New Amp Build

[Jana]

Martin,

Yes, I have seen the schematic at Mark Huss' site--that is one of the higher voltage models.

Check out this site for some historical data: http://marstran.com/Historic%20Data%20Base.htm

The 1202-118 PT is the lay down transformer that the early 50W amps had. They started to transition to the 1202-164 about 1969 with some amps using the -118 for a few more years. It looks like about 1980 is when they went to the 1202-324.

The 1202-164 is the one that gives about 375 B+. The -324 is the one that gives about 460 to 470 B+. This is the transformer that the majority of transformer vendors sell as the 50W replacement.

Marstran has the 1202-164 as well as the 1202-118. Brian has gone to great lengths to duplicate the transformers correctly.
I used the 1202-164 PT and the 784-139 OT from Marstran in this amp. I have been playing the amp a lot lately and it is now my main amp--it is really sweet. When Mark F. is feeling better, I hope to bring it over and let him give it a strum or two. He can give you an unbiased report on it. Hey Mark, I just volunteered you!

You are right, the higher B+ and the low OT impedance does stress the tubes. To my ears, it also sounds harsher and less musical.

[martin manning]

Hey, thanks for the details! Did you use the plexi-style C1999 3H choke?

[Jana]

I'm not sure what the Henries is of the choke I used. It is one I had on hand--one that is used in Marshalls--4 mounting bolts, 110 ohms DCR. I think it's 3H but not entirely sure.

[martin manning]

Thanks, again. That style of choke seems to be spec'd at 5H in the replecement parts.

[Colossal]

Hi Jana,

Great looking build! Is that one of RJ's chassis? or your own?

[Jana]

It's a custom sized, blank chassis from RJ. I punched all the holes.

[Colossal]

Very, very nice.

[RJ Guitars]

Wow Jana, that is a very cool build. I knew that you were finished but missed the thread when it first came out - sometimes I get so Trainwreck focused sometimes I forget there are other options.

Nice looking cord bundling - I've heard some folks like dental floss for that but I am too impatient either way and tend to gravitate toward zip ties. It's way cool to see those old mustard caps in there too... wish I had a stash of those.

I've got an old Marshall in the shop that a guy asked me to rebuild, this is certainly inspiration for that. Do you think the aluminum chassis makes any difference in the sound of the amp? I sure do prefer to work with aluminum myself.

Very cool amp and thanks for the kind words on the chassis. The local shops can do some amazing things and are used to mil-spec and nuclear industry standards so occasionally some of that quality shows up in my stuff... it's hard to tell a certified welder that a mediocre weld will be strong enough for our purposes.

rj

[Jana]

Thanks RJ--it all started with a good chassis.

I never thought about using dental floss--good idea. I have a roll of heavy, waxed thread that I got from Tandy Leather quite a few years ago--that's what I use. If I was building a lot of amps, one after the other, then I would probably gravitate towards zip ties too. But, in the last decade or so, my amp building has changed from building a lot of them to just building one every few years as a therapy type thing. I take my time and enjoy the process.

I like working with aluminum too. I don't know if there is any appreciable difference between an aluminum chassis and a steel one. I think the more important things are grounding scheme, layout, lead dress, etc. If it was me, I wouldn't worry about using an aluminum chassis to rebuild an amp that once had a steel chassis. Is there something wrong with the steel chassis so that it can't be reused? With the skilled shop you have, extra holes and stuff like that could certainly be welded up with plugs, sanded down and replated if you wanted to keep it as original as possible (original serial number stamped in chassis, etc.).

[gingertube]

A big hint from Jana's work on this amp. I have also found that EL34s are just stunning at around the 375 volts mark. Only get 35 Watts from the pair but the sound is just more "relaxed" and "stress free" than pushing them to 50+ Watts at 480V.

My "Junkbox Trainwreck" runs cathode biased EL34s at 380V into the usual 3K4 Raa Output 50W Output Tranny. (Well actually I'm running some NOS Philips 6CA7 in it now but it did run JJ EL34 and some fat bottle Chinese EL34 at various times - all were good.

Lacing Cord - reminds me of my apprentice days.

Cheers,
Ian

[dorrisant]

Late to the show ,but... Wow, Jana, very nice looking amp! Thanks for the build details as well.

Tony

[sepulchre]

Boy, if I had a nickel for every yard of cable I laced . . .

Beautiful work!

[sepulchre]

Btw, I didn't want to buy 100 yards of waxed lacing so I looked around. Hobby Lobby has "Waxed Linen" which is perfect for the job and relatively cheap.

[Structo]

My late father worked for the Bell company for 34 years.

I remember seeing that waxed string in his tool box.

I think it may be linen string that is wax coating.
He said the was helped it lock into place when you were lacing wires.

I can't remember I may still have some.

I think most of the great techniques regarding lead dress is from the military.
We had to get good quickly and so rugged design or military quality construction was born.

Some of the greats like Marshall Hi Watt and other manufacturers adopted the mil spec standard and the rest is history.