Trainwreck Express Build

Littlewyan · Post by **Littlewyan** » Thu Mar 06, 2014 10:25 pm

Right I'm going to add to the confusion here. When I took a test lead and grounded the grid connected to the volume pot the noise DIDN'T stop. However, when I grounded the grid on V2 it did. But if I pull V1 the noise stops as well............o my head hurts!

Littlewyan · Post by **Littlewyan** » Thu Mar 06, 2014 10:37 pm

I lifted the negative side of the cathode bypass cap, put my meter between the cap and ground and I got a reading of 1.5V. Tried a new cap and got the same reading. Am I missing something?

Also did I mention that if I run my pliers down the back of the volume pot then I get a crackling noise through the amp? Same if I touch the ground bus wire near the volume pot.

Tillydog · Post by **Tillydog** » Thu Mar 06, 2014 11:21 pm

Littlewyan wrote:... if I run my pliers down the back of the volume pot then I get a crackling noise through the amp? Same if I touch the ground bus wire near the volume pot.

I don't know if that means much... Metal on metal always seems to generate some noise... It could mean that the ground wire isn't making good contact somewhere.

What did you do with the PSU cap grounds?

Littlewyan · Post by **Littlewyan** » Thu Mar 06, 2014 11:24 pm

Here are a few audio clips of the issue. To record these I sat the amp on top of the cabinet on top of foil.

Volume Up and Down - This is me turning the volume up to 10 and then down again. You'll hear that the amp gets louder at first but then quieter on 10.

All Dimed Except Volume on 5 - All knobs are dimed except for the volume which is on 5. Couldn't go above 5 as it would feedback (amp not in head cabinet yet)

All knobs 0 - Everything turned down, you can't hear it very well here but there is a buzz and sometimes a pulsing noise.

Littlewyan · Post by **Littlewyan** » Thu Mar 06, 2014 11:32 pm

With regards to the PS Grounds here are some pics. The filter caps are grounded in the same place. PS center taps are grounded by the power switches. I realise this may have been a bad idea though.

geetarpicker · Post by **geetarpicker** » Fri Mar 07, 2014 2:10 am

Usually with these amps IF the amp gets louder as turned up but at the end of the dial actually then gets quieter, it's a sign that the amp is actually going into oscillation when fully turned up. The oscillation can be so high you can't actually hear it, but it truly messes up the tone. Not good. Once the amp starts running away with an ultrasonic oscillation it sucks all the power out of the lower (audible) registers making the hiss actually less apparent. That said, when it is oscillating it is probably fully clipping the amp and IF you play your guitar it won't clean up even if you back the guitar down to almost off.

Anyway, it's just a sign to "keep at it" to find 100% stability because then your clean tones will really shine. An unstable amp can sound somewhat usable for dirty tones, but it will never clean up well and sound very ratty.

Another way to verify a run away situation is to dim the room lights and watch the power tubes as you ramp up the amp knobs, with the guitar unplugged. If the amp runs away you may not be able to hear it, but you will see the tube grids light up strongly, and any blue glow will get stronger. That said, if the amp is stable the tubes shouldn't show any difference in color or brightness when the knobs are cranked.

I just listened to your clips. The hum is a problem. Yes the hiss will get louder as you turn up, but the hum will barely change from having the knobs fully off. I suspect ground loop problems, or maybe even your layout and transformer placement is going to be a no go. Still I'm hoping for the best, and I'd keep at it and see what progress you can make...

Littlewyan · Post by **Littlewyan** » Fri Mar 07, 2014 7:29 am

Well tonight I was going to try connecting the PT CT directly to the filter caps ground to see if that makes any difference. I read that Paul Ruby had an issue with that once. Other than that I'll just have to look at all my ground points and go through it logically as it definitely seems like a ground loop causing the issue.

Edit: Perhaps another thing to try would be to connect the negative of the 40x20x20x20 cap to the back of the pots. Did I mention btw that I can change the noise with my hand over the amp? Which leads me to believe that current is flowing through the chassis causing thr interference. Just need to find where....

I've also heard of a few members having to connect the volume pot ground to a local cathode ground on the board so i'll give that a go later as that sounds like a winner.

