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

[Transmogrifox]

IMO the best way to scale the gain would be to find out how much distortion the triode stages generate. For example a 60V p-p (say at 300hz) signal enters a triode stage, the gain is 60, the output signal should be 60X60=3600V, but the plate voltage is say 300V so we have 20xlog(3600/300)=21db of distortion. Now, we should design the fet stage so that is has the same amount of distortion and do that for every stage. The first stage would be a little more tricky, I think it would need an additional 'input trim' control to make it work with both single coils and humbuckers (assume input voltage 100mV p-p for single coils).

We assume the guitar and pickup types are irrelevant.  If the amp itself behaves similar to the real thing, then it will have the same response to single coils as the real amp would have, and likewise with the humbuckers.

Secondly you don't have to think in dB for this.  It's as simple as my Alice in Wonderland story.  You just scale the magnitude of the input by the ratio of (clipped amplitude low voltage stompbox)/(clipped amplitude of high voltage tube amp).   It's like making a doll for a doll house.  If a room in the dollhouse is 1/96th the size of a real room, then you make the doll 1/96th the size of a real human, then the doll will fit properly into every room in the house as long as it's truly built to scale.

In other words, you don't need to adjust every single stage as long as the gain on each stage matches the gain on each stage in the tube amp.  We will assume these FET's are close (although I suspect the J201 may be a little higher).  A typical 12AX7 stage has a gain of about 100.  You can find out from a datasheet how this compares to the gain at the currents you're using.

Would Alice need an additional 1:10 mushroom at the end to bring back the level ?

No.  At least not if you don't need 100V peak to peak to drive a power amp stage.  You don't want the output to burn up your $400 Line6 delay station.  Just think through what we're trying to do here.  On the output you want the waveform to look like a scaled-down version of the waveform that comes out of the tube preamp stage. 

[Auke Haarsma]

No.  At least not if you don't need 100V peak to peak to drive a power amp stage.  You don't want the output to burn up your $400 Line6 delay station.  Just think through what we're trying to do here.  On the output you want the waveform to look like a scaled-down version of the waveform that comes out of the tube preamp stage. 

Just tested it on the breadboard: you are right! There is no need to have another gain stage to compensate for the lower input level. And yes, 470k/82k (values I used) did lower the gain and 'sweetened' the sound up a bit. I am going to add this into the schematic and pcb as a mod. Higher gain, just  omit the 470k and replace 82k with 1M, lower gain (more 'real' amp like), use Transmogrifox's suggestion. (see edit at end of this post)

Btw, with this circuit realy responds well to the guitar volume. You can go from clean to crunch with just your volume knob on the guitar!

I am now going to look for a suitable 1-knob tone section to include as a possible mod.

edit:
I just tried the '470k-at-the-input'-mod with a single coil guitar: it does have quite an effect on the available distortion. With humbuckers you can still go from pretty clean to distortion, with SCs it goes in to OD / mild distortion max. But hey, maybe that is what the amp is supposed to do

[Auke Haarsma]

Well, the BMP tonestack can be used. Since the TS is followed by two gain stages, there is no need for a gain recovery stage after the BMP tonestack.

[MadMac]

Whoo

This looks like alot of fun!  Looks like this will be on my "must build" list.

[Lurco]

IMO the best way to scale the gain would be to find out how much distortion the triode stages generate. For example a 60V p-p (say at 300hz) signal enters a triode stage, the gain is 60, the output signal should be 60X60=3600V, but the plate voltage is say 300V so we have 20xlog(3600/300)=21db of distortion. Now, we should design the fet stage so that is has the same amount of distortion and do that for every stage. The first stage would be a little more tricky, I think it would need an additional 'input trim' control to make it work with both single coils and humbuckers (assume input voltage 100mV p-p for single coils).

As soon as the triode entering voltage exceeds the grid-cathode voltage, distortion starts (roughly between 1.5 and 3v in typical applications)
and absolute voltage amplification factor drops rapidly.

[Auke Haarsma]

Ok, here is the updated schematic and the proposed PCB layout.

Schematic has the added input resistor as mentioned above.

A note about the PCB: I have not attempted to make it as small as possible. But instead, I have made it as small as possible while trying to keep it not a very tight, complex fit. My goal is that even a relatively unexperienced builder should be able to use this pcb layout. That is also the reason why, with the final version, I want to offer a full-out 7 knob version as well as a simple three-knob version (and notes for anything in between).

I have tried to keep the traces thick and the spacing a bit wider than I normally do.

C1      0.1uF    C2      15uF    C3      15uF    C4      15uF    C5      15uF    C6      4n7    C7      2n5    C8      5n    C9      2n5    C10      1uF    C11      0.47uF    C12      120pF    C13      220pF    C14      220pF    C15      220pF    C16      220pF    C17      47nF    C18      120pF    C19      47nF    C20      10n    C21      1n2    C22      100uF    C23      5n

R1-A      82k / 2M2 R1-B      470k R2      2M2 R3      68K R4      1K5 R5      1K5 R6      1K5 R7      1K5 R8      1M R9      220K R10      100K optional R11      220K R12      10K R13      10M R14      10M R15      100K optional R16      82k R17      100k R18      2M2 R19      1M R20      220k R21      100R

Q1      J201 Q2      J201 Q3      J201 Q4      J201 J1-J2      jumper    D1      1N4001 POT1      1M   drive POT2      25k   treb POT3      25k   bass POT4      1k   mid POT5      1M   gain POT6      100k   master POT7      100k   presence SW1      On/Off/On TRIM1      100k TRIM2      100k TRIM3      100k TRIM4      100k

[Transmogrifox]

Nice work!

