Please check 12xxx build diagrams

[Night Owl]

Hi,

I've been combing through the forum and wiki, trying to pull together the details for building my 12xxx kit. Of course, I relied heavily on the 100w kit instructions. I think I have most of it right, but I'm sure there must be errors. I wrote this up for myself and it is kinda rough in spots, but hopefully it is clear enough to tell whether I have it right. If someone could review the diagrams for the wiring and components, that would be great. Or if you really don't have anything else to do, check out the text and let me know if you see anything obviously wrong there. There's an optional section on installing some shielding - you can ignore that. And I won't get to the MV and loop until the rest is working first. I'll add pictures as I build.

Many thanks!

Night Owl

[neikeel]

You have been busy

Only way to check it out thoroughly would be to build one as a paint by numbers from a Newbie

Must be some volunteers

[Night Owl]

Yes - I'm that newbie, for sure! I wouldn't want anyone to ruin some components (like maybe their OT or their heart) because I don't know what I'm doing.

I just wanted to get a sanity check on the circuit board and power board before I toast something. I'm on Step 8 right now. Step 9 is mostly/all from the 100w kit, so there shouldn't be much risk until I get to Step 10, where things differ from the basic 100w kit.

I already found a couple of minor things that I've adjusted. Like putting on the plexi panels before putting the indicator lamp in. And you only need 6, not 7, of the red and black wire sets to wire the heaters. (BTW it's not easy to get the 6v lamp to stay in place snugly with that metal locking tab - not sure what I'm going to do with that yet.) I've made those updates, so they will be in my updated version. I've also taken a few pictures as I go.

The things I most need reviewed are the power board and circuit board wiring and components, along with some of the details of the wiring of the power section (fuses, switches, etc). I'm hoping someone that has built one will chime in before I get there. Otherwise my next posting will probably be something like "Is smoke and a really bad smell normal when you power up for the first time?"

[neikeel]

Check against this:

http://forum.metroamp.com/viewtopic.php?f=26&t=2941

and this

http://forum.metroamp.com/viewtopic.php?f=26&t=2887

I personally use a 2k7 with the 0.68uF on V1 (rather than period correct 820R) and a 0.0022uF channel 1 coupling cap - 2nd one in from left). I use a 47k NFB resistor hooked up to the 8ohm tap for NFB.

I use a 5000pF bright cap and use either PPIMV to taste on my clones or attenuator on my '68 to be able to put volumes on 7-8 and enjoy

[Night Owl]

Thanks, Neikeel! I checked out those threads again.

I see that in the power board wiring, George has an inconsistency or change between the photo of the back of the board and the photo of the front where he added the lines for the leads. The yellow lead going off to the PT COM is attached at different points. That may not matter, so I updated my diagrams to match his picture of the back of the board, instead of the edited picture (since that's probably what he actually did). He also shows a red lead going over to the circuit board where the "choke, output screens, and 10K 2 watt" meet. Most diagrams show that as a yellow lead (minor detail), but it goes to where the "choke, output screens, and 8.2K 2 watt" meet, not the 10K 2 watt.

I also had the red lead going to the HT fuse at the other end of the red jumper on the board. Again, that may not matter, but I updated my diagram to match George's.

So there are a couple of corrections (hopefully!) resulting from your help - Thanks!!

On the main circuit board, I think all is well. I had selected a 100pf cap instead of a 47pf in one position, but now I'm switching it back to 47pf. I think either will work, but I'm not sure what the difference in the sound would be. Do you know? I think in my picture one 820 resistor was cut off on the far left. I've put that back in there, too.

I hadn't seen the updated 68 layout in that thread, just the original 68 layout. I see the .0022 cap instead of the .022 one. My kit with the Sozo upgrade has the .022 instead of the .0022, so I guess I'll start with the .022. Do you know what the difference in sound would be?

All the diagrams I saw were pretty consistent on a 47K NFB resistor, although I was planning to connect to the 16ohm tap. The 100watt instructions mention that 16 ohm gives a cleaner and tighter sound, while the 8 ohm and 4 ohm are progressively looser and hairier. Have you tried the other taps to see how they compare to the 8 ohm? I thought I'd give the 16 ohm one a try...

I have an OK attenuator (Gibson Stealth, which is basically a THD Hot Plate) but it sucks some tone as I use higher levels of attenuation. What works on my 1959HW is a 250pF bright cap with that setup and the volumes I use, so I'll give that a try. I do plan to put in a LAR/MAR PPIMV, so maybe that bright cap value won't matter so much after I do that. Then I can crank it without the attentuator or bleeding ears! Adding the Zero-Loss loop will let me get my delay in there without having to run wet/dry with a second amp, so life is going to be simpler when I get through this!!!

I'll post an updated doc as I get a little further along. Time to warm up the soldering iron...

[neikeel]

It is important to check the arrangement of your on board bus wires (some board have the bus wire to the screens/PI crossed. The screens wire is usually yellow, fuse connection red, PI blue.

A 100pF is said to get rid of 'PI fizz' not sure I have heard it using stock cap but using bigger cap can lose some highs.
A 0.022uF coupler on bright channel is fatter and thicker but not quite as cutting in the lead role. I usually use a humbucker guitar with switchable coil splits. I prefer the brighter setting but the 0.022 is smoother, but a lot will depend on the rest of your rig and what you are playing. I find that I can darken my sound with tone controls etc.

I have a build with a loop kit to be incorporated and may use one myself later if I like it, my delay and flanger never sound particularly good unless I run wet/dry which is as you say a nuisance to drag around.

[Night Owl]

Thanks for the info on the other cap values!

