First Build Loud Buzz

[WilliamDouglas]

Hello all,

I just finished my first amp build using the Trainwreck express schematics and I am having a terrible noise problem. With the volume all the way down I get a nasty loud 120hz buzz. With a dummy load connected and the O'Scope connected to the output and the heater supply, I see Spikes that line up with the 60hz heater sine wave; It looks like Diode switching. The spikes are in the range of 20vpp. I have tried to isolate by removing one tube at a time. When I remove the first stage, the problem is greatly reduced. When I remove the the second stage or the PI, the problem goes away. I did put the scope on the grid of the power tube and I do see the same trace as the output so I dont think it's related to the OT or power tubes. I have double checked every connection but everything looks good. At this point I am about to rewire the whole thing to rule out faulty construction. Before I do that though, I wanted to ask if anyone here has a suggestion or troubleshooting tip they could offer. Thanks

[Jana]

It sounds like something power supply related.

Are your filter caps good?

Are all your ground connections for filter caps good?

Do you have a ground connection someplace that is floating?

[WilliamDouglas]

Hi Jana,

"Are all your ground connections for filter caps good? "

All Caps are Brand new. and I have them grounded to a solid bus bar which is also grounded to the chassis.

"Do you have a ground connection someplace that is floating?"

This is what I was thinking but I have checked and checked . This is whats making me think to start over.

[Jana]

Don't tear it down and start over just yet.

With your scope and multimeter, check every node of the supply rail. Check the DC voltage as well as the AC voltage. What are you seeing?

[KellyBass]

You say it's when your volume control is turned ALL the way down? Does it change at all when you turn it up just a hair? Pay very special attention to every detail regarding V1 and the volume control. And, like Jana said, post your PS numbers

[Tillydog]

All Caps are Brand new. and I have them grounded to a solid bus bar which is also grounded to the chassis.
.

Note that in the conventional Express layout the filter caps are divided into 3 groups, each with a separate ground connection. I am of the opinion that this is important to avoid feeding noise into the preamp (YMMV).

Are you using the chasis for the ground connections (Ken Fisher style)? If not, the filter caps shouldn't be connected to the chassis.

Have you included any heater elevation?

Decent, high resolution photos always help

Andy

[WilliamDouglas]

So here are my PS measurements.

B+1 428VDC
B+2 412VDC
B+3 323VDC
B+4 304VDC
B+5 287VDC

-All Grounds are are chassis grounded.

-I am not using heater elevation.

I included some images.

The Noise trace shows the scope Channel 1 connected to heater supply and Ch2 connected to the output.

The B+1 Trace shows shows the power supply B+1 supply ripple.

The "Caps" image just shows my filter caps (a little messy after some troubleshooting and resistor changes)

[WilliamDouglas]

You say it's when your volume control is turned ALL the way down? Does it change at all when you turn it up just a hair? Pay very special attention to every detail regarding V1 and the volume control. And, like Jana said, post your PS numbers

Yes it does change. In fact if I turn it up just a bit I start to hear funny little noises that I believe is coming from one of the tubes. If I put signal through the amp, I see the noise riding on the signal.

[Tillydog]

-All Grounds are are chassis grounded.

-I am not using heater elevation.

Is the sole ground connection from your filter cap busbar the tinned wire that goes through the hole in the board with the two red wires in top right in that photo?

If so, I think that's a big part of your buzz problem - definitely something to resolve by separating the grounds from different sections.

Merlin Blencowe (the Valve Wizard) has an excellent article which explains it. It's a pdf file linked from his web site:

http://www.freewebs.com/valvewizard2/grounding.html

In particular look at page 4 onwards in the context of my concerns and your smoothing capacitor connections.

This circuit is reasonably high gain, and demands good grounding practices. Using a formal scheme like star or bus grounding (also covered in Merlin's article) is one way to achieve this. Getting a chassis grounding scheme to work well is much more of a dark art. Quite why the traditional TW grounding scheme works, I don't know, but deviate from it at your peril.

There's no point tearing it down and starting again unless and until you understand the principles in Merlin's document - when you do, it may be quite simple to modify what you've got rather than start again. (Either that, or follow a proven layout ! - I'm assuming that you're at least aware of the "standard" layout?)

Unfortunately details matter a lot in this circuit - it's a brave choice for a first build - congratulations on getting this far.

Andy

[WilliamDouglas]

Is the sole ground connection from your filter cap busbar the tinned wire that goes through the hole in the board with the two red wires in top right in that photo?

Yes, The ground wire goes down to to another bussbar on my main turret board which is chassis grounded.

Merlin Blencowe (the Valve Wizard) has an excellent article which explains it. It's a pdf file linked from his web site:

Yeah he is a great technical writer, I just got his book on power supplies. I also have his book on preamps.

Getting a chassis grounding scheme to work well is much more of a dark art. Quite why the traditional TW grounding scheme works, I don't know, but deviate from it at your peril.

lol..yes I seem to be learning this the hard way.

There's no point tearing it down and starting again unless and until you understand the principles in Merlin's document - when you do, it may be quite simple to modify what you've got rather than start again. (Either that, or follow a proven layout ! - I'm assuming that you're at least aware of the "standard" layout?)

Yea, when I wrote the origional post I was very frusturated. Now I am looking at this as a great learning oppurtinity.

Unfortunately details matter a lot in this circuit - it's a brave choice for a first build - congratulations on getting this far.

So it all gets easier after this right..

Andy[/quote]

[Tillydog]

Yea, when I wrote the origional post I was very frusturated. Now I am looking at this as a great learning oppurtinity.

[...]

So it all gets easier after this right..

In all honesty: yes

You're on a steep learning curve because:

a) The Express is a high gain amp which makes construction practices such lead dress, etc. important, even on a 'known good' layout

b) You've chosen to plough your own furrow regarding layout.

Get through this and you will have learned *a lot*

The 4-4-0 amplifier at AX84 (roughly: an express preamp with a triode power section); uses a bus grounding scheme which might give you some ideas (not so much for the power supply which uses a multi section capacitor, but for everything else).

http://ax84.com/4-4-0/AX84_4-4-0_Studio_100216.pdf

(layout on last page)

Happy to help if you want suggestions on how to proceed, but need to know a bit more about what you've already got.

Andy

[WilliamDouglas]

Is the sole ground connection from your filter cap busbar the tinned wire that goes through the hole in the board with the two red wires in top right in that photo?

If so, I think that's a big part of your buzz problem - definitely something to resolve by separating the grounds from different sections.

So it turns out that I had violated all the rules regarding good grounding procedures. I re did my grounding scheme after reading the grounding article from Merlin Blencowe and the problem went away.


Thanks for getting me on the right path!

[passfan]

For the record, I'm not a big fan of the grounding scheme's used by the AX84 project. Everyone I've attempted was noisy. I shy away from "buss" style schemes in favor of multiple chassis points. The OT is the only one I return to the "main" point and my issues have disappeared.

[Tillydog]

I re did my grounding scheme after reading the grounding article from Merlin Blencowe and the problem went away.

Excellent, well done!

It's all easy now

Andy

[rwijaya]

I have this same issue with my express also, i didn't build the amp, anyone knows any tech in LA that can do this sort of fix?