Tillydog · Post by **Tillydog** » Fri Mar 07, 2014 1:37 pm

What are the wires with crimps on at the 'top' of the power supply board [1]? Is one of them the power transformer centre tap?

I think you have two separate problems:

1) The 'going quiet' is likely oscillation as Glenn has suggested - likely to be a lead dress issue around the preamp.

2) The buzz:-

I think the power transformer centre tap connection is a big part of your buzz issue (simply because I can't see it anywhere near the first smoothing cap) - The best place to put it is *directly* onto the -Ve terminal of the first two smoothing caps (I guess it is a 40/40 can cap??). Do not connect it to the chassis, other than the connection it will have via the ground connection to the 40/20/20/20 cap. IMO this will minimise the infuence that it has on the noise.

(The standard Express has all the 40uf caps grounded separately to the 20uf ones. You can't exactly recreate that with your 40/20/20/20 can cap.)

I would expect a substantial improvement by making that change.

[1] Just a note on the PTFE tag boards - the material 'creeps' under load, so anything clamped to it (like those crimps) will eventually slacken off and make a bad connection - bad practice.

Littlewyan · Post by **Littlewyan** » Fri Mar 07, 2014 1:59 pm

Guilty, my heater ct and ht ct are grounded on the ps board. Ill undo the ht ct and use a test lead to connect it to the 50x50 cap negative terminal to see what happens. I agree there are two seperate issues here.

I'll also move the heater ct once these issues are sorted.

Tillydog · Post by **Tillydog** » Fri Mar 07, 2014 2:56 pm

Littlewyan wrote:Ill undo the ht ct and use a test lead to connect it to the 50x50 cap negative terminal to see what happens.

FWIW, I would suggest a short, direct, heavy gauge wire for this connection, as it has high current pulses on it which could transmit noise into the preamp - my image of 'test lead' is of something long, thin and indirect!

(Oh, I would also solder it, rather than crimp, too - I know that a properly made crimp connection should be as good, but I don't trust 'em for audio!)

Littlewyan · Post by **Littlewyan** » Fri Mar 07, 2014 5:35 pm

Right an update. I move the HT CT to the cap can negative terminal but it didn't make a difference. Moved the volume ground, no difference (used a test lead). Now I then moved the bass wires behind the buss wire n held them down against the chassis n the amp got quieter. Whether this is because my hand was over them I don't know. The solid core wire I've used is quite thin so I'm going to replace those leads with thicker stranded wire and glue them down if need be.

So basically I'm thinking its the thinness of the solid wire i'm using, as in Ken's amps its solid wire but its very thick. I'm also using the same wire for the cap can grounding so will replace that as well. I'm also going to turn the volume control round to make the bright cap leads shorter, sorry Glen I now see what you mean about the caps affecting the volume control. I think I just spent so long yday on the amp that I started to go mad!

geetarpicker · Post by **geetarpicker** » Fri Mar 07, 2014 7:35 pm

The wire Ken used was 20awg 1000v rated solid core w/PVC insulation.

Littlewyan · Post by **Littlewyan** » Fri Mar 07, 2014 7:39 pm

Ah mines 0.6mm which is 22/23awg I believe. The only stuff I have thats thicker is the stranded wire I used for the heaters.

tictac · Post by **tictac** » Fri Mar 07, 2014 11:00 pm

Observation about SAFETY!

Those four un-insulated wires going from the power supply board to the preamp board stick out like a sore thumb... What an invitation for trouble if you dropped a tool of accidentally brushed up against those while working inside your amp.

It would take very little effort and expense to make those insulated wires; not a bad price to save your life...

Also; non-metalic substances are very poor heat sinks. Your 1K/25W resistor is more like a 1K/5W mounted on your power supply board like that. Not the type in an actual TrainWreck amp. If the 25W rating is a requirement for this amp you'll be replacing that resistor when it frys... Mount it with the slightest dab of heatsink compound on the metal chassis and you'll have no worries....

My .02

TT

Tillydog · Post by **Tillydog** » Fri Mar 07, 2014 11:29 pm

Littlewyan wrote:Right an update. I move the HT CT to the cap can negative terminal but it didn't make a difference.

Hmm.... That's a shame!

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