Trainwreck Express Build

[M Fowler]

Twist the secondary OT wires going to speaker jacks/impedance switch together go between octal tube and PI tube sockets. The OT CT will be on it's own to filter cap or standby switch and twist primary OT leads together over to the octal tube sockets. They won't be together at all.

[Littlewyan]

Does twisting them not have an affect on the sound though? As surely the Primaries would cancel themselves out due to them being out of phase?

Edit: Just looked inside my Marshall 1987xl and the primary and secondary leads are wrapped together and run inbetween the two output valves. I'll still do this but will twist to reduce interferance if any. Better safe than sorry. Then I might untwist them at a later date if I feel like experimenting.

Not ignoring your advice Mark but because of the layout of my chassis if I ran the OT secondary where you said then it'd be quite long. Compared my chassis to my Marshall 1987xl and it is different. Your method would of worked better on the 1987xl.

[donzoid]

That's a very nice looking build. Be thankful you have the extra real estate under there.

[Littlewyan]

Thank you

[Littlewyan]

Well guys I finally received the remaining parts yesterday for the amp and managed to get it going last night. I had little time to tinker though as it was quite late when I finished.

First off when I flick the standby on I get a squeal that gradually grows and I get mains hum. This is with all knobs turned to 0 so can only assume at this point that the output phase is incorrect for the nfb. I then ran the amp with an attenuator and it didnt squeal, so I plugged my guitar in and it sounded pretty distorted straight away on low volume so I'm guessing thats the nfb. The mains hum changes as I put my hand inside the amp so may be that a wire needs pushing down to the deck or i need a cover.

Other than that pretty chuffed it made a noise first time

[Littlewyan]

NFB fixed, the amp is now playable and sounds great! Installed a 2.7K Grid Stopper on V1b which I think made it a bit quieter. Only issue I have now is a possible ground loop. There is a buzzy raspy pulsing noise and it gets louder if I turn the bass and middle down. So thats why I assume its a ground loop. Its also still audible if I have all knobs turned to 0. I've got an oscilloscope so will see where the noise is coming in. I'll post gut shots later for you guys.

Edit: The noise disappears if I pull V1. Tried disconnecting the Input and the anode wire for V1A (Input) but the noise remains, so narrowed it down a bit.

[Colossal]

When you are idling the amp on the bench and observing hum and potential ground loop issues, are you covering the chassis with an aluminum shield plate? If you have any solid state high frequency power supplies in the area such as those found in compact fluorescent lamps, etc. make sure they are powered off. 'Scoping the amp would be good to ensure that you don't have oscillation. The amp is very sensitive to lead dress!

[Littlewyan]

I did try covering it with foil which helped a bit. On my scope I found 50hz and 100hz signals. I should also add that I get extra hiss when the volume is between 3 and 7, but above 7 it goes quiet

There are two noises coming through the speaker, both pulsing at different rates, I'm guessing this could be the difference between 50 and 100hz?

[Colossal]

I did try covering it with foil which helped a bit. On my scope I found 50hz and 100hz signals.

Ok. Those frequencies do indicate loop issues.

[Littlewyan]

Sorry just edited my last post.

[Littlewyan]

Here are some pics

[Littlewyan]

Now as you can see my grounds for my cathodes are a bit bundled together. Now I tried disconnecting the Input wire from the grid, the anode for that but still the noise remained. The noise goes away if I remove V1, so perhaps its the V1b cathode ground?

EDIT: Just found that if I touch the back of the volume pot with my pliers I can hear noise through my speaker, same if I touch the shield for V1B. Also found that after disconnecting the volume pot from the treble pot I still had the noise. So its obviously between the volume pot and V1B.

[Colossal]

I dig that Marshall knob on the bias adjust

Usually with the Express, you want to stay very close to the original lead dress layout, especially with the tone stack leads. Sorry for the 20 questions, but have you tried moving your tones tack leads around with a chopstick and observed the change in the background (if any)?

Also, this might be a dumb question, but it's hard to see in the photo, but is your input jack grounded?

[M Fowler]

The way the input jack is wired doesn't seem right to me?

[Littlewyan]

Haha yer I thought it was a nice little touch

Right, the noise definitely changes if I move the Volume to V1B Wire, if I move the anode wire for V1B and if I go near the volume pot with anything metallic. The input jack is grounded, there is a little solder lug near it that the input jack ground and the pots ground buss goes to.

M Fowler: I believe it is but will check it again. Its not causing my issue anyway as I did try disconnecting it. The noise seems to be around the volume pot and V1b. It also gets worse if I turn the treble up. Perhaps I should shorten all of the tone stack leads? Did notice that the wire for the treble pot is very sensitive and is picking up the same interference.