jwyse's first build: 50W

[jwyse]

I've built a few pedals since December 2009, and really enjoyed the whole learning process, and getting something usable out of it when I was done. I liked having the ability to modify (even if only slightly) the circuits to make them "my own" in some way. After drooling over a few of the amp-related threads in the BYOC forums, I decided to step it up and build an amp.

I went back and forth between several amp kit makers, and finally decided on Metro because the consensus on many other forums seems to be that Metro provides great quality kits/parts, great instructions, etc. I haven't finished the build yet, but so far I agree 100% with that opinion. I can't compare Metro to anyone else from personal experience, but I can say that it was a good choice for a first build.

After I decided on Metro, I eventually decided on the 50 watt plexi kit (1968 "1987" lead specs) with Mercury transformers, MV, effects loop, and AB NOS CCs. I'm building the Lar/Mar PPI MV in from the start, and adding the effects loop as soon as I get it all finished/tested/biased. I'm also making a couple of very minor modifications to the Metro instructions: I'm using an IEC power connector, and I'm wiring the mains fuse to the hot wire instead of neutral, as explained in these threads.

Here's my progress so far. Please excuse my craptastic photography.

Unboxing:






Assembly:




















That's it so far. Ready to start wiring and populating the board next.

[shakti]

If that's your first build, then I am mighty impressed. Looks *super* nice so far! Ground buss on the pots is superb, well done!

[somethin'else]

Yeah, really nice work there. It shows you enjoy it. Metro kits are tops all the way. What kind of pedals have you done? Cheers.

[TheFullNelsons]

Built the same amp a few months ago and I can tell you that you are going to love it. Looks good so far!

[syscokid]

Metro is at the top-of-the-game when it comes to these Marshall kits and replica amps. Your work looks really-really good...I'm jealous
I guess the MV is going in between the power tubes. The pre-drilled holes look good, too!

[jwyse]

Yeah, really nice work there. It shows you enjoy it. Metro kits are tops all the way. What kind of pedals have you done? Cheers.

I started with a modified Tube Screamer for myself, then a couple of compressors (one based on the Ross, and one optical compressor), then a more modified Tube Screamer for a buddy, and I'm about to wrap up a Dr Boogey ("transistorized" Mesa Dual Rectifier preamp circuit, in a pedal). My Metro kit arrived the same day I finished painting the Dr Boogey enclosure, so I've been working on that and haven't wired up the board/pots in the enclosure yet.

Here's a pic of my first Tube Screamer (green), optical compressor (blue), and the "5-knob" comp based on the Ross (aluminum). I've swapped out the knobs on the TS and the aluminum comp since this picture.

Metro is at the top-of-the-game when it comes to these Marshall kits and replica amps. Your work looks really-really good...I'm jealous
I guess the MV is going in between the power tubes. The pre-drilled holes look good, too!

Yep, the MV is going between the power tubes in that unmarked hole. The FX loop is going in the 3 holes on the preamp-tube end of the chassis. Drilling/filing the holes for those (and the big one for the IEC socket) was fun...

[jwyse]

I just finished wiring the board for the off-board connections.

I stuck the stripped end of wires up through the bottom of the board, tightly bent them over the tops of the turrets, and put just a tiny bit of solder on the outside of each connection to hold them in place for now. The solder doesn't really show up in these pictures, but it's there. When I add the component leads in the turrets, the additional mound of solder on top should hold the wires even better. I hope this is a good way to do it...

[OnTheFritz]

Lookin' great!
Golfer by any chance?

[jonamojo]

looks real professional, seems like it might get a little messy doing the turrets that way though.

[54strat]

NICE looking work. Still plenty of room in the turrets for your components, should be fine.

[jwyse]

So, after about 8 months of not being able to work on the 50W very much, I've finally finished this weekend. I put the Lar-Mar PPIMV in right off the bat, and I'm going to hold off on the effects loop for just a little while.

It sounds INCREDIBLE.

The testing and biasing went well, but the voltages are all a little high -- the documentation says to expect +/- 5% on the voltages, but I'm seeing more like 13-25% on most of them. I'm putting all the voltages in a spreadsheet to see exactly how much they're varying from the expected values, and I'll post them all tomorrow.

In the meantime though, some highlights:
Input voltage (house wiring) is 122 VAC measured at the hot terminal of the mains switch
Heater voltage is spot on, 3.15 VAC
Rectifier (+) is 509 VDC (expected: ~450 VDC)
Output tube pins 3,4,6 are 510-511 VDC (expected: ~435-440 VDC)

Bias readings were initially EXACTLY 0.0 mV with the bias pot fully clockwise; 33.8 and 30.9 mV fully counter-clockwise. Expected range is 35-40 mV. I'm going to put a few more hours on it and then try to bias it again, since that might drift as the components burn in.

So, the documentation says to expect +/- 5% variation on the voltages, and that components/etc may vary, but it doesn't really say what the 'limit' is before I should know that something is wrong. Is 13-25% variation ok?

I should mention that I have (I think) a pretty decent DMM: http://www.service.kleintools.com/Tools ... uct/MM1000" onclick="window.open(this.href);return false;
I just picked it up today because my other DMM was maxxed out at 500 VDC, and I could tell that my voltages were too high for it to read. [edited to add: My point is, I don't have any reason to suspect that the readings are wrong because of a cheap meter.]

Transformers are Mercury Magnetics [from Metro]: P4550JT-G2 and O50JM.

Thanks in advance for any input.

[syscokid]

I was wondering what happened to you!
Your voltage readings are interesting ...
Your work looked so good in your previous pics. I'm surprised you didn't post pics of the finished amp...hint-hint!

[jwyse]

Ah yes, pictures.

Reading 509 VDC on the rectifier (+) terminal


Board

The holes in the upper left are for the effects loop mod, which I'll wire up later. I mounted the loop board and switch in the chassis just to fill the holes in the meantime.



Sitting on top of my 4x12 and 2x12. Still deciding which speakers to fill them with to replace the stock G12T75s.

[jwyse]

And here are my voltages.

(columns are expected / actual / % difference)



Could it be that the transformer (Mercury Magnetics P4550JT-G2) is just out of spec, and running everything else a little hot? The AC voltages all seem good, and the DC voltages are all off by similar percentages (~15% and ~25%, unless I'm just imagining a pattern where there is none).

[syscokid]

I hope the "more-educated" chime in here, soon. Those Mercs are suppose to be putting out about 420 volts B+.....

Are you sure that PT isn't marked "G3"?