le Chargeur (yet another EF86 AC4 kind of amp)

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ChopSauce
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le Chargeur (yet another EF86 AC4 kind of amp)

Post by ChopSauce » Tue Sep 26, 2017 9:09 pm

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.

Image

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.

Image

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...

Image

(pics from an earlier version)

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... 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.
Last edited by ChopSauce on Tue Feb 13, 2018 2:51 pm, edited 3 times in total.

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Re: le Chargeur (yet anoter EF86 AC4 kind of amp)

Post by joeboo88 » Tue Sep 26, 2017 10:59 pm

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

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pompeiisneaks
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Re: le Chargeur (yet anoter EF86 AC4 kind of amp)

Post by pompeiisneaks » Thu Sep 28, 2017 7:10 pm

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

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M Fowler
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Re: le Chargeur (yet anoter EF86 AC4 kind of amp)

Post by M Fowler » Thu Sep 28, 2017 7:54 pm

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

Mark

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ChopSauce
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Re: le Chargeur (yet anoter EF86 AC4 kind of amp)

Post by ChopSauce » Thu Sep 28, 2017 8:26 pm

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 :lol:

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Re: le Chargeur (yet anoter EF86 AC4 kind of amp)

Post by sluckey » Thu Sep 28, 2017 9:29 pm

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.

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Re: le Chargeur (yet another EF86 AC4 kind of amp)

Post by ChopSauce » Sun Dec 31, 2017 6:26 pm

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 :wink:

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.

Image

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. 8)

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Re: le Chargeur (yet another EF86 AC4 kind of amp)

Post by pompeiisneaks » Mon Jan 01, 2018 2:32 am

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

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Re: le Chargeur (yet another EF86 AC4 kind of amp)

Post by Guy77 » Mon Jan 01, 2018 4:35 pm

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

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Re: le Chargeur (yet another EF86 AC4 kind of amp)

Post by ChopSauce » Tue Jan 02, 2018 5:20 pm

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.
Guy77 wrote:
Mon Jan 01, 2018 4:35 pm
Getting tired of these loud 50 watt and 100 watt amps in my basement!
Well, this Chargeur is somewhat "refreshing" in that respect :wink:

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.

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Re: le Chargeur (yet another EF86 AC4 kind of amp)

Post by pompeiisneaks » Tue Jan 02, 2018 6:00 pm

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.

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ChopSauce
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Re: le Chargeur (yet another EF86 AC4 kind of amp)

Post by ChopSauce » Tue Feb 13, 2018 2:49 pm

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 8)

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)

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