Trainwreck Express Build

[Littlewyan]

Well if you look at V1 the black heater lead is wrapped tightly around the socket, so I'm going to move it over the top of the socket.

[Colossal]

I would vote for elevating the heaters, but that means finding a little space for a few more components.

I second the vote for a humdinger/balance pot as well as heater elevation. I don't think that's the issue but heater elevation is always a bonus and not hard to do.

Something I noticed in your build photos is that you bundled your negative feedback wire with the secondary impedance taps from the OT. I would consider separating that wire and running it away from any other leads. Look at the TW Express layout for cues on this.

Glad you are making progress taming the beast.

[geetarpicker]

I agree on working with your feedback wire and OT wires in general. My original '89 (when it was new) sometimes had an issue with oscillation and the fix was to put a small piece of foam under the power supply board to specifically hold the loosely twisted OT primary wires down tight against the chassis. Interestingly these days the foam is long gone and the wires aren't quite held down but the amp is actually now stable without the fix. Anyway, your routing of your OT wires is quite different than an original however you might experiment with OT wire placement a bit, and you might try to keep the primary and secondary wires apart if possible.

That said your buzz problem is probably more a ground and filament circuit issue.

[Littlewyan]

Ok moved the black wire and I think it helped a bit but still got buzz.

I just checked the amp with my scope and found that 50Hz is present on V1b's Anode (Volume Pot Side) but not on V1a's Anode. So surely a Hum Balance Pot wouldn't help? Although the heater elevation might. I can't see it being the wiring as it doesn't go away if I push down on the heater wires so it must be happening inside the valve. Then again I find it very weird that its only happening on one side, surely it'd affect both sides?

[M Fowler]

Is that .0022uf coupling cap bad?

[Littlewyan]

I'll check it but I can't see it being that as its affected by the volume control. Just twisted some of the heater wires a bit more, might even try a shielded wire again between the volume pot and V1b. I'm trying the easier stuff before I go into the artificial heater tap etc.

Checked the .002uF Cap and its fine, no DC on the grid of V2. Do you think having a grid stopper on the volume pot wire would make it worse? I know it could be more prone to AM signals inside the Amp.

[Littlewyan]

Right I'll confess that when I put the amp in the head cabinet it was quiet and I didn't bother with a shield. Just tried the shield again and it reduced the buzz by a noticeable amount. So perhaps it is something in the wiring in this house? Anyway I'm not sure how to mount this foil in the cab, I was looking at perhaps buying some self adhesive aluminum sheets?

I also might take the amp round my mate's apartment next week to see if its any quieter there. Like I said earlier I'm sure my Marshall has a slight buzz when I use it in the house, which I'm certain I've never heard when at band practice.

[geetarpicker]

Any light dimmers or fluorescent lights in the house? Both don't get along with high gain amps. I once lived in a townhouse where at dinner time my amps would pick up the neighbor's light dimmers. It appears the noise is radiated in the air and not just on the AC line.

[Littlewyan]

We have two light dimmers in the house, one is very close to my room but its switched off and the other is upstairs and probably on full. I got the same noise when moving my amp to the lounge but thats just next door to my room. I was originally getting a LOT of noise when running my amp on my desk as I have a TV, PC, Xbox, Wireless Router and set of Cordless Phones on it! Probably the worst possible place for a high gain amp.

http://www.ebay.co.uk/itm/121213676743? ... 1423.l2649

Looking to order two sheets of this stuff for shielding the amp. Currently got some of my mum's best Kitchen Foil underneath it in the Cab but obviously I'd rather it be stuck down so I don't have to mess about when taking the amp out.

[redshark]

If you have a wireless router next to an express or Liverpool you will definitely gonna get some noise!!

[Littlewyan]

Yer I'm keeping the Pre Amp Valve Covers on. Aluminum Sheets are on order so they will give me a good shield in the case. Should be all good by the end of the week

I've also ordered some 1 ohm resistors to stick on the Cathodes of the Output Valves. Shorted the Anode to the heater pin trying to measure the voltage last night, yikes! Luckily my B+ fuse blew and nothing got damaged. Replaced the fuse, started up with the current limiter to check it and it was fine, just had to blow off some soot. So I'm not checking the Bias again until I get those resistors in!!

I'm also replacing the Volume Pots on my Guitar with Audio Taper as they are the usual Linear Taper and unless I really carefully raise the volume from 0 I can't get any clean from the amp! The only other mods I may look at are messing with the .002uF Coupling Cap off V2 but to be honest as it is the amp sounds pretty awesome! Once the Cathode resistors are in I'm going to put EL34s in just to see what they sound like. Thanks for all your help again guys, this amp kicks my Marshall's ass!

Also if anyones interested to know what Transformers I used it was a Hammond 372JX and a Danbury VT1548 6.6K.

[Littlewyan]

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