The way the PCB is designed now it just seems like a turret/eyelet layout but on a PCB board...
This is not a knock at you RJ, just thinking out loud...
I just think the PCB could be smaller and improved a bit.
Chris - I'll give you good technical merit on those thoughts and your commentary is all taken as helpful input. I didn't want to go too far from the original design in all aspects since I did set out to deliberately change other "sacred" aspects of the Express design. I also wanted to keep it as a DIY friendly project so lots of things could be improved from the engineering and manufacturing perspective but would not be as agreeable to the average home amp builders - familiarity brings us comfort and confidence.
One driver for building an amp in this giant over simplified PCB board style is that there is a stigma associated with PCB boards, radial caps, and surface mount parts, so selling this idea requires a bit of finesse.... familiar components and layout help to associate this with the original amp it is based on. For me that association needs to be transparent in circuit and ultimately tone. Eventually you might see and I'd be pleased if you contributed to the smaller and better version, but it's an iterative process for a variety of reasons.
I approached this from the perspective of an experimentalist and one thing you learn in that business is that you can't change too many variables and understand the outcome. That might not matter if the results are good, but it's game over if it doesn't work. Modern engineering can step in and make great decisions and predictions and take away all the fun from progressive experimentation... but it can be comical, painful, and even career changing when real data doesn't fit the theory.
Zippy wrote:Given the history of the Express and instability, going to as tight a layout as physically possible may be inviting "issues".
I agree... baby steps... especially with a hairy beast of an amp.
Once it is a proven design change, try something else. rj proved that the original design will work... I want to push a little further with the Presonance...
I would like to see a board with the pots mounted to it to see how it affects the tone... also, I like the radial caps and for the same reasons mentioned... I would rather use cans myself. I still can't see any benefit to mounting the sockets anywhere but directly to the chassis.
Just my two cents.
Octal sockets and =C=s to fill them will be here Monday.
"Education is what you're left with after you have forgotten what you have learned" - Enzo
I'm not a fan of pots-mounted-to-PCB construction. There are advantages, but it seems like there is too much opportunity for the connections to be stressed due to misalignment and/or cyclic stresses due to differential thermal expansion of the board and chassis. It makes lifting or removing the board more difficult too. Where do PCB's in amps fail most often? Cracked solder joints due to mechanical stress and heating.
Using radial caps might be problematic because of chassis height, and stacked filters may be required to get the voltage rating needed, increasing part count. Replacement is more difficult too since the solder pads will be covered by the cases. As it is everything is accessible from above.
I don't think rj was going for speed of assembly so much as consistency and predictable stability, and I think he's reached a nice compromise. Distributing the filters is an improvement, and keeping the high-current filament and plate leads off of the PCB is good for isolation and for reliability. I also agree with keeping the tube sockets mounted to the chassis, but I think I'd rather see all the leaded components mounted to the board (screen resistors and grid stoppers included) with only jumper wires connected to the sockets. I'm thinking about swapping of components and the possibility of needing to lift the board at some point. A possible negative is that screen resistors can get very hot in the event of a tube failure, and mounting both leads to the board would increase the possibility of colateral damage.
Finished the PCB Express and fired her up for a spin. At low volume she is perfect for tone not bright like other Express builds of mine. At higher volume it has that distinct Express gain and will tear your head off.
I need to find a good speaker cab match and let the amp burn in for a while.
Wish would of had Martin's presonance circuit layout before I started so I could have inserted some eyelets on the board. Nice function works great.
Mark
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M Fowler wrote:Finished the PCB Express and fired her up for a spin. At low volume she is perfect for tone not bright like other Express builds of mine. At higher volume it has that distinct Express gain and will tear your head off.
I need to find a good speaker cab match and let the amp burn in for a while.
Wish would of had Martin's presonance circuit layout before I started so I could have inserted some eyelets on the board. Nice function works great.
Mark
Hi Mark, looking good there! That was actually Dorrisant's layout for the presonance mod, I just made a slightly prettier picture :^) Ultimately it would make sense to move the ground point over to get rid of the wire run to the existing one and shorten the wire to the pot. I guess there would be room to put both 0.047's on the board and avoid the flying cap too. It's great to hear that the presonance works with the Express circuit!
I was thinking of another turret for the cap to the pot but didn't for the sake of making the run shorter... and one less turret to install. Don't know if it really would make a difference or not.
