Squeal and Bass Boom from Trainwreck Express

[apache]

Hello.

I recently completed my first Express build. Following the recommended start-up procedure, I measured all the voltages and found them to generally be satisfactory (though the power tube screen voltages were about 10V too high). However, the amp lets out a high pitched and deafening squeal when the presence knob is set anywhere between 0 and 2, and increasing presence to around 8 or 9 also causes a deafening low pitched boom. All of this seems to be independent of the volume control.

Apart from the squeal and boom on those particular presence settings, my guitar sounds quite nice through the amp. There is quite a bit of hiss, but it's not incredibly annoying and I consider it a secondary problem to the deafening squeal.

I've tried moving around the wire connecting the presence control to the impedance selector, but it didn't seem to have any effect on the squeal.

The tubes I have in the amp are very good NOS and I did some rolling to find the quietest and least microphonic ones. One interesting thing I noticed was that using a 12AT7 as the PI made the unseemly sounds disappear. I'm not sure what to make of that.

Any advice?

[M Fowler]

Reverse the primary OT wires to pin 3 of EL34's.

Try different PI and preamp tubes and see if there are any noticeable changes with noise issues. Will always have high gain hiss with this circuit.

[apache]

Thanks a ton, M Fowler.

I tried swapping the OT primaries and the squeal is now totally gone. The noise is also noticeably lower.

What a great forum!

[M Fowler]

Great now just get a bunch of different tubes and spend a day tube rolling to find out what tubes work best in the three preamp/PI positions.

EDIT I see you did this already never mind.

[Colossal]

Something else in your post that stands out is that you said that your screen voltages are about 10V too high. Are you plate voltages 10V high as well relative to say the Kelly 90 voltages, or, are your screens 10V higher than the plates?

[apache]

It's crazy how picky this amp is when it comes to tubes! My mazda silver plates become microphonic and noisy in the trainwreck. Does anyone know a quantitative way to measure noise from an amp? I saw someone mentioning using a multimeter to measure output but I wasn't sure exactly what procedure to use. I don't think my ear has good enough memory to pick the best tubes.

About the power tube voltages, I measured them again and got 415V on the plates and 403/404V on the screens. When I measured the screens yesterday I got 408. Should I be worried about having the screen voltage ~7V higher than expected? I have 6V6s in the amp right now (I've got a pair of EL34s somewhere, but can't find them for the life of me).

[M Fowler]

I wouldn't be worried about your voltages, having the screen voltage lower than the plate voltage is what you want. The suggested voltages verses your amp's actual voltage is okay since none of us get the same voltages.

I don't have any luck with NOS or old used tubes for the power amp I use mostly JJ EL34L or sometimes get GT EL34M.

[apache]

What problems exactly did you run into with NOS power tubes?

[M Fowler]

What problems exactly did you run into with NOS power tubes?

Finding good ones, i'm not going to pay high prices for Express recommended power tube types but if I run across some by chance locally or some used ones I try them. The NOS are suppose to be built better then new production but I haven't invested enough cash to find the good ones.

I prefer to work with current production tubes in my price range, the deep pocket owners can buy the expensive NOS tubes.

[apache]

NOS tubes seem to be quite hit and miss. My current production tung-sols and mullard 12ax7s are quieter than most of my NOS tubes!

[Mark]

Regarding fluctuating voltages, how constant is your mains voltage?

This will determine your DC voltages on a given day unless you are using an auto transformer.

[apache]

I'm using a variac to keep the voltage as close to 220V (the transformer primary voltage) as I can.

Following the logic that hiss that increases with the volume control must be caused by circuitry before the volume control, I tried substituting some resistors in that area with 1W and 2W ones (including replacing the 1M input jack resistor with a 3W wirewound). It didn't seem to affect the hiss by any noticeable amount, although I must admit that I always have a hard time remembering how much hiss there is before each change. As you can probably see in the images, I have the grid wires on V1 shielded. I don't know why I'm getting all this hiss... perhaps I just haven't landed on the right V1 tube?

People say that trainwrecks naturally have hiss because they're high gain amps, but the hiss on my amp sounds stronger than what I would deem acceptable.

[Mark]

So is there any difference in hiss between the guitar being plugged into the amp (the guitars volume set to zero) and no guitar being plugged in?

I would hope there is very little difference. The other thing is, the input jack, is it earthing the input to the amp when nothing is plugged in?

(I have a Switchcraft jack in my Champ 5F1 clone that makes a horrible noise when I pull the instrument cable out of the amp. I should replace the jack, but I usually only have to give it a clean event once and a while.)

What is the hiss like when the volume control is on zero. I wouldn't expect to hear much hiss at all when the amp is in this state.

Is the hiss you are hearing occurring at full volume?

I doubt you could use the amp at full volume, it tried using a 12 watt version at full volume and the humbuckers shook in it mount from feedback.

[apache]

There isn't any difference in hiss with the guitar plugged in. The jack is also working correctly.

I spent another hour rolling tubes and finally found a nice and quiet combination: Tesla 12ax7s in V1 and V2, with an American Amperex 7025 in V3. Putting the amp on its aluminium bottom cover also helped to reduce hiss. The level of hiss right now sounds pretty reasonable, especially when you compare it to how much volume you're getting out of the amp.

I feel a bit stupid saying this, but I can't for the life of me work out how to attach the bottom cover to the chassis. This is the bottom cover http://au.element14.com/productimages/f ... 559-40.jpg

The chassis just has four holes in it for the screws. Is there a special type of bolt for this or something?

Now all I need to do is build an attenuator so that I can actually take advantage of the distortion...

[Mark]

I forgot the cover does make quite a difference.

As far attaching the bottom panel goes, there are probably several options. I'd be inclined to use a long bolt going through the chassis allowing to be attached to a cab as well.

I'm sure others will chime in with other options. I'm why you used Element 14 when Evatco make a very good chassis and a cover.

http://www.evatco.com.au/product-list/c ... s-1444-32/