Express squeals when touching the OT

[Dries]

I have a TW Express clone that sounds pretty good, but I think it's very close to the point of oscillation. When you touch the OT or go close to a output tube, it starts oscillating with a loud squeal. Is this normal? Somethings I have the idea that there is a little oscillation fizz on top of the notes. There are EL34's in it and has hammond transformers.

[IMG768]http://i59.tinypic.com/eaia2r.jpg[/img]

Does anyone has suggestions?

Thanks,
Dries

[geetarpicker]

Not normal. Your lead dress is a bit different and not as short point to point as an original. Are you using single conductor signal wire? It allows for tight bends and a neater lead dress. You may have to experiment with things to stabilize the amp. There should be no buzz on top of the notes, and the amp should be stable with all the knobs dimed though you will rarely run it that way. Your build looks nice and I hope it doesn't take much work to get it fully dialed in.

[fishy]

Cool.....always interesting to see a different layout.
That also makes it more difficult to comment.

If it's only squealing under the specific circumstance, it's probable not a phase issue and the OT primaries are probably correct.

It does look like your negative feedback back to the imp selector goes under the power board. This might be worth tinkering with. This is generally kept away from power and crosses them at 90.

Maybe a couple of other pics so people can see around the pots, bright switch and associated wiring.

[geetarpicker]

I'd work with the wiring around the pots and bright switch. Since this layout shifts the board components down it may have it's own set of rules. That said, the way Ken wired the pots was with very short leads and some of the wires are tucked right down against the chassis as opposed to up in the air. You might take a look at these pics of an original Express or two>
http://ampgarage.com/forum/viewtopic.php?t=19

[Dries]

Thanks anyone for the help ! I recently rewired the pots because some leads were indeed a bit long, but as I look to the originals, it might even go shorter. The NFB wire is indeed going under the power supply board.

Here are some extra pictures;

[geetarpicker]

I not sure if it matters, however on an original the wires that run off the bass pot are tucked back against the rear of the front panel, then run down to the corner and kept agains the chassis on their way back to the board. If you look at a pic of an original it's hard to even see these wires they are tucked away.

You might also experiment with the placement of the OT primary wires. On my '89 original when the amp was fairly new it would sometime oscillate unless the primary wires were held down tightly against the chassis where they run under the power supply board. You might reroute yours right after the tube sockets so wires are right against the chassis. You temporarily might try some electrical tape, foam, or hot glue to hold the wires down. On my original I had to add a little piece of foam rubber to keep the wires down under the power supply board, but oddly over the years the amp isn't as finicky about it and the foam is now not needed. Maybe it was somewhat dependent on the gain levels of particular tubes.

[geetarpicker]

Duplicate post...

[Tillydog]

I have a TW Express clone ... When you touch the OT or go close to a output tube, it starts oscillating with a loud squeal.

Are you holding the guitar when you do this?

[geetarpicker]

Don't forget you need a ground plate of some sort covering the bottom opening of the chassis.

[Dries]

I have a TW Express clone ... When you touch the OT or go close to a output tube, it starts oscillating with a loud squeal.

Are you holding the guitar when you do this?

Yes I am. And there is indeed no grounding plate at this moment. Does it has effect?

Thanks! I will experiment with the primairy OT wires.

[Tillydog]

Are you holding the guitar when you do this?

Yes I am...

IMVHO, that's why you're getting feedback - I don't think it's a "fault" as such. Without a cover over the bottom of the chassis it'll be much worse.

[Dries]

Are you holding the guitar when you do this?

Yes I am...

IMVHO, that's why you're getting feedback - I don't think it's a "fault" as such. Without a cover over the bottom of the chassis it'll be much worse.

Okay that's something I can experiment with. Will try this afternoon! Thanks everyone!

[Dries]

I rewired the pots more like the original layout. It seems more stable now. But touching the OT still results in oscillation while holding the guitar. Only when the guitar volume pot is turned open though. But it sounds killer, it is just better than expected. It has a bit the same character of a BF fender circuit cranked, but with more gain.

[Littlewyan]

I have to say I've never touched the OT whilst holding the guitar but I have leant over mine to check the valves before with my guitar in my hand and noticed it would make a strange noise as it got close to the head.

[donzoid]

A bottom cover is much less important on ANY other amp- I highly recommend you get one as it makes a huge difference in mine to "faraday cage" the beast. For me it's a night n day difference.