Express noise problem

[toskrlec]

Hi, I finaly build my Express, amp work, and sound what I expect, but have some issues.

1. Noise and/or hiss is to high even on idle, and increase with volume. It is not 60-cycle hum, but something higher freq., I was try decribed methods, but with no results. When pull out preamp and PI tubes, amp is dead quiet, but when put in tubes, one by one, noise increases whit every tube added.

2. I have signal on input whit volume on zero and hear guitar in speaker, how it is possible, is it normal for Express ? or something is wrong

3. Presence pot scratch when rotate (I have not install VVR yet)

Perhaps these problems are linked ?

I will send few guts.

[Allynmey]

White noise hiss is normal on an express.

High output preamps tend to give you a little volume (very little) with the volume off.

There is a fix to take the voltage off the presence knob...lookit up on the site using the search feature.

[PCollen]

...(snip)...

There is a fix to take the voltage off the presence knob...lookit up on the site using the search feature.

http://www.drtube.com/schematics/marshall/1987pwrm.gif

See lower left, 0.1uF/22K pot in parallel with 4.7K resistor..that does it.

[WhopperPlate]

Check out the old posts; Expresses are very susceptible to oscillations and electronic interference due to lead dress and or micro phonic / low quality tubes. They generally bring out the worst in low quality components, especially the tubes. First and foremost make sure you're using reliable glass.

[Gibsonman63]

Some thoughts about your volume control issue. I was not able to solder to the back of my pots. Your volume control basically bleeds your signal to ground. You may want to test your grounds.

[ampdoc1]

I love the Express amps and have built 3 or 4 with modifications. But the design has too much hiss. So, I have always put in lower gain tubes in the front end.

I just finished one a couple of weeks using the Two Rock Ruby chassis and transformers "kit" I bought 5 years ago. After some trials with an added gain stage I changed the circuit back to stock and I'm using:


V1 - 5751
V2 - 12AU7
V3 - 7025

The amp will still feedback on just about any note on the neck, and there is virtually no hiss or hum.

ampdoc

[toskrlec]

Thanks for advice. I know that high gain amps like TW have something increased hum, and hiss,expectet that but in my case it is way to much. Now i suspect on pot grounding.
Because I didnt solder bus direct on pot, but use separate bus wire for grounding, and conect it in one point on chassis. Maybe that is problematically?
Images are not up to date,now I use schielded cable from second stage to vol pot, but ground bus is betwen pots, and turet board.

Thanks

[img655]http://img705.imageshack.us/img705/5540/preamp122.jpg[/img]

[img673]http://img832.imageshack.us/img832/2519/preamp123.jpg[/img]

[ampdoc1]

I can't tell where your pots "ground buss" is connected to the power supply ground. On my amp I also have the pots on a "buss" that connects directly to the power supply board. The chassis ground is made thru the input jack.

I'd check to see how good your ground connection between the PS and pot buss are. You should have maybe .3 ohms or less between the two.

And, again, when you get the hum problem fixed, try some different preamp tube combinations. You can get rid of 90% of the hiss this way.

a'doc

[tjauernig]

I'm always confused when the forums talk about "hiss" with TW designs. I used to build an amp I called the "Luxury Liner" (still do actually) based on a TW Express. I never had problems with noise/hiss regardless of tubes or lead dress. As far as lead dress goes, there is so little going on in these things, that lead dress is ALMOST a non-issue. I tend to think solder joints are a bigger issue.

All amps, especially tube amps have some amount of noise, usually referred to as "hiss". Does anybody have a noise measurement as to the amount of hiss. I've heard a lot of Boogies and Marshalls that have way more "hiss" issues than any TW style amps I've heard. Even a lot of Fenders for that matter.

Am I missing something here? I've been reading about the noise for years on forums. I've sold about 50 Luxury Liners, and not one single noise complaint. These type of amps just are not that hard.

[Structo]

I think hiss is inherent in any high gain amp.
You are amplifying the crap out of the input so the hiss is to be expected.
Not sure if you can bleed any of it off with fizz caps but it may be worth a try to take a 47pf ceramic and go from one of the later plate resistors to ground with it.
I did this on an amp that had a bad fizz in the top end and it fixed it.

Ampdoc1's solution is probably the only realistic practice unless you can find a very special V1 12ax7 that doesn't hiss very much.
And if it doesn't, it is probably a very weak tube.

[toskrlec]

It is my first express, I was build some other projects in past, low gain like 18 watt Marshall, Deluxe reverb...,and never have any issue with noise, but on my Express it is to much noise, and interference.
Yesterday I was hear radio signal from amp ,so something is definitely wrong there.

I am planning some actions:

1. bus wire solder direct on pot, and lead dress like original TW, on preamp section
2. 0.01uF ceramic capacitor betwen chassis, and input jack schield
3. Lift OT sencondary from chassis, and take referenc for Presence
4. Check and resolder, all related joints to grounding !
5. Maybe add 33K grid stoper

Is there any measurable method to check grounding, or everyithing is trial and error ? (oscilloscope, freq generator)

Thanks

[geetarpicker]

Sounds like it could simply be normal. Compared to a Deluxe Reverb or even an 18 watt Marshall a Trainwreck is a bit noisier for sure. You have build yourself a Ferrari now, and you can expect it be quite on the wild side! If you want to tame it down put a 12au7 in V1, and a 12ay7 in V2. That said I did some comparisons to vintage Marshalls in the studio a few years back. A fully dimed 4 input Marshall Plexi Superlead 100 (split cathode), or lead spec metal panel 4 input Marshall 50 has noticably more noise when compared side by side with a Trainwreck Express set at similar gain levels which is all knobs on the Express on about 1/2. I even found a JTM45 to be a bit noisier when dimed. Also if you take a lower gain amp and hit it with a clean boast I have yet to find a combo that can match the gain of an Express AND actually be quieter noise wise. If you dime an Express it wil be quite a hiss machine, but at that point it's crazy gain so you can't stop playing to hear the noise anyway!

Keep in mind these days folks are used to the noise levels of channel switchers. You play those at high gain but then click them to lower settings for clean. However with a wreck it's basically stuck in the high gain mode, however it should clean up very nicely. But, you will have more than Fender Deluxe levels of noise for sure.

As far as radio pickup I've had issues with that before with my Express/Liverpool but only with specific guitar cords. I've never picked up radio since I started using cables with Mogami wire. Hope that helps! GK

[steviea55]

I know I'm going to take some heat for this but here it goes, Proper lead dress is very important in this amp and cannot be overlooked. Ken chose to use cheaper Pots that could be soldered on the back so he could attach the ground buss directly to the pots. IMHO this is a requirment(very effective) for attenuation of this circuit.

[geetarpicker]

Very true! Also Ken ran things just about as short as possible darn near everywhere.

[toskrlec]

Maybe I will take amp to friend who has osciloscope, and try messure frequency and amplitude of noises, hum, hiss and find source of problem. It would be cool to have some oscilogram of good working Express as reference. Sorry I am engineer, admittedly mechanical but....

I'm sure I was made some mistake, noise level is too high, 30% of signal
is noise, it can not be tolerated. Thank You for Your suggestions, really appreciate them.