BramK said:
I've enjoyed following the builds here. Glad that things are finally coming together Potato Cakes.
I'm getting ready to start my build. I ordered most of the components way back when. I quickly perused my stock this evening and had a few questions come up.
1. 372X mounting holes don't fit the chassis cutout vertically. Two holes will line up, but not all 4. Are folks just mounting the top two screws?
2. C6 & C7 - Looks like I ordered some old PIO Gudeman caps, they're axial ones, not the square cans. Mine are rated 400V - will that be sufficient or is 600V needed here?
3. Resistors - I'm not concerned with visual originality here, but I want to do good on the character sound. I went almost entirely metal film with my resistor purchases last year. Which spots in this design would benefit sonically from the Carbon Comp distortion?
4. What is a recommended order to approach this build? Mount chassis back panel transformers/sockets/hardware first? Populate PCBs? Wiring?
Thanks!
Some suggested answers to your inquiries:
1 - see moltenwork's answer
2 - On the schematic, it shows a test voltage of 310V that goes from R44 on the PSU through both primary windings and connects to C6 and 7. Using modern components this might not be an issue, but whether you are using NOS or new parts it would be safe to use 630V rated ones.
3 - If you read page one of the 175b manual posted by Martin from Sowter's website, it says in the first paragraph that the unit was designed for low distortion to be used in high quality audio systems and in the second paragraph that it was designed with overrated components and precision 1% resistors. It seems that the whole series of these limiters (175x, 176) were designed for fidelity and not tone (distortion). I wouldn't get too hung up on where to install carbon comp for character. When you use this unit with fast attack and release settings you naturally start to get distortion/artifacts/character/tone or similar quality. Also, can you even buy 1% carbon comp resistors? I've never seen anything below 5%.
4 - As a general rule, I would start on the PSU side and work the left as you look at the chassis from the front. Here's my recommended order of operations:
1. Build PCBs (this can be done at any point up to actually installing them). I would strongly recommend before you install any components that you solder any and all rivets where they connect to their respective traces. Apply solder directly to the trace and then work it to the rivet. With your iron touching the side of the rivet, leave it and work your solder on the side of the rivet till you see it flow around it and adhere to it's trace. Obviously you don't want it to go inside of the rivet. Do this for every one, even if there will be no component placed there as some of the rivets are vias to make connections on the other side of the board.
2. Install Power transformer. Get a hand drill to do the twisted pairs per the build guide. Grip the leads being twisted at the transformer so you don't rip out the windings. Use the slowest setting, naturally.
3. Install the inlet AC cable, GZ34 socket, OB2 socket. Wire the 325V leads to the GZ34 socket. Any connections on the OB2 and GZ34 should be made with appropriate wire lengths to their destination per the wiring guide. You want this whole section prewired as you won't be able to get to them when the choke and PSU board is installed.
4. PSU board - I would recommend making your connections between components on the PSU board on the back using the appropriate turrets.
5. Install 6AL5 socket and C6,7 if you are using the designated mounting holes.
6. Install Input, Interstage, and output transformers. If any of these are original you need to prewire these as well.
7. Barrier strip. I put shrink tubing over each terminal where it went through to chassis to ensure no contact was being made there.
8. Input/Output jack assembly. Prewire this as well and double check to make sure the terminals for passing signal do indeed have continuity. Same for the ground
9. Install front panel. You could do this before the jacks if you wanted. One of the nuts is under the jack assembly which you may find difficult to tighten with the jacks in place. For me it didn't matter, so 8 and 9 are which ever you prefer.
10. Install PCBs. I would recommend using standoffs that are slightly varying in length to allow the appropriate leads from the output transformer to make connections to the ratio switch buy passing between the top and bottom PCBs. Otherwise you will have to extend them to reach with the front panel fully open.
11. Install front panel components.
12. Install tubes.
13. Calibrate/Test. Be sure to have several 6BC8 tubes as I've had 2 out of 7 test/perform poorly. A good gauge is to set the ratio to 12:1 and attack/release to 12 o'clock and run some known program material that hasn't been smashed to pieces to be earbud optimized. Bad tubes will compress (if they even pass sound) in a dramatic fashion on the transients with extreme swings in gain reduction, almost as if there is an intermittent audio connection. I'm sure someone here with a tube tester can give you objective measurements for what range to expect in a quality functioning 6C8B tube.
These are my recommendations and in no way any sort of official guide. Use at your discretion/risk. Let us know how you get on.
Thanks!
Paul