Express clone noise on high notes

[markr14850]

I'm hearing a noise in a used express clone that I recently picked up.

With the volume set loud but still clean, if I start picking a relatively high note softly and then harder and harder, eventually I pass some threshold and this noise starts. If I let the note ring, when the level drops below that threshold, the noise cuts out very quickly. I'm not sure how to best describe the noise - it sounds like a low frequency hum/buzz/gurgling superimposed on the note. It's like dimmer or motor noise being suddenly turned on and off.

This noise is most noticable on high notes, say high E string, 12th fret, but I can still hear some of it when I play notes an octave lower. Below that, I have a hard time detecting it.

It is not incredibly loud - but it makes the notes I play sound out of tune, and generally crappy. (That is to say, more crappy than the same notes through some other amplifier. I'm just a living room player.)

I was wondering if it could be 'cone cry' but it happens on a 1x12 and a 2x12 with different speaker types, and other amps through those same speakers don't make this noise.

I've swaped all the tubes, with no change. So, it's probably not a bad tube - unless I'm just really unlucky!

No combination of tone/presence control settings cause it to stop.

One other note... There seems to be a lessening of the noise (or maybe just raising the activation threshold) if I mismatch the impedence.

Any ideas what this might be, or what tests I should run? I don't have a scope, but this problem has been making me want to buy one.

Thanks in advance,
--Mark

[philmanatee]

Try isolating the amp from the speaker cabinet, if you haven't already. If it stops when the amp isn't on the cab it could be a cold solder joint. Phil

[LeftyStrat]

Another thing is ultra high frequency oscillations that may be caused by bad layout/ lead dress. Take the pictures of Francesca and print them out and make sure the lead dress is similar.

The wreck is a very touchy design. Get the dress close with the pics of Francesca, then with chopstick and the amp on and guitar plugged in, play a note and then use the chopstick to more around wires.

Also, if you can, post some detailed gut shots, someone here might see the problem right off the bat.

[markr14850]

Here are a few shots:

[markr14850]

and a couple more:

[mdroberts1243]

I would look at your blue grid wire on pin 2 of V1... I tried to keep this as short as possible and even resorted to a grid-stopper and tried out coax. When my trainwreck was unstable/edgy I found changes in this area can tame the amp quite a bit. (Coax is gone, but the grid stopper is still in place, although I'm not sure I need it any more).

I think the old rule of thumb is keep grid wires short, but plate/anode wires can be long.

[markr14850]

Thanks. I'll try replacing it. That particular wire is fairly microphonic.

Also, the 0.0022uF cap between V1 and V2 is pretty microphonic. Could microphonics make low buzzing noises? The chassis is several feet away from the speaker cabinet.

Finally, I've just noticed that there is no 1M resistor from input to ground. That's just for RF rejection, right?

--Mark

[mdroberts1243]

Thanks. I'll try replacing it. That particular wire is fairly microphonic.

Also, the 0.0022uF cap between V1 and V2 is pretty microphonic. Could microphonics make low buzzing noises? The chassis is several feet away from the speaker cabinet.

Finally, I've just noticed that there is no 1M resistor from input to ground. That's just for RF rejection, right?

--Mark

Everything seems to be microphonic in a TW build!
I did comment on another thread that I found the 150M caps quite microphonic... changed all mine to OD 6PS series for a smoother tone... and the bonus is the new caps aren't microphonic at all.

Hmmm.... I think you need that 1M resistor to reference the grid to ground and bias the triode properly!

[LeftyStrat]

Yes, add the 1M grid ref resister, you can solder it between signal and ground directly to the input jack.

Next, here are some things to try:

- Shorten the wire connected to v1 pin 2 and route it under the other wires. I believe in some wrecks this wire may be shielded.

- Route the wire to v1 pin 6 clockwise around the socket.

Again look at Francesca pics.

- The resistor you're thinking of for RF suppression is a grid stopper which would be soldered in series between the input and pin 7. The closer to the pin the better. Depending on your first name (Leo, Ken, Alexander), you can use 68k, 33k, or 22k. Ken would use 33k and only sometimes used it.

Don't make all these changes unless necessary. Do each one and then audition the change. You don't want to completely break the horse, just calm it down enough to hop on for a wild ride.

[mac0611]

the first thing you need to do is put a 1M grid leak to ground resistor at your input jack. you should notice a big difference.

[drhulsey]

...look at Francesca pics...

I made myself a big composite of the Francesca chassis guts to use as a reference for my build. I tried to route my wires exactly as I saw them, having heard how important lead dress was in this amp. Here it is if you wish to use it.

[jurgen]

looks like you might want to get the NFB wire away from the grid wires

[Fischerman]

The grid wires look to be shielded so I wouldn't expect that to be the problem. But maybe it is?

One thing that seems to be very sensitive (in just about any amp) is the treble wire from the board to the Treble control. Notice how it is separate from the Bass/Mid wires? The Bass and Mid wires are kept next to each other and run down close to the chassis. All 3 of those wires look like they are all parallel for a significant distance in op's clone.

[jurgen]

The grid wires look to be shielded so I wouldn't expect that to be the problem. But maybe it is?

One thing that seems to be very sensitive (in just about any amp) is the treble wire from the board to the Treble control. Notice how it is separate from the Bass/Mid wires? The Bass and Mid wires are kept next to each other and run down close to the chassis. All 3 of those wires look like they are all parallel for a significant distance in op's clone.

Can't really see from the picture if the shield is truly tied to anything.
Maybe I'm superstitous.

In my wreck clone the grid wire from the volume pot is practically a ribbon mic.

[Fischerman]

jurgen,
I'm just guessing because I can't really see it but if you look at the ground buss right behind where the grid wires connect to the board, there is what appears to be the ground connection for that shield.

I used shielded for the input wire, the wire from the volume to the second stage, and for my power tube grid wires (I used the 2-conductor+shield just like this amp did) and my amp isn't microphonic and it doesn't appear to be oscillating (nothing audible and the amp puts out full power)...and it's very quiet regarding noise/hum. I also used a 22k grid stopper on the input wire (although it wasn't microphonic/oscillating without it...there was just so much gain and brightness I felt it was needed). But it's never sounded that great so...whatever.