...and yes, Martin's picture looks much better than my "edited with MS Paint quick and dirty drawing." Besides his has the NFB and ground leads oriented correctly. Thanks for keeping it correct for us Martin.
Mark, beautiful job as always. Glad to hear that it is working well for you.
I suspect mine will be finished within a couple hours of getting home Monday after work. Until then I'll just look at it.
I have to say again... great job and hats off to rj for pushing the envelope!
Thanks guys, One thing I did was add an 820R oscillation resistor off the volume pot wiper to V1. Never used that before, thought I would try it on this build.
I also have a 33k grid blocker on V1 as well. Used shielded wires.
Got some ground buzz with guitar plugged in so I'll poke around for that.
Finally - done with my taxes... my second favorite thing besides everything else in the world. Only 12 months until i get to do that again!
Protection diodes will not cause any ill effects that I am aware of. Once I lose that first output transformer I'll probably get more inclined to get them in place to start with.
For adding power scaling I can make that an easy addition by just putting a gap in the B+ lead coming off the diodes so you can either jumper it or run it through a VVR unit. I tend to like the attenuator best for taking some of the fire out of big amps, but I really don't like anything as well as the amp all by itself... The VVR has a good track record and the PPIMV has a similar following so I think that it's down to taste to some extent.
I was thinking of running a thread on isolation cabs. Would any of you cab makers be interested in trading for a rev-2 board and chassis for a quality isolation cab? I would really like to play this amp at church but it's far too ferocious for my venue.
I also used a 33K grid blocker on the input to V1, I was really expecting more of a battle in getting this thing stable but I'm not convinced there is a down side to a grid blocker in any of the gain stages.
It's great to see the Fowler amp snorting fire. Sounds like it is similar to mine. Mine is not brassy or harsh either which was a pleasant outcome but I still have no need for all the treble you get with the brite caps.
Last I talked to Tony he was about to sneak up on his PCB Express and we'll see if he makes it a hat trick and we can also learn how that presonance is going to treat him in this layout.
The one and only Colossal should be looking at a parts package any day now and his will have a little different flavor yet... should be fun!
RJ Guitars wrote:The one and only Colossal should be looking at a parts package any day now and his will have a little different flavor yet... should be fun!
rj
And a box did indeed arrive today. Thanks man! I'm listening to some Floyd getting parts set aside and making a list of things to be done. Will start tomorrow as time permits! Really nice board RJ. Been looking forward to this build. I will be adding the presonance circuit, PPIMV, and will probably add an OT primary and a high/low voltage switch just for fun and heresy. I'm debating whether to go with polyester film foil signal caps and carbon film resistors or polyethylene and metal film.
Well,
Parts were there... amp got finished about 8:30... We rocked out until about 12:15... the neighbors need their sleep.
Our happy guinea pig, Justin, couldn't quit saying OMG, even though he kinda knew what to expect.
My ears are still ringing and I sat a good distance behind the cab.
I did have one problem. I had the hot and ground backwards on the input jack... at least I knew what it was instantly.
It was amazing how responsive this amp is... on the verge of meltdown.
No real issues... except for a bad rumbling noise. We swapped tube to no avail. I guessed it was vibration from the cabinet. I moved the chassis over to a cabinet not being used (thus no vibration), and the noise went away.
Justin said he couldn't get away from 12:00 on everything, even the Presonance control. Found it to be very useful to dial in a plethora of tones.
I might get a chance to play it myself in the next couple of days. More to report then.
Thanks to all involved!
Tony
"Education is what you're left with after you have forgotten what you have learned" - Enzo
That is awesome. Did you do the Presonance control? I also want to make up a voltage chart. I think these are a few volts shy of the Express standard. I only got 394V coming out of the rectifier so I have a thought of adding about 10v of AC to the next transformer iteration.... although it certainly doesn't seem to hurt anything in the results right where it is at.
I'm listening to some Floyd getting parts set aside and making a list of things to be done. Will start tomorrow as time permits! Really nice board RJ. Been looking forward to this build. I will be adding the presonance circuit, PPIMV, and will probably add an OT primary and a high/low voltage switch just for fun and heresy. I'm debating whether to go with polyester film foil signal caps and carbon film resistors or polyethylene and metal film.
