Rex Everything wrote:I have my SDS and Poseidon boards finished and am ready to start on the amps. My basic knowledge and skill level is as a parts replacer. I'm not so great with schematics but can solder well and follow instructions(for the most part)
I've noticed that my lack of planning has left me without replacements for the two 50uF@75v caps and the diodes to replace the selenium rectifier. Any and all suggestions for the parts will be appreciated.
Here is what I'll be working with. It looks like someone has already been inside the one to replace part of the can cap.
Use 100uF, 100v rated caps for the bias supply. Sprague Atoms are nice, but expensive. I used Mouser
140-TG101M2A-1322-RC in mine and they work fine. You'll need two for each amp (four total).
The diodes should be something like a
UF4007. You could use a UF5408, but you really don't need the amp rating. There's almost no current drawn from the bias supply. You need one diode for each amp (two total).
You will want a bunch of multi-lug terminal strips. If you bought four of the 3-lug strips (Mouser
158-1003), you'd probably do just fine. You'll use one 3-lug strip to replace the original one in the Mk3 (where the
ceramic disc cap is soldered - did you buy them already?) and another 3-lug strip where the selenium rectifier is located. This one is used as a place to solder the UF4007 diode. Look at the photos of my Mk3 (
here and
here) to see how they fit in. Just don't fly the end of the Dale like I did - tie it to something solid, use a 4-lug strip if you have to. It was supposed to get tied to the grounding lug on the nearby tube socket, but the replacement sockets I used didn't have grounding lugs.
You'll also need two 4.7K and two 18K ohm resistors (one of each for each amp). Some people report the 4.7K resistor doesn't work for them, and they end up needing to swap it out for something smaller (2K or so). You might save yourself some trouble and just start out with 2K. Use whatever is economical - I used 1/2 watt Dale parts
71-RN65D-F-18.2K and
71-RN65D-F-4.75K.
Finally, you should replace the 10K one-turn pot. If yours still has the original pot, I wouldn't trust the carbon track any more. I bought a cheap Alpha pot (
31VF401-F) but I wish I had spent the extra money and bought a nice PEC hot-molded carbon pot. These things are expensive, but they are really nice pots. I think this is the right one, but check dimensions carefully as I have not actually used this part:
RV4L103C-ND.
Oh, almost forgot. You should add a 100K "safety" resistor between the wiper and the top side of the bias pot (where it meets the 4.7K part). Geek detailed the changes
here. Don't look for it in my photos - I haven't put it in mine yet, but I will real soon. I promise. :)
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