M-125 Build - Frank111

I thought I'd post some progress on my build of the M-125 mono block amps.

Note: I'm going to build the driver boards after I complete the build of the amp chassis.

I started this weekend. First I removed the bells on the power and audio transformers and painted them with Duplicolor heat paint in what looks like a satin black color. I decided to not paint the 'iron'. I sort of like that look, and with the fresh paint of the bells, new hardware and the white spacers, it all seems to work well on the stainless chassis.

I read through the instructions, and broke them into sub groups where I could work on both chassis in smaller 'bites'. Since I have a habit of reviewing things several times just to make sure that I did the step correctly, I think I'll be ok doing both chassis at the same time. I'm not in a rush, and am going very slow on this. The quality control is at a very high level!

I have all of the chassis parts mounted, and am at the point in the manual where the title says 'WIRING'. I'll get started there probably this coming weekend. One thing I noticed is that there is a lot of extra wire coming from the power transformer. What I'm thinking of doing is using the extra filament wire to make the connection from the lower tube sockets to the upper tube sockets (V1 to V3; V2 to V4). It may be a small thing, but it seems to make sense to use this wire and keep things the same. Any comments on this would be appreciated, as I don't see any harm in doing that. (Filament connections only).

The jumper wire on the multi cap - I'm just going to use some 18 gauge bus wire to make that connection rather than the loop shown in the plan.

Regarding the wires that jumper between V1 to V3, and V2 to V4 - would it be beneficial to make those connections with 18 gauge bus wire covered with sleeving rather than use the 20 gauge wire? Any harm or benefit to doing this? I'd like to hear what you think. (Non-filament connections).

I'd like comments on some of the questions I'm going to be bringing up as I go through the build.
Now is the time to ask! Thanks in advance!

Frank

Frank, when I built my ST-70 last year I used the cut off remains of the transformer filament leads to make the heater connections between the output tube sockets.  I then used regular hookup wire to make the PC board heater connections.  I also used the cutoffs from the output transformer leads, too, where possible to make the triode-ultralinear switch connections.  The only one I had to do with 'regular' wire was the solid blue, that remnant was a bit too short.





The connections to the board heaters are barely visible in the image below; the left side is the thin twisted green and the right side is the thin twisted brown.



As for the other interconnects between the output tubes, I see no harm in using sleeved bus wire to make them.  It might turn out more neatly if you dress it carefully.

I'll be getting to the chassis assembly stage on my 125s soon - powdercoating the transformer end bells are almost finished.

-Pat

Thank you Pat. This is very helpful.  I'll be doing the filaments using that excess transformer filament wire. Thanks for showing me the triode switch wiring too. I didn't think of that. I'll see if I have enough excess audio transformer wire to use there.

I think that using 18 gauge bus wire and sleeving for the V1, V3 and V2, V4 parallel connections
would come out nicer because the extra thickness of the wire will hold it's shape better. Also provide more surface area for an electronic advantage, albeit a very small one. If there would be a negative to that, I'd like to know about it before I wire it this way.

Frank.

Using the bus wire for the interconnects comes with no downside that I can think of other than being a bit more work since you'll be sleeving it rather than simply stripping standard wire. I really don't see any electrical benefit since the currents involved are so small. Done carefully, I think it will look very good.

-Pat

FYI ... a good source for solid hookup wire is:

http://www.elexp.com/ProductListing.aspx?CatId=d168a4e3-3de0-4715-a75b-59e9db8175d1

Their 20 and 22 gauge six color kits should give you everything you need for most any project.

Update - I have both chassis wired, and am now building the boards. The instructions for these amps were very clear. It made the build process virtually painless. I took my time and did a lot of rechecking as I went from step to step. And worked on it only as long as I was in the right mood to do it. So, it's getting close. Another weekend and I should be able to perform initial
testing on them.

On another note, I took advantage of the March sale and have an SP-14 pre-amp on order. Looks like a nice match for these amps.

Frank.

3/30/15 Update:  Completed all wiring of the M-125 amps. Performed a preliminary test by using a very small test speaker and two 6550 tubes. I ran an old Craftsmen FM tuner into the amps. Both worked without any issues. I ran them for about half an hour each. So far, so good. I have eight Kt-120 tubes due to arrive this week. I also have an SP-14 pre-amp kit to build next. Once that gets done, I will then hook this all up to a pair of Klipsch Quartet speakers and run a CD player into it. Then I can really check it all out.

I have a question regarding the way the bottom panel attaches to the sides of the amp chassis then gets compressed. I'd like to install some type of spacing so that the gap does not compress, but remains even along the top of the chassis. Has anyone inserted anything as a spacer in there? I'm trying to come up with something. Would appreciate any ideas.

Frank D.

No spacers, just don't torque screws down so hard.

Maybe add a few small drops of silicone caulk either side of the screws and let those dry. Vibration damping, doncha know ...

As Bob suggests, I put flat washers in that space on my ST-70.  It can be a bit tricky to get them positioned properly, but they work perfectly to prevent the bending of the side panel of the chassis bottom.

If memory serves, I placed the washer in the slot, then poked the screw in through the hole and pressed the bottom edge of the washer against the inside of the chassis.  It was then a simple matter to, while applying gentle pressure to the screw, press the washer down.  As soon as the hole gets in line with the screw, the screw passes through it and traps the washer.  You can then start the screw into the threads and install the second washer on the other screw.  Once all the washers are in place, tighten the screws.  It's much easier to do than to explain.

-Pat

About all I could come up with were washers too. And like Pat said, the challenge is getting them to hold while you install the screw. I'll see what size washers I have and maybe try to glue one to each hole on the bottom panel. They have to be close enough in size to do the job of leaving the gap right but not too big as to inhibit the clearance between the panel and the chassis.

I also located some nice feet for the bottom from Antique Electronic Supply that will provide a substantial support for the amps. I need to get some # 8-32 x 3/4 screws, nuts and washers to mount them. They will lift the amps up about 3/4 inch or so.

KT-120's should arrive on Wednesday. I went with Weber's. They add to the look of the amp as well as seem to be a perfect match for the current requirements of it. Good suggestion from Bob on this.

Frank D.