mil-spec style.

LOL. If you wanted true mil-spec you could use all white wires.

I tend to oversize wire when I build automation controls for my employer. If something burns up, it isn't going to be because I used wire that was too small.

[BrownSound1]

Double post

[vh junkie]

The only place I would consider critical to use heavier wire is between the PT and the v7 socket. Of course this is only an issue with what they call self-leaded PTs... yours has the wires built in to the PT so it is not an issue. The smaller wire (22awg) lets you get the winds tighter, which I think is more important for hum avoidance.

[emmjaydubya]

Ahh so maybe I don't have enough winds in the power tube heaters? Uh oh...

[emmjaydubya]

Worked on the OT primaries today, thinking I got them right. Measured the resistance between each combination of all three wires and the center tap seems to be brown just like the Marstran diagram says. I wasn't sure however about which wire (red and white) went to which set of tube sockets. The M****n diagram for his 132 looks identical to the Marstran, and I found out in the 12,000 Larry ground thread, that red goes to V4 & V5 and white goes to V6 & V7 on the M****n, so just used common sense that the Marstran is the same. Again, hope it's right!

Also put in the Welwyn screen resistors and a couple of Iskra 1K 5 grid stoppers...

[Strat78]

Wow, really nice looking leed dress. Keep the the heater wires as they are to see if noise is not a problem, because they look really good that way. I just finished another build and put the little tag board where you have yours, now I'm copying you!
Inspirational work here!

[emmjaydubya]

Ha, thanks! You gonna post any clips of that new one? Is this the one with the 2.2k MM trans?

At this point I think I'm stuck with what I've got on the heaters since I've moved forward and changing them now would mean practically starting over. Last night I wired the speaker jacks and impedance selector. I used 22 awg topcoat for the speaker ground and it seems so wimpy next to the solid 18 awg, maybe I'll change it, opinions? Put the purple wire on the 4 ohm tap, thought I would start with 100k NFB!

[vh junkie]

I think you have the OT primaries right (red towards v4 & v5, white towards v6 & v7). You can give yourself a little insurance in the future by extending the v5 wire to v4 instead. That way if you have to reverse them you have enough length. Remember to keep the purple NFB wire from running close and parallel to the twisted (red/white) OT primaries. I guess they need to cross, so it should be at right angles?