I'm hoping to finish my construction this weekend. I still have to wire the main circuit board, but that's about it. I'm having a little technical difficulty right now. I have a 25w soldering iron, but it didn't seem to get quite hot enough. So I got a 45w one a while back. That one is great, except it's really easy to melt the insulation on my wires, even when I use a heatsink clip. I figure a 30-35w iron should be about right, but Dallas seems sold out of the one I was looking for. Grrrr...

I found a couple more errors in my write-up, but mostly it seems on track. I'll know soon

[Night Owl]

I finished up the initial build of my amp this weekend. It took me a little while to debug it, as I made a couple of mistakes on the circuit board. I laid everything out, checked it 3-4 times, and then soldered two things to the wrong spot anyway. Doh!

I still have a little tweaking to do. My PI socket suffered an embarassing solder drip right straight down a pin-hole. I doubt I could have hit that hole if I was trying, but it went right straight in. I think that is probably causing the only problem I have. After I play for 30 minutes or more at pretty high volumes, my sound fades out to nothing. Seems to happen quicker if I'm loading the front end with pedals, but that may be coincidental. If I let it cool off a little, it'll fade back in again after 5-10 minutes. I noticed if I touch the PI tube, it comes back immediately. I'm assuming that drip of solder in there is the culprit when it gets warmed up.

First time I powered it up, I had a loud humming sound through the cab. I noticed the PI tube wasn't very warm, so I swapped it for a different one. That seemed to solve the problem. I think that was probably also that solder. After a while, I put the first PI tube back in just to see if it was a bad tube. It's not - that tube is working fine now, except for the fading problem noted above. I think a new socket will be necessary.

The tone is pretty good, but I need to work on it a bit. Sometimes I get a ringing sound on some of the higher notes. It is a very regular repeating sound, sort of like a delay. It only happens once in a while. If anyone has any suggestions - let me know!

I have a quad of the re-issue Mullards for power, and re-issue Mullard 12AX7's in V1 and V2. Balanced JJ in V3. There's very little clean tone in it. It's breaking up pretty early - maybe 3 or so if I remember right. The distortion is nice, though! That was with humbuckers, bridging the channels, and playing through the top jack on the first channel.

The touch sensitivity is pretty good, but the harmonics may be a little weak. I'll have to check that again. It may be because I had to turn up my noise suppression just a tad. It's a little noisier than my 1959HW, but not much. I'll poke around and see if I can figure that out. My location is pretty noisy, though.

Power tubes were around 470-480 on the full voltage setting, around 415 on low voltage. I biased it at 32 for both high and low voltage and will check it again in a day or two.

Build options include hooking the negative feedback to the 16ohm connection and a 250pf bright cap.

I updated my build journal. I think I captured it pretty well, as that is what I built from. I took quite a few pictures along the way, but together all of that is much too large to post.

I'll add the LAR/MAR MV and FX loop as soon as I fix these issues.

[neikeel]

Try it with no bright cap for a bit - you will get more clean zone on the volume, I would stay with 47k on 8ohm for nfb.

Play it. Get a few hours in once you have sorted the PI.

Stick it in a room with an attenuator and run a signal through it for 24-48hours best with a small speaker so you can 'keep an ear on it'.

ONC IT IS RUN IN THEN TWEAK

[Night Owl]

Wow! You are *really* right about that!

I've probably seen The Who once or twice too many, too close, so I wasn't sure how much difference I could tell after "breaking in" an amp. Since I last posted, I've probably only been able to get in 6 hours or so on that amp. But just in that time the difference is already big - and all good. I have more clean headroom, and the noise level has also decreased to acceptable levels. If there was a "smiley" for "mile wide, ****-eating grin" I'd use it now.

I'm waiting for my replacement PI socket to arrive, so I'll still swap that. It isn't causing problems any more, so maybe that tube is soldered securely in place by now...

I'll give it a few more days and then pop in the new socket, LAR/MAR MV, loop and check the bias again. Once that all settles in, I'll start trying some of my vintage pre-amp tubes in there. Since I seem to have plenty of gain on tap, I may even try something in there with a little less gain than a 12AX7 to increase my headroom a bit. Or maybe cool the bias a touch.

Thanks again for your suggestions!

[tone chaser]

Hi hight Owl I'm folloing your directionswith my 12xxxbuild. Is there anything that isn't correct on there. I also don't have any pics that you posted within your directions. Thanks hope to hear from you!!!

[Night Owl]

Yes, there were a few minor things incorrect in the last version I posted, and one connection I forgot to include to the standby switch. An updated version is attached. I didn't do Step 13, so skip that. Maybe next time.

My pictures are too big to post - my resolution was much too high on many of them. I don't appear to have any software that will lower the resolution for me, so I'm not quite sure what I'll do with them. I'll see if I can divide them up and upload them or something.

Good luck with your build! Mine went OK, but my soldering isn't anything to brag about. Now in my next build, I'll...

[Night Owl]

Here come some pictures...

[Night Owl]

... and the rest.

I got lazy at the end and didn't title the pictures and reference them in the document properly. Sorry, but I'm not much of a photographer...

I also forgot to update the step where you put the knobs on. That was almost the hardest part. I had to bore them out a bit with a drill bit, and they weren't very cooperative about it. But it all worked out with a bit of patience and a cool, refreshing beverage.

After my break-in period finishes up, I'll add the MV and loop, and look it all over again.

[tone chaser]

Thanks Night Owl for the step by step directions. I am not that good at this forum so with my build I spend so much time looking at other builds and all the directions I get lost in all the info, but the learning curve is huge. I spend 5 hours a day on the comp. finding info. and about 2 two 3 on the amp. This is my first build and it is so freaking fun, but very slow because I get stuck allot. Anyway thanks again. The pics are great and hopefully I can take it one wire at a time........