I bought the 18W TMB kit a couple of years ago and, for reasons I won't bore you with, just got around to building it this past week. Nice kit, excellent instructions, and it fired right up the first time I powered it up. I was checking the amp's noise floor with nothing pugged into the inputs, and noticed that the TMB channel squeals pretty badly when the gain is dialed up and the volume is increased much past halfway. At this point, I haven't started troubleshooting the problem and have just been poking around here in the forum for ideas on what to check. The amp is built pretty much exactly as the layout diagram and instructions indicated. Some (I think) relevant facts:
- Used the shielded cable for all 4 inputs, and the gain pot wiper connection to V2. The 68K input resistors are mounted right up against the pins on V1, as is the 470K grid stopper on V2. Shields are all grounded at the front end and floating back at the tube end. The input cables are routed above/around the board, and the gain pot cable runs under the board to V2. I will check continuity on all of the ground connections to the preamp buss up to the ground connection, but I was very careful in making these connections and doubt that I will find anything amiss there.
- OT primaries are braided back to the output tube sockets; brown to V4 and blue to V5. The secondaries are braided back as close to the impedance rotary as I could get them.
I can post photos here in the forum as needed. There are a bunch that I took during the build posted at
http://duhvoodooman.com/miscimages/musical/18W/ but are not the kind of detailed close-ups that would be most useful for troubleshooting.
Questions based upon some of what I read here in the forum:
- I saw a couple of references to grounding the "common" wire of the OT secondary. This is not shown on the layout diagram, so I didn't do it. Should I? Is this just a matter of running a connection from one of the output jacks back to the power ground?
- While I tied off the 5K conections of the OT primary, I left them inside the chassis, tucked under the braided secondary as shown in THIS PHOTO. Should this be changed?
- Is it OK for the shielded cable from the gain pot to V2 to be run under the board?
- I saw a reference made to also running shielded cable for the connection from the board to the wiper of the TMB volume pot. Instructions didn't show this. Is it a good idea?
Any assistance appreciated....