You have turned around the PCB pretty fast

[Auke Haarsma]

Thanks!

I must say that I now see that I again failed to put the lead-switch correct in the schematic. SW_B should ofcourse be mirrored 

But other than that, does the PCB look like it is 'project-ready'? If so, I'll go and make the PCB to verify it probably this weekend.

[Jimmy-H]

Ziet er goed uit Auke!
Ik ben benieuwd hoe het uiteindelijk wordt en klinkt!
(weer een project voor de toekomst  )

Just some Dutch chit chat!

[Auke Haarsma]

Dank je

Andre etched me a PCB very quickly! I haven't received it yet, but he made a photo, which ofcourse, I cannot keep just for myself!

[Jimmy-H]

Andre etched me a PCB very quickly! I haven't received it yet, but he made a photo, which of course, I cannot keep just for myself!

Very Nice PCB Auke!
So with the layout you were already aware that the PCB would become blue!

[Scruffie]

So, which one of you distinguished veroboarders feels like veroboarding this circuit? I'd like to try it against my Dr Boogey I just made.

[Auke Haarsma]

Sounds like a great plan! However, I still need to verify the build. In a couple of days I can tell if the circuit really works well.

[Auke Haarsma]

So with the layout you were already aware that the PCB would become blue!

Well, I wasn't aware of that. Just a lucky guess. In my opinion gold/copper looks better against a blue background

I'd like to try it against my Dr Boogey I just made.

Also, keep in mind the Dr Boogie is high gain, the lonestar has less gain. It still has plenty of gain on tap for my liking.

[Scruffie]

Yea I want something with a little less gain to go with it, I just wanna compare its Mesa sound in comparisson with the boogey, but as I say need a vero to test it.

[Auke Haarsma]

Well, Andre shipped the PCB fast... however I had supplied him with my old address. How stupid can I be??? I moved months ago, and still I mixed up the addresses... But, I got the PCB finally and man, it is gorgeous!!






And another close up to show how good this etch is. This is a really close close up



I plan on populating the board this coming weekend. Stay tuned!

[Auke Haarsma]

The board is being populated. Next step is to connect all the pots...

[Auke Haarsma]

All pots wired: it works on first attempt! I have been playing around with the pots and been tweaking some. Ofcourse, despite many breadboard verifications, I still manage to flip the lugs of the pots on the pcb some corrections need here. The pots are very interactive, but you can get some nice tones out of it.

I wasn't completely blown away by the sound though... until I tweaked the 2M2/120pF (R18, C18) after Q3. The sound much improved when I just bypassed those. So, I checked the amp schematic. The 2M2/120pf is in between the send and return of the Reverb. Send is before R18/C18, Return right after R18/C18

What is the function of the 2M2/120pF in the original schematic?

Since we are not using the reverb (I left it completely out), can we also ditch the 2M2/120pF?  (I am gonna do that anyway, it sounds much fuller! but I do like to understand the function).

[Auke Haarsma]

So, I deigned a new version of the PCB. The changes:

• Dropped the 2M2/120pF (R18, C18). This gives a much fuller sound.
• Relocated some components. The PCB is now smaller, but still not extremely tight. Should be an easy build, component-placement wise. I gues it could fit in a B sized enclosure. (In case you want to build the 3-knob version...
• Added a switch. Yeah, another one! I made the input resistors switchable. You can use the reduced level input, or the 'hot' input. The hot input sounds good with weaker pickups, or if you want extra gain
• Added an (optional) resistor and pad for a status LED.
• The current version now has: 7 pots, 2 footswichtes, 2 toggle switches...

Any suggestions?

[kurtlives]

All pots wired: it works on first attempt! I have been playing around with the pots and been tweaking some. Ofcourse, despite many breadboard verifications, I still manage to flip the lugs of the pots on the pcb some corrections need here. The pots are very interactive, but you can get some nice tones out of it.

I wasn't completely blown away by the sound though... until I tweaked the 2M2/120pF (R18, C18) after Q3. The sound much improved when I just bypassed those. So, I checked the amp schematic. The 2M2/120pf is in between the send and return of the Reverb. Send is before R18/C18, Return right after R18/C18

What is the function of the 2M2/120pF in the original schematic?

Since we are not using the reverb (I left it completely out), can we also ditch the 2M2/120pF?  (I am gonna do that anyway, it sounds much fuller! but I do like to understand the function).

Try a smaller value resistor than 2M2.

I like the Marshall 470pF/470K. 300pF/470K also sounds very good imo.