Trainwreck Preamp Pedal..built it.Sounds good. Has one mega problem...need help!

[zambo]

Ok so I built the trainwreck express preamp into a pedal and it sounds pretty good. I tried it at 200 volts and it sounded good. I tried it at 9 volts and it STILL sounded good....weird i know, But every time I roll the volume knob of my guitar the scratching noise is crazy loud...anyone know what would cause that? I dont think the amps did that....any help would be cool! I would post a schematic but...all you have to do is build the first 4 preamp stages of the express...pretty easy. I actualy put two volumes on either side of a 3pdt as a lead rythm switch and then gave it a master volume as well. I will try to post a vid tomorow if anyone is interested.

[merlinb]

Under 9V conditions you will get DC grid current flowing in your guitar's volume pot, which causes the scratching. Just add an input coupling capacitor- 100nF say.

[GibsonGM]

....and be aware that the cap CAN be all that will prevent the supply voltage from going right to your hand under worst conditions....like, at 50V....so, I would get a cap rated at the full voltage (greater than 250V) for those 'in between' conditions.  Probably overkill, but it's worth thinking about.

I often have wondered why most tube amps don't use an input cap, when if there was a significant failure you possibly could have the full HT on the grid (not likely with a preamp, but then, not 100% UNlikely, either). 

[zambo]

yeah, i tried the cap in the initial build along with a series resister of 33k but i got no sound so i took them out as they were the variation from schematic and it worked. I just re installed a .047 cap and no more scratchy...thanks guys. I would probably do well to read Merlins book..i am guessing this is the same merlin?

[merlinb]

I often have wondered why most tube amps don't use an input cap, when if there was a significant failure you possibly could have the full HT on the grid (not likely with a preamp, but then, not 100% UNlikely, either). 

The chances of an anode-grid short are exceedingly small. I have never heard of a single case. Even if it did happen, you will be isolated by about 168k of anode and grid-stopper resistance, rendering the shock painful but harmless.

[Brymus]

Yeah post a vid .
I would love to hear a TW running at 9 volts.
As well as 200V,I bet it sounds great.

[deadastronaut]

me too!..

[zambo]

ok here is a vid if i got it to upload right .... finaly....sorry about the delay. and the quality of sound etc.  http://www.youtube.com/watch?v=T-sNT_xjJ9E   this one is running on 45 volts. ( sounded good but 45 sounded better. 9 didnt have enough gain for me. Great for mid gain blue type of sound though.

[phector2004]

Sounds great!

What schematic are you using?

[zambo]

thanks phector! its the one at ampgarage and its under trainwreck express. I wouldnt feel cool posting it but you can go over there and look..I kept it almost the exact same except i used a 3pdt stomp switch for the dual gain knob for cleaner and dirty switching, ran it at 45 volts instead of real plate voltages and I also put a .047uf input cap. It blocks most of the dc but i think i need a bigger one cause my volume knob is still just a little scratchy. How do you determine the right size cap for dc blocking but not changing tone to much? anyone?

[markeebee]

I like that!

I also like the idea of running it at, oh, 12v from the same power supply as a chip amp. Less gain is ok for me.

So, am I misunderstanding, did you run it at different voltages without changing the component values?  And did the pre play nicely with the power amp without any tinkering?

Also, how much did you have to pay Owen Wilson to be in the video?

[DougH]

I often have wondered why most tube amps don't use an input cap, when if there was a significant failure you possibly could have the full HT on the grid (not likely with a preamp, but then, not 100% UNlikely, either). 

I have an amp that failed this way. The cathode ground connection on the first stage came loose and the grid floated up to around 100-200V (whatever the plate voltage was). The input jack ground was isolated from the chassis too. So if there had not been an isolation cap on the input, the jack would not have shorted the voltage to chassis ground and that voltage would have been on the input cable and guitar. I have had problems with scratchy guitar pots with some amps so I put input caps on them to fix that. I'll probably start adding those on all amps now.

[DougH]

ok here is a vid if i got it to upload right .... finaly....sorry about the delay. and the quality of sound etc.  http://www.youtube.com/watch?v=T-sNT_xjJ9E   this one is running on 45 volts. ( sounded good but 45 sounded better. 9 didnt have enough gain for me. Great for mid gain blue type of sound though.

Sounds real good!

The trainwreck pre in and of itself doesn't develop a lot of gain and clipping. The "trainwreck sound" is due to how the pre drives the power stage. I'm guessing the reason you are getting a good "gainy" sound out of the pre is because you are running it at a reduced voltage. It really sounds nice! 

