18 watt trainwreck

[Richie]

Geek use to have some posted of his amp. I may have some of a 36 watter.


i found some old info or clips.

and i think the amp in the youtube clip geek built that for his friend,which is an 18/trex

http://mhuss.com/Clips/18wSteel.mp3
http://mhuss.com/Clips/18wZZ.mp3
http://mhuss.com/Clips/18wTom.mp3

http://www.youtube.com/watch?v=9KF0KjjS ... re=related

Mark Also had a schem for a switcher for the feedback circuit,should be on his site.
I don't know if this layout one is accurate or without mistakes,but should give you an idea. Mark should have some answers to his drawings.

[ampbldr2]

Thanks Richie I redrew the board again and plan to use this layout.

[edited] I deleted this board layout and will repost a new one.

[ampbldr2]

I finished rebuilding my 18w Trex. I am still getting out the bugs. I was not after good looks. I wanted a better sounding amp and so far I am really liking the trex channel but it does have a bit of hum that I may or may not be able to reduce but the overall sound quality and gain in the trex channel is to my liking.

On the stock channel I am not getting the same amount of volume as the trex channel. It is greatly reduced and not sure why just yet. I already tried different tubes to see if that would fix it and even changed back the tone stack to original 18w tmb setup but still get a reduced volume level on this channel. I can crank the volume pot to full 10 and not get the same amount as i do in the trex channel. Lets just say if i crank the trex to 10 the neighbors could hear and crank the stock and its good basement level practice level. I have never seen anything like this before. When I get some more time I will test out with a scope.

[JamesHealey]

A standard express with 6v6?

[Richie]

I have never seen anything like this before. When I get some more time I will test out with a scope.

Sounds like the feedback circuit is not switching, or may be reversed.
it is important on this amp. The wreck channel needs the feedback circuit to help calm it down.
The marshall channel is just the opposite,it does not use the feedback circuit,and will make the channel very clean instead of the more raw type 18 watt.
Also like in most fender amps that have the mix resistors for the channels
if the values are off much there will be a level of volume difference. So check the value of those resistors. The wreck channel will be a bit more lively.. Hope this gives you a few things to check.

Just curious, i see the tape on the chassis covering a socket hole?
probably would have been better to have the tube mounted there.You could cut down on the longer leads going to that tube.

As for the hum, if your useing current made tubes, try using some different preamp tubes for V1. Alot of tubes now days have hum in various levels.

[ampbldr2]

I have never seen anything like this before. When I get some more time I will test out with a scope.

Sounds like the feedback circuit is not switching, or may be reversed.
it is important on this amp. The wreck channel needs the feedback circuit to help calm it down.
The marshall channel is just the opposite,it does not use the feedback circuit,and will make the channel very clean instead of the more raw type 18 watt.
Also like in most fender amps that have the mix resistors for the channels
if the values are off much there will be a level of volume difference. So check the value of those resistors. The wreck channel will be a bit more lively.. Hope this gives you a few things to check.

Just curious, i see the tape on the chassis covering a socket hole?
probably would have been better to have the tube mounted there.You could cut down on the longer leads going to that tube.

As for the hum, if your useing current made tubes, try using some different preamp tubes for V1. Alot of tubes now days have hum in various levels.

I made a mistake and used a 1inch hole punch instead of a 7/8 orbital bit on V3 and thats why I could not use that hole. I may see if a Belton Jack will work for V3. I measured all the resistors when installing and nothing was out of the normal 5% tolerance. I did have a issue last night where the stock channel had a fizzing sound and when i banged the chassis it went away. Maybe a cold solder somewhere.

I will check the feedback circuit again.

[ampbldr2]

[edited] Here is the voltage chart.

[ampbldr2]

Richie,
What "mix resistors" are you talking about? Are you refering to the 220K into the PI?

[soma_hero]

I built a TRex awhile back, but have since turned it into a Rocket/Liverpool (switchable extra stage with Liverpool values). I had some blocking issues, and a whole host of layout issues with my Trex since I crammed it into a weird small organ chassis. I don't remember how much gain the Marshall channel had, but keep in mind there is one gain stage with a tone stack loss going into the PI, while the Trainwreck side has three gain stages with a tone stack going into the PI. There will be way more volume available on the trainwreck side, and there won't be near as much gain on the Marshall side.

Your voltages look ok too

[ampbldr2]

[deleted]

[geekmacdaddy]

It is not part of the design. This was the first amp I ever built, from the same layout, and had no such problems.

[ampbldr2]

I finally figured it out. I had mistaken a 82k instead of 820ohm in the PI of v3. Now this thing really cranks. Thanks to all for your support. I still have a few more things to try out to try and bring down the hum. I star grounded everything to one lug on the PT and probably should have separated some of the grounding. I think the ground right from AC in should drop right to the chassis and the buss ground goes to the pt lug. Does anyone care to share there grounding methods to reduce hum or am I wrong thinking this would help?

[Ian444]

Good find on that resistor value. Do you have hum in the clean channel, or both? The TW is said to be a bit noisy due to the high gain. Do the heaters have a ground reference?
If you can handle 15 pages, this shows some good grounding methods.
http://valvewizard2.webs.com/Grounding.pdf

[ampbldr2]

Do you have hum in the clean channel, or both? The TW is said to be a bit noisy due to the high gain. Do the heaters have a ground reference?

Thanks for the doc. The Hum is on both. If no input is used the hum is there and loud enough to be annoying. I can try shortening the v1 input resistors and get them at an entry point 90 degree angle and see if i can probe around with a scope to find what is going on.

The heaters ground reference is connected.

[M Fowler]

Take the amp to another part of your building or to a friends and see if the noise is still there. Believe me I had severe noise and it wasn't my building layout it was line noise.

Also if the noise persists you should look at the input grounding in reference to the V1 and preamp grounding to one point near the input separated from the other ground points.

Mark