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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:

1. 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?
2. 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?
3. Is it OK for the shielded cable from the gain pot to V2 to be run under the board?
4. 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....

1. No don't do this.
2. I'd tuck them under the turret board myself. Make sure they are heat-shrinked and tied off.
3. Yep, many builds done that way.
4. Shielded cable never hurts. The captenience of the cable can take away some high end though.

Try chop-sticking the amp first and moving around some of your wires (especially shielded runs).
I would also try some different tubes, you may have a microphonic or high gain tube that is causing the oscillation.
If all that fails I would shorten your shielded wires, they are overly long.

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Thanks for the quick response, Chris! Yeah, I'm planning to "chopstick" the wiring this evening and see if my lead dress is the issue. I saw a post from a guy who solved his squeal issue by making sure the wires connecting the pots to the board were as far from the TMB input jacks as possible.

Ya it will probably be a lead dress thing that solves the issue.

Try to space the wires out while keeping them as short as you can. Pay attention to shielded wires, signal wires and wires going to the tube's grid's.


Here is a pic from an 18W I built. It may help with an idea of routing of wires and such.

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Improving the lead dress around the TMB input jacks and shortening the shielded cables to V1 helped a bit--the squeal started a bit higher up on the volume pot--but that wasn't the main problem. Turned out to be the V1 tube itself. I had a JJ ECC803S (long-plate 12AX7) in there. When I pulled it out and replaced it with a regular 12AX7 (JJ ECC83S) the squeal disappeared. The 803S runs fine in V2, but for whatever reason, it doesn't get along with the V1 position. Go figure....

The V1 is the most sensitive position for tubes. The input signal is small and the gain is relatively large. Ideally your best most stable tube goes in V1.

The long plate structure (such as your JJ) are more prone to microphonincs and noise so I can see why it did not work in the V1 position.

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