New Trainwreck Inspired 2 channel amp

[UR12]

Yeah I've checked it, waiting to build the plexi they mentioned. anyway, why scale the whole amp? any advantages?

I don't know if I would call it an advantage. This is just my take on it and I am sure that others would disagree. But hey that's what makes the world go round. No matter where you scale an amp you run into the problem of overdriving the scaled part of the amp. If you do just the power section then your preamp is running full blast and you need some way to attenuate the pre from going completely overboard with signal driving your power section. If you scale the PI and power section you get the same thing. So somewhere down the line you need to install a gain/MV/PPIMV etc etc. This involves making 2 adjustment and the extra knob. If you scale the whole amp your guitar volume control becomes that extra knob. The first preamp stage, say V1a has it's bias set by the cathode resistor. It's usually a 820 ohm - 2.7k resistor. That resistor self biases the tube with about 1 - 2 volts by making the cathode more positive than the grid. As you dial down the plate voltage on that tube, your bias becomes less. Looking at your guitar, Your pickups generate somewhere between say 250mv to maybe 1v, if you have some hot ones. So lets just pick 500mv or 1/2 a volt as our guitar's output with the vol control cranked. When the voltage on the grid of the 1st preamp tube drops below that 500mv that out guitar is puttnig out then you start to overdrive the amp. To compensate , just turn down your guitar a little.

Having said all of that , some peope like having the extra knob to fiddle with. The say it gives them extra settings that you don't get by scaling the whole amp. So I guess you need to find what works best for you and go with that.

Hey Dana. Wanna fit a 240V trannie and send it to me? I'll look after her real good...

But seriously. Has this amp been built to order? Do you think you'll sell it?

Would you be kind enough to post a BOM when you post the schemo and layout?

Love your work, thanks for sharing.

Mark Abbott would probably have something to say about what address I sent to in Aussey land. Hope this doesn't come a shock to you guys but I don't play guitar. I am a retired Drummer. But don't fret (Pun intended) I have a few guitarst friends and my brother who I loan these things out to, to get their feedback. So I guess to answer your question I would probably sell it after I have spent some time with the amp perfecting the design and wringing out every bit of available tone out of her. I do this as a hobby and for the enjoyment of creating and playing with new things. After I'm done with it , what use would I have for a guitar amp? I only sell about 2 - 3 amps a year. Last year I built 6 and I still own 4. This particular amp was built just to see if I could pull it off and for no other reason than to give me someting to play with.

I'll try and do a BOM but I'm not much into doing layouts. Once I get her tweeked in the next week or so I will post the final schematic. Maybe if we can find someone good at doing layouts I would be happy to work with them to get a layout made.

[bnwitt]

I'll try and do a BOM but I'm not much into doing layouts. Once I get her tweeked in the next week or so I will post the final schematic. Maybe if we can find someone good at doing layouts I would be happy to work with them to get a layout made.

Ummm, I'm pretty good at layouts

[UR12]

Well, there you have it. If your up for it, then it sounds good to me. What do you need from me?

[Beerman]

Mark Abbott would probably have something to say about what address I sent to in Aussey land.

I don't know him personally.... Sydney is about a thousand kilometres from Melbourne... but I think he could make his own. I however have never built a 'Wreck, and find the idea a little daunting.... I just keep hearing about how sensitive they are.

I'll get around to it soon. I think a single ended variation is my next build, 30 odd watts is more than I need, but 6v6's sounds like a great idea.

[UR12]

. Sydney is about a thousand kilometres from Melbourne...

Barry Marie, if I'm not mistaken, is from Melbourne. He has done some beautiful amps and might be willing to take on a project.

[Beerman]

Yes. You are quite right. And his Barwatt amps (Barry's Marwatt) are if not the talk of the town, at least very nice. I've played through one anyway...

[philmanatee]

Impressive as always Dana! I may have to give this one a try. Phil

[Fischerman]

Hope this doesn't come a shock to you guys but I don't play guitar. I am a retired Drummer.

That worked for at least one big guitar amp company founder...his name escapes me.

