I can't seem to add a pic, I'll try a link to my "spare parts" limiter.
http://images.lilypix.com/displayimage.php?pid=62743&fullsize=1

I built one of those, even bought a metric tape measure. It works well, but, (since I don't think in metric), I didn't make it wide enough. I currently have a DR build in it, and it just fits!
Another thing, don't paint it where the boards pivot. After being clamped for a short time, it's stuck!

Bear, I just paralled the speaker output. (It really sounds great through a 15" JBL)! It uses T03 output transistors, and decent heatsinks, and a knowledgable tech said there should be no problem with a 4 ohm load.
I plan on building a matching speaker cab, and have a Marshall "stack".

Bear, I picked up one of these a while back, and had to wedge toothpicks under the knobs to get it to play. Do to the age, I replaced all the pots, and the jacks, (it's already apart, why not). I also replaced the 30 yo filter  electrolytics, and added an aux output jack. The switch was also intermittant, so it too was replaced. I enclosed the rear to gain a little more bottom from the 10" speaker. All parts are availlable from CE, or AES.
What a great little amp!

Thanks for the info! I lost my K-20 to my wife's sister, but if it ever comes back home, I have a few more ideas, (reverb would be cool!)

Just had to post about the latest orphan to find my house. One of my brother in laws was cleaning out his rent house, and brought me a little Marshall Lead 12 that was in pieces. I replaced all the pots, input jacks, and PS caps. Cleaned the thing up, and made a full cover for the rear, (with an oval cutout). I put it together last night, and got to play it some, even with the stock speaker, it sounds pretty good. I added a speaker out jack, and also played it plugged into a single 15" JBL cabinet I have, seems louder that 12 watts!
It really can sound like a real Marshall!

I have, what I believe is the head version of your amp, an FM100H. I have had no problems since I bought it new, but I did change the reverb tank for a 4 series Accutronics, (4EB3C1B), which gave it more of the reverb sound I was wanting.

I may have lost mine! My sister in law was by over the weekend, and I showed it to her, she thought it was so "cute". I told her to take it home, and play with it for a while. My wife talked to her on sunday, and she couldn't say enough about the thing, maybe wondering if it was for sale.

Some of my customers have a shop that buys estates, and I tell them to find old amps for me to tinker with. A few weeks ago, one of the fellows came in with an "old" amp, and gave it to me. I guess it's probably 80's, Dean Markley, K20. It's really clean, (brown tolex), but would cut in and out while playing. I pulled the chassis, and cleaned the pots, now as good as new! Surprisingly good sound, with a little 8" speaker, and has decent built in OD. Now if I can get my grandkids more interested in guitar, I have an amp for one of them!

I probably will never need the high power position, but the procedure to set bias, is to use the high setting, with no input. (Of course I had to measure the voltage after Enzo said what it would be)!

Thank you Roly, I found some articles on class B operation.
I reset the bias this weekend, and had a chance to play it for a while. Granted, I'm not trying to fill a 1K seat house, but in my practice room, it sounds sweet. I A/B'd it with my "Twin" which has an EV SRO15, and feel that set clean, there's not much difference in the Sho-Bud's sound.
Like Enzo said, there is approx. 720V on the plates at idle with the power switch in the HI position.

Sorry, here is the schematic for this model.

I am no tech, but enjoy tinkering. I recently picked up an 81 Music Man 100RD amplifier that was dead. Replaced the caps/diodes in the HV, and LV power supplies, and retested it with a bulb limiter. Still had a problem, finally figured out that one of the 5KV diodes at the output tubes was shorted. Found out the function was to protect the OT, and eventually settled on some availlable 3KV diodes.
My question is, how does it generate 100W with only 2 6L6 tubes. The PI is a pair of transistors that are biased at 25mv. The center tap of the OT shows 715V, (in high power mode), but only 350V feeds the power tubes. It is loud!

I had a couple of old tube amps I worked on last year with the cap cans. They are availlable, but pricey. I bought a capacitance meter from AES, (about $45), so I could check the cans, and didn't replace either. It's also handy to determine values in the box of parts we all have.

I also have an MT-2 that I'm deciding what to do (mod or not). I have experience with a few other Boss pedals, a couple I installed the "Keeley" mods, and another was a ? that the guy I bought the pedal from included. On all three, the reduction of noise (hiss) at idle was really noticeable. Also on all three, the tone control was more useable after the mods, (not off or on), and I think I can get more of the sound I was searching for. In the case of the DS-1, and the BD-2, there is also another option for sound adjustment, with an added switch.
Not trying to suggest you need any mods, but I wondered why Boss didn't put a little better components in their boxes, just for the noise reduction alone.