le Chargeur (yet another EF86 AC4 kind of amp)

[ChopSauce]

The plan was to build a "desktop amp" to play guitar while listening to the computer through the AUX input. Originally the input circuitry was something "à la" Fender/Greta, but it has now evolved into a common anode mixer for both inputs.

The amp has been built into a battery charger enclosure - hence the name.



As obvious, the outcome was built from spares, recycled and traded parts.

The circuit design has been elaborated by the (french) projet G5 forum(1) team.

In a nutshell: Denney & Jennings' AC4 circuit has been revised as per Merlin Blencowe's insights. The "classic" Vox tonestack was preceded by a MOSFET follower.



NOTE: the above schematic does not necessarily provides for a recommended build and it has no even been checked, despite my intend to faithfully reproduce the actual "Chargeur" circuit. The design was elaborated as a pure exercice and I degraded it at will to use what I had already available, or what I could buy for cheap.

That made for a tight build...



(pics from an earlier version)



... since the power tube does not even stand right in this low box. So, I'm glad I did it - since it perfectly suits its purpose, but I won't do this again!
_______

(1) For more insights, see http://www.projetg5.com/phpbb3/viewtopic.php?t=5347 - in french though - and feel free to ask any questions, here or by e-mail to chopsauce@free.fr. I'll do my best to reply &/or translate as needed.

EDIT: typographical error corrected

EDIT2: changed Aux/GTR input circuit to common anode mixer - in an attempt to reduce both background noise & interaction between inputs. Yet I am missing a triode gain stage now, the outcome is even closer to the design requirements.

[joeboo88]

Wow Chopsauce, this thing is way cool...one of a kind.

[pompeiisneaks]

Oh man there's no way my fat fingers could build that! Well done

~Phil

[M Fowler]

That's a very cool little amp. Mosfet follower something I should mess around with one of these days.

Mark

[ChopSauce]

Thanks for your comments. I have been lucky that the guys at the projet G5 forum found this one of a kind challenge interesting, for sure, and...

(this is not a joke)

... Indeed I have been using tweezers, pliers and several other fine surgery kinda tools to build this one

[sluckey]

I like this kind of stuff. But I know it's a real challenge. Kinda like a layer cake. I bet you spent as much time figuring out which layer to put down first as you did on the actual layout. I salute you!

I bet it will never fail.

[ChopSauce]

Thanks!

I spent even more time on mock-ups than you suggest, assembling and dis-assembling it, to make sure it is fairly serviceable...

... in a Vox-ish manner

So I spend some more time on it sorting the grounds, increasing the filter caps, adding some shielded cable and ... well, I am totally pleased with it as a bedroom/practicing amp - but it is still not usable (too noisy) cranked.

It looks like the preamp collects too much noise, and I suspect this is due to the 2nd triode being used as an early power-amp stage.

My concern is that I will "loose" the NFB, which I am really pleased with, if I simply disconnect the second triode - to see if that reduces the noise.

I also engraved/punched-painted & sanded the faceplate.



I wont' say that I am totally pleased with the faceplate, which I need to sand it better, but that was an experience and it kept its rough looking.

[pompeiisneaks]

Oh man that thing looks incredible! I love hte lettering etc. Tres bien fait mon ami!!!!

~Phil

[Guy77]

I love the AC4! EL84 is one of my fav tubes. I will have to try and build something small like that .
Getting tired of these loud 50 watt and 100 watt amps in my basement!

Happy New Year!

Guy

[ChopSauce]

Thanks Phil! You're definitely too kind. I'm really not playing in the same league as many garagists here, but anyway here it is, which sounds well enough for me - yet not really as I expected.

Getting tired of these loud 50 watt and 100 watt amps in my basement!

Well, this Chargeur is somewhat "refreshing" in that respect

So, I also wanted to mention that the EF86 at such moderate gain (around 70, roughly), in this position, is more reminiscent of an overdrive section than of the original AC4 by Denney & Jennings.

I am not disapointed by that anymore (my first reaction was more negative, though) but - due to the likely interferences within and around V1, I think I would rather built it as the EF86 being an overdrive section after the first triodes in a classic arrangement.

[pompeiisneaks]

I've actually been thinking of a design doing that as well. I know the Merlin talked about how the EF86 really calmed down when you put a first stage before it as far as microphonics go etc.

~Phil

[ChopSauce]

Yes, and pushing further, my current temptation is to build an "Hot Rodded Vox" with an EF86 as an overdrive section on its own. Still not sure what I'll do for the first two stages, though. I am tempted by a Blackface alike arrangement, whose tone stack could be lifted (PAB)...

... but I have many things to do first. So, so far, I just turned the input circuit into a common anode mixer for the Aux and GTR inputs - as per Merlin, as usual

That makes for much less noise and interactions, but the thing still is under development.

I have been wondering what the settings for the common anode mixer had to be, because it seems there is some confusion p111 of Merlin's book.

However, it - https://www.ampbooks.com/mobile/classic ... ins-mixer/ - looked like I could just go with the already soldered (Ra=110k, Rk=1k5) so, this is what I did.

To be cont'd...
(my plan is to change P1 for 1M and to decrease filtering, because the sound is a bit harsh when the gain goes to 11)

[rooster]

That cathode follower is easily bypassed, what does it sound like when you bypass it? Frankly, putting that tone stack (AC30TB) in the circuit is totally against the EF86 premise and path design. If you insist using this in the path, I would suggest subbing a different value for the treble cap, probably 500pf, but not 50pf. Yeah, I think 'harsh' is what I see.

This said, I do appreciate your efforts and work. Tweezers? Surgical tools? Ha, no, I save that for my mic building bench, not the amp bench.

[ChopSauce]

EDIT (still looking forward to viewing your mic building bench, though:)

I have been toying with the tone stack (TS) simulator and yet upping the cap value lowers the upper mids, still the treble remain high, almost at the same level. I could read here that the TS simulator is worth what it costs, still I'm not sure this will solve the harshness issue ...

(but I'll try this, anyway)

... also, do you mean bypassing the TS or really the mosfet-follower with the TS behind it?

I really like the mosfet because it drains the caps, and I was planning to use the switch for the bright cap, which I never use, as a TS lift switch

So thank you very much for your feedback and please note that any pointer to further reading upon the subject would be most welcome, as I am somewhat definitely puzzled at this point.

[tictac]

I wonder if your harshness issue is due to the EL84 grid voltage being too high when overdriven?

There have been some disscussions about this and there was a circuit created (Paul Ruby?) that puts a zener diode in the signal path of the power tube grid to limit the voltage to somewhere around 15 to 18 VAC. You might do a web search for Ruby Zener Mod or 18 watter buzz....

TT