FWIW - I recently tried scaling the PI along with the power tubes in my power scaled Express (which actually had been modded to be a power scaled K60 when I did it...then I made it a power scaled Concorde afterwards). It certainly made the circuitry easier and uses less components since the MV is so much simpler. And when I dial in the 'best' tone it sounds pretty much the same either way. I'm still not sure which I like better...when scaling just the power tubes I got lucky with my (made up just for this amp) post-PI MV in that the MV knob and the Scale knob 'tracked' each other very well (i.e. put the Scaling control on 5, put the MV on 5...Scale on 3, MV on 3...etc.). That wasn't the case when I scaled the PI...but I didn't piddle with that much...I just wanted to hear it to see if there was any difference.

I will say this though...when you try both ways of scaling (I've never tried scaling the whole amp...yet) you really get to hear what the preamp is doing in isolation...then the preamp+PI in isolation...then the whole amp. And just like most non-MV Marshalls...most of the crunch is happening in the PI.

Also FWIW...so far I like the (tweeked) Concorde preamp the best. But I'm still not there yet...amp still won't sing like it's supposed to.

[UR12]

Hope this doesn't come a shock to you guys but I don't play guitar. I am a retired Drummer.

That worked for at least one big guitar amp company founder...his name escapes me.[/quote]

Ahhh you must mean Mr. Marshall. If I remember correctly Leo didn't play guitar either.

Also FWIW...so far I like the (tweeked) Concorde preamp the best. But I'm still not there yet...amp still won't sing like it's supposed to.

Are you saying you don't think it's singing because of the preamp or the Power Scaling?

[bnwitt]

Well, there you have it. If your up for it, then it sounds good to me. What do you need from me?

lots of pics, a hand sketch if possible and question answering when I am doing it.

[Jimy]

Ahhh you must mean Mr. Marshall. If I remember correctly Leo didn't play guitar either.

Jim Marshall was a retired drummer, like you. I think Leo played a little clarinet in High School, but that was about it. He couldn't even tune a guitar and Forrest White (Fender plant manager) tells a story in which he was forced to tune Leo's "test guitar" to an open G because the racket of Leo banging on the open strings at full volume was driving him crazy. It would seem that musical ability is not a pre-requisite for great amp building. In fact, great playing and great amp building may be mutually exclusive...

[Fischerman]

Are you saying you don't think it's singing because of the preamp or the Power Scaling?

Dana,
One of the main reasons I wanted to try scaling the PI along with the power tubes was so I could remove my post-PI MV and run it using a pre-PI MV to see if my post-PI MV was messing with the tone in any way. But the post-PI MV doesn't seem to be the problem...when I dial it in similarly it sounds pretty much identical. So I don't think it's the scaling and I'm beginning to think it's not the PI either.

Using the pre-PI MV allows me to turn down the signal going into the PI yet leave the preamp volume cranked (with Scale control cranked)...so I can hear what the preamp is doing 'in isolation' so to speak. I don't expect that to sound very good (sounds crappy on Marshalls too) but I don't expect to hear so much blocking for so little actual crunch. I've also done a few other tweeks...late in the preamp*...and I can get more gain or less gain but the 'problem' seems unaffected. So I'm going to go back earlier in the preamp and tweek. In tweeking other high gain preamps, my limited experience is that often the first stage and the circuitry just after it is crucial...but I was hoping to not have to mess with that (was hoping some minor tweek would magically bring it all together lol). So my hypothesis is that there's a 'gain logjam' somewhere early in the pre, and the blocking is sort of 'choking' the amp from singing the way it should.

* - it's a tweeked Concorde pre...I tried adjusting the 330K grid load up/down, adjusting the 'Fast/Slow' split-load, and adjusting Rk for the third gain stage feeding the CF. They all affected the gain and tone to some degree...but the blocking (or whatever it is) is there no matter what and doesn't seem affected at similar levels of distortion. With a 330K grid load, Fast (i.e. no split-load), and a low-ish Rk on the third stage...there is gain out the wazoo...so I've got gain to spare.

[Fischerman]

I was tempted by the dark side this past weekend...tried a Bogner Ecstasy Blue channel preamp and like it a lot better. I tried making the early-tone-stack-thing work for me but I think I just like the tone stack at the tail end of the preamp (and I like a CF at the end of the preamp too).

[gkelm]

Brilliant...love it!

Greg

[paulruby]

Very cool!

I would consider current limiting the variable power supply or, at minimum, include HT fuse. A shorted power tube will blow the mosfet in about 1 second.