[zambo]

Hey Thanks guys...I didnt change any componenet values to run it at low voltages and I suspect I could run all of this off of the same onespot 9volt adaptor.. It is hitting the little power amp so hard that its causing some power amp distortion...just not tube distortion. I noticed when I backed off the master that the gain goes down. So it has retained some of the trainwreck essence that way. Today I am going to run it straight into my hotrod deluxe power amp and then i will try the deville etc. I will also start the whole tube switching for tone process. I know 12au7 are lower gain but i really like the tone of them. They work better at lower volts too... Thanks for the help and the compliments. I will update as the prototype continues...G

[zambo]

ps...do you think I could take a epi valve jr and tear out the guts, salvage the iron and power tube and chassis etc. and make a dirty little monster out of it this way? I hear its a SE el84 amp with the express preamp...Thoughts on this? Hard to pass up for 129 bucks....and you would have the smokenest valve jr. anywhere on earth...

[defaced]

I often have wondered why most tube amps don't use an input cap, when if there was a significant failure you possibly could have the full HT on the grid (not likely with a preamp, but then, not 100% UNlikely, either). 

I have an amp that failed this way. The cathode ground connection on the first stage came loose and the grid floated up to around 100-200V (whatever the plate voltage was). The input jack ground was isolated from the chassis too. So if there had not been an isolation cap on the input, the jack would not have shorted the voltage to chassis ground and that voltage would have been on the input cable and guitar. I have had problems with scratchy guitar pots with some amps so I put input caps on them to fix that. I'll probably start adding those on all amps now.

Dumb question.  Any reason not to put the cap inside the guitar so that you know your guitar is always safe if you're using an amp you don't own? 

[DougH]

No reason not to do that at all. But understand that won't protect against high voltage on your input cable in the event of this kind of failure.

[DougH]

ps...do you think I could take a epi valve jr and tear out the guts, salvage the iron and power tube and chassis etc. and make a dirty little monster out of it this way? I hear its a SE el84 amp with the express preamp...Thoughts on this? Hard to pass up for 129 bucks....and you would have the smokenest valve jr. anywhere on earth...

The only thing I can think of is make sure you have enough current capacity in the power transformer heater windings to add another 12ax7 tube.

Re. your SS power amp distortion- I think people make too much of that as a "bad thing" IMO. I plugged a keyboard in to test one of the power amps of our new "dumpster dive sound system" and when I cranked up the keyboard it overdrove the power amp. With the organ patches it sounded pretty cool, like Jon Lord to me. If you turned it up too high it could get nasty but a little bit sounded great. I think your setup with that 'wreck pre and your SS power amp sounds just fine.

[Brymus]

ps...do you think I could take a epi valve jr and tear out the guts, salvage the iron and power tube and chassis etc. and make a dirty little monster out of it this way? I hear its a SE el84 amp with the express preamp...Thoughts on this? Hard to pass up for 129 bucks....and you would have the smokenest valve jr. anywhere on earth...

Yes
I have an EVJ with 2 x 12ax7 in the pre (3 gain stages and CF) and 2 x EL84 in parallel for the PA.
Wth a Hammond 125ESE for the OT it kicks butt.

Lots of people have bought the Valve Jr head and a Hammond 125E OT and built an 18 watt Marshall LiteIIb.
There is just enough current in the PT to support the 4 tubes heaters.
Here is the PB album for that build,I did it in steps to try a variety of mods including diode clipping,NFB,ect.
You can see the progress on the eyelet board pics.
Schematic in album. I dont recall if I took pics with the second PA tube in,but it will run two EL 84s or 6V6s or one of each blended together.
http://s419.photobucket.com/albums/pp276/Bry928/High%20Gain%20EVJ/

PS I like the way your new pre amp sounds too.

I have also tried a TW pre amp in the EVJ platform,but you really need the phase inverter and PP PA to get the TW sound.

[zambo]

@Brymus. That is the most monster EVJ I have ever seen. Very cool! Very super cool! I might have to build one.

I plugged the pramp into my fender hot rod deluxe power amp in and cranked it up really loud. I have a somewhat mushy feeling down below and a little tear in my eye as I write this....It was really nice. I really like the gain on this at super loud volume. And it was really really loud. Very juicey and deliciouse.....just thought i would throw that out there....

ok really though...is there a problem going straight in through the PA in? It sounds WAY better! I just dont want to kill my amps doing it. And what happens if I hit bypass on accident and its now just my guitar signal straight in? We ll my amp be a full blast like my little solid state ones do? I dont mean to sound dumb but i really am quite new to all this over the last year...thanks for all the help and if you can answer these questions I would really apreciate it!