[Lonely Star] An adaption of the Mesa Boogie Lonestar amp. Seeking comments!

[Auke Haarsma]

So, it looks OK I guess?

It will be the third PCB I will populate for this project. Wanna make sure this is the final one!

[Ripthorn]

It's looking pretty good, though if it were me (and probably just for me) I would put in a voltage select switch so I could run it at either 9 or 18V with the flick of a switch.  Maybe that is why my submini amp has taken 4 months so far...

[Auke Haarsma]

as if we do not have enough switches already 

Why would one want to switch from 9V to 18V? Or from 18V to 9V?

[Ripthorn]

I would include a switch to change the headroom and thus the amount of distortion available.

[Auke Haarsma]

Ah yeah, I understand.

I'll stick to the current pcb I think, there is already plenty of 'gain-shaping' available (lead/normal chan, hot/norm input, norm/thick/thicker-switch). However, it sure is a great add-on, for those interested in even more 'shaping' options!

[kurtlives]

as if we do not have enough switches already 

Why would one want to switch from 9V to 18V? Or from 18V to 9V?

Ya I was thinking that as well.

I think a little DIP switch on the PCB would be nice. Kind of set it and forget it kinda thing.

[Ripthorn]

as if we do not have enough switches already 

Why would one want to switch from 9V to 18V? Or from 18V to 9V?

Ya I was thinking that as well.

I think a little DIP switch on the PCB would be nice. Kind of set it and forget it kinda thing.

exactly, but still you would have the option of changing it without any desoldering, etc.

[flo]

Looking good Auke!  
I would like to build one when you're finished and happy with it and André has the final PCB available!  

Did the members from Run Off Groove react to this design yet? They have lots of experience with this amp simulation stuff...

[Auke Haarsma]

as if we do not have enough switches already 

Why would one want to switch from 9V to 18V? Or from 18V to 9V?

Ya I was thinking that as well.

I think a little DIP switch on the PCB would be nice. Kind of set it and forget it kinda thing.

exactly, but still you would have the option of changing it without any desoldering, etc.

I like this idea, using a dipswitch. However, it will be a challenge to fit it onto the pcb. I'll have a look at it. The pcb *must* fit in a Hammond B-sized enclosure. As you can see on the pics of the V1.1 PCB, there is some room left to enlarge the pcb, but I doubt there is enough room...  We'll see

Looking good Auke!  
I would like to build one when you're finished and happy with it and André has the final PCB available!  

Did the members from Run Off Groove react to this design yet? They have lots of experience with this amp simulation stuff...

Thanks Flo. Not sure if the ROG-crew has seen this thread. However, I have read most articles and checked most 'amp-sim' designs by ROG. They do have made great amp-sim pedals.

[Auke Haarsma]

I've been trying to get a charge pump working. My initial test at 16V was with my own PSU, not an onboard charge pump. I have now breadboarded several versions of the chargepump with several IC's, but each an all of them give me squels when I bias either Q3 or Q4 around 1/2 Vcc. Unworkable sadly. And I have no clue why.

At first I thought it may have to do with the (way too) long wires I used in my test build. But no, that's not it. It must something related to using a charge pump. So for now I think I will go back to the previous version of the pcb (without charge pump, running at plain 9V, sounding great!) unless someone has an explanation for my 'squeels' at biassing Q3/4.

[Ripthorn]

Did you make sure to engage the boost function on the charge pumps?  You might be getting the switching frequency into your nearest jfets which would then get amplified and could become a squeal.

[Auke Haarsma]

Yeah, that is not it. I've tried acouple of variants of the MAX1044, all with boost function.

And, I must correct my previous post: I now notice the squeels also happen with my own PSU's. So it is not charge pump related.

It must be something in:
1) the circuit itself: it squeels when fed with 18V
2) my test-setup (chaos to the outsider, very organized to me )

Option 2) seems unlikely.

[Auke Haarsma]

Just to add some more info to this mystery...

This is what happens when I put a higher voltage on the circuit.

Vcc = 18V
Q1 Drain: biasses nicely at ~10V
Q2 Drain: no probs to get it to ~10V
Q3 Drain: trimpot is still on the 9V setting, so it starts high (15V). I trim it down and I can hear the squeel coming up. If I keep it at 15V, I do not have a squeel. Just a not so nicely biased sound.
Q4 = same as Q3.

[Auke Haarsma]

well, I've had a bad vibe. Sometimes I have that, nothing seems to work... plain bad luck all the time. That vibe is over. It is time for gooood vibrations!

I abandoned the charge pump, went back to the 9V design. Changed the thick/thicker-switch so that there is a bigger difference between thick and thicker and a smaller difference between normal-thick.

Oh, I also received some submini tubes yesterday.... (no, I won't try to fit them into this design )

[Ripthorn]

If the problem happens with any kind of 18V power supply, then it is probably just oscillation.  This is because 1) 18V provides more headroom and thus more volume and 2) biasing the jfets at 1/2 supply voltage is the ballpark for max gain on the fets.  So what you are probably seeing is that you are getting tons of boost through those last two stages when all is biased "correctly" and that starts the circuit oscillating.  Still looks like a good project.

[Auke Haarsma]

Thanks Brian, that's reassuring!

What I don't understand is that it even happens even at mininum gain. At that setting you have to dial up the volume control to not drop the output level compared to the bypass signal.

[kurtlives]

Try increasing the value of the source resistors, just a bit?

[Auke Haarsma]

It's my inderstanding that increasing the source caps will even further increase the gain.

What do you expect from raising the source caps?

Maybe lowerling the caps (currently 15uF to 10uF or 4,7uF) could help? But, that would also drop the gain in 'normal 9V-mode'.

[kurtlives]

My last post said resistors not capacitors.

Ya you would loose a bit of gain, thats why I said only increase them a little bit. I am just throwing ideas out here to try and help.

[Auke Haarsma]

Ah apologies, I didn't read that carefully.

I am just throwing ideas out here to try and help.

And that is much appreciated!

If my question "what do you expect from..." sounded a bit hars, that was unintentionally. I try to understand the reasons for changes to the circuit in an attempt to overcome my lack of knowledge Especially if some1 knowledgeable like you gives a hint/tip I'd like to understand the reasoning behind that.

What I not yet understand is why increasing the supply voltage leads to oscilliation. I do get that :
-increasing supply voltage and biasing around 1/2 Vcc increases the available headroom
-more headroom means increased signal level before clipping

What I don't get is why it happens even at minimum gain. At the 9V supply voltage there is hardly any gain, I'd say it pretty much is a nice warm clean tone, with even a drop in volume (unless you set the volume pot at max). I'd expect to just have a nice clean tone at min gain settings and 18V.