First TW Express Build Issues

[bal704]

As requested:

[IMG574]http://i755.photobucket.com/albums/xx198/bal704/PICT2123.jpg[/img]

[IMG574]http://i755.photobucket.com/albums/xx198/bal704/PICT2120.jpg[/img]

[IMG574]http://i755.photobucket.com/albums/xx198/bal704/PICT2126.jpg[/img]

[LOUDthud]

Is that the output transformer right next to the first preamp tube? I'm sorry to say that if that's the case, you will never get it to work until you move the OT to the other end of the chassis. You can try shielding the OT wires and erecting sheet metal shields around the preamp, but with limited resources, your chances are not good, maybe one in ten.

[bal704]

Sigh. Thanks for the advice. Perhaps I'll repurpose this build for something else then.

[M Fowler]

Did you ever try swapping the OT primary wires for out of phase squeel?

Also, after that if the presence is out of control try disconnecting and ground the tail resistor to test the amp's stability without NFB.

The OT should be ok there just make sure those wires go to the back of the chassis.

Mark

[ampdoc1]

Before you quit on this amp, I hope you try some of the previous suggestions. I built my first (somewhat modified) Express in 2005. It had all the good things that I had heard when one I got to play with one for a couple of weeks in 1995.

But, I wanted the amp to respond a little differently. I subsequently used different 12A*7s in the preamp to lower the noise, and make the transition between clean and OD smoother. I now use:

V1 - 12AU7
V2- 12AT7
V3//V4 - 12AX7

This sequence may not work for you, as I us both halves of V2, but trying different combination's is the key.


The first amp I ever built circa 1990 (a 50W version of an American Vox), worked like a charm, but had a LOT of hum. When probing with a PLASTIC HANDLE screwdriver, I noticed that when I inserted it at a particular spot, the noise disappeared. I cut and bolted down a thin metal sheet in that position, and ALL the hum was gone. Took about 30 minutes from problem to fixed.

Don't give up because everything is not perfect right off the bat. Ken's basic design is the Holy Grail of amplifiers, bar none.

adoc1

[bal704]

I've built probably 8 amps, and I did try swapping the OT wires, but it's not THAT squeal.

I'll try disconnecting and grounding the tail as you suggested.

This isn't my typical 'noise' problem really. The squeals and odd behavior with the volume and presence knobs tell me I've probably got something reversed/loose etc. I've dealt with the classic hum/hiss/buzz issues, but I've never had this problem before.

I've considered lower gain tubes for V1 and V2, but until I get past the current issue, I won't learn anything.

Did you ever try swapping the OT primary wires for out of phase squeel?

Also, after that if the presence is out of control try disconnecting and ground the tail resistor to test the amp's stability without NFB.

The OT should be ok there just make sure those wires go to the back of the chassis.

Mark

[rock_mumbles]

In my lower power Wreck amp the Presence pot lead has to be ran from the board straight toward the pot and be held against the chassis or it has a high frequency squeal.

I'd pull the excess OT leads out of the inside of the chassis and have them sitting on the top of the chassis bunched up and tied to the OT ... and like Mark said run the leads to the output jacks in the chassis bend

[bal704]

I lifted the tail and grounded it, which removes the presence and NFB. No improvement.

[Firestorm]

You might be able to get away with the OT on that side, but lead routing is critical. It looks like have the output plate leads on a terminal strip right to V1. One of those leads will be in phase with your input signal, so that's a recipe for oscillation. You've got to get the OT leads far away from everything on the preamp side.

[bal704]

I moved the 3 OT connections as far away from V1 as possible. That seems to have gotten rid of the oscillations when I turn the volume knob. However, I'm still getting squealing when I turn the volume knob.

I also reconnected the tail resistor, and I still get squeal. I don't get any squeal when I try the bright switch however, so things are improving. Twiddling the TMB knobs doesn't seem to have much affect on the squeal.

[Firestorm]

Leads on the tone stack are crazy sensitive. Also, as has been observed, leads on the presence pot.

[bal704]

Success!

I appear to have a working Express!

I moved my tone stack wires around to minimize squeal, re-routed the shield wire from the input jack to V1 and changed a lead on the presence knob to give me more length to move around.

The result was squeal free, and really loud. I re-installed my LarMar, and it sounds great. Lots of gain on tap. I adjust the volume and LarMar to get a crunchy sound at a divorce-free volume. Cleans up nice, and the coil tap on the Mira sounds great.

Thanks for the help!

[Firestorm]

There you go! Somebody should write a lead dress book. On Expresses and other high gain amps, I used to disconnect the mid pot ground and let the preamp run free. You will never get it to behave, but you can tweak it to minimize the unwanted oscillations. Then, when you reground the stack, the amp will be well behaved. It's useful to look at the tone leads on one of Ken's amps: mid and bass are together and go down to the chassis corner (these could actually be shielded with no ill effect - I think). But the treble lead is exquisitely short and hangs next to the ground buss. I shield this too, but wonder if I killed some magic by doing so.

[rock_mumbles]

Sweet!!