AX84 Moonlight Amp Build Project

Hi Everyone,

I've decided to go ahead and try building an AX84 Moonlight Amp as a build project for next year, you can find details to build one on the AX84 website, here's a link to it:

http://ax84.rru.com/moonlight.html

The AX84 Moonlight amp is basically a three valve, low-wattage guitar amp, the valves used to make the amp are as follows:

1X EF184 Small Signal Pentode (9-pin Base).

1X 12SL7 High-Gain Twin Triode (8-Pin Base).

1X 12SN7 Medium-Gain Twin Triode (8-Pin Base).


I don't actually have any of those valves, but I do have some substitutes that I know should work, for the EF184 I'm going to substitute an EF86 (functionally the same as an EF184), for the 12SL7 I'm going to substitute a 12AX7 (same specs as a 12SL7 but in a 9-pin base format), and finally for the 12SN7 I'm going to substitute a 6SN7 (same as a 12SN7 except for a 6.3V Heater).

I'm going to be using a commercially manufactured output and power transformer which I'll order online from Evatco, and the amp will be built using a Diecast Aluminium box from Jaycar Electronics.


Note: All comments and posts,etc are most welcome.

Cool build plans.

On the face of it your tube substitutions all seem good. The 12SL7 and 6SL7 are the same except for the heater voltage (so nice when the ID numbers actually mean something and have relevance to what's inside the thing). From memory my Radiotron book refers searches on the 12AX7 to the 12SL7 page for operating specs so you should be right there. Have to have a look in the book on the EF184 to EF86 swap but I presume you have done your research.

I did have a cheeky 'a little heater for when in the dog house' comment floating in my head but I'll let out only the nice civil ones... Looking forward to see how your build turns out.... ;-) ...

Quote:

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Originally Posted by Marcel

Cool build plans.

On the face of it your tube substitutions all seem good. The 12SL7 and 6SL7 are the same except for the heater voltage (so nice when the ID numbers actually mean something and have relevance to what's inside the thing). From memory my Radiotron book refers searches on the 12AX7 to the 12SL7 page for operating specs so you should be right there. Have to have a look in the book on the EF184 to EF86 swap but I presume you have done your research.

I did have a cheeky 'a little heater for when in the dog house' comment floating in my head but I'll let out only the nice civil ones... Looking forward to see how your build turns out.... ;-) ...

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As far as I can understand, both the EF184 and EF86 are small signal Pentodes that have 6.3V AC heaters and indirectly heated cathodes, although the pin connections may be different, I seem to remember reading somewhere that the EF184 had a mu of about 150, or thereabouts, the EF86 might have a higher mu spec, I do remember building the Moonlight amp circuit before moving to Darwin, and substituting a 12AX7 for the 12SL7, and a 6SN7GT for a 12SN7 actually did work and I got pretty good results, even with the home made output transformer that I made for the amp.

I'm having a debate with myself on whether to use tagstrips to mount the resistors and caps point-to-point style, or mount them on a piece of perfboard or pad-per-hole board, or even make a custom turret board for the amp.



Note: For those of you unfamiliar with the term "mu", in this case it refers to the theoretical voltage-gain of a valve, a 12AX7 is a high mu twin triode (there are two triodes within the glass envelope) with a mu of 100, the 6SN7 is a medium mu twin triode with a mu of 70.


Incidentally, the EF86 was one of the valve types used in the Vox AC30 preamp circuit.

I know it's your project Doc, however if it were mine and I was intending to use the EF86 I'd be looking at mimicking the AC30 circuit in terms of gain and biasing. The clean channel AC30 input is pretty special and would be a very nice to have.

My AC15 clone build will only be the Tremolo channel with two different gain inputs as that channel uses only 12AX7's of which I have plenty.

In the past I found tag strips pretty easy and very flexible. A bit more planning and earlier commitment is needed with any form of turret board layout. On my tag strip 50W 2204 build I got the circuit and built it from input to output in that order by adding tag strips and components as required. The AC15 clone has a generic turret board so will need much more early planning and adjustments to component placement as I build it. Turret board pretty well defines where your connections are right from the start but the result looks neater and is much easier to repair and service. At the moment my biggest headaches with the AC15 clone turret board planning is wire lengths minimisation and then separation of HT and the earth points, and there are a lot of earth points...

Quote:

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Originally Posted by Marcel

I know it's your project Doc, however if it were mine and I was intending to use the EF86 I'd be looking at mimicking the AC30 circuit in terms of gain and biasing. The clean channel AC30 input is pretty special and would be a very nice to have.

My AC15 clone build will only be the Tremolo channel with two different gain inputs as that channel uses only 12AX7's of which I have plenty.

In the past I found tag strips pretty easy and very flexible. A bit more planning and earlier commitment is needed with any form of turret board layout. On my tag strip 50W 2204 build I got the circuit and built it from input to output in that order by adding tag strips and components as required. The AC15 clone has a generic turret board so will need much more early planning and adjustments to component placement as I build it. Turret board pretty well defines where your connections are right from the start but the result looks neater and is much easier to repair and service. At the moment my biggest headaches with the AC15 clone turret board planning is wire lengths minimisation and then separation of HT and the earth points, and there are a lot of earth points...

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What if I modded the preamp circuit of the Moonlight Amp so that it was more like the AC30's preamp, wouldn't that be cool?, I'd end up with a 1 watt AC30 style amp which I'd be able to crank up to get that nice AC30 chiming overdrive, the power amp section is a self-biasing setup like the AC30, I'll definitely have to look into that for sure, I'm currently in the process of fixing an intermittent power switch in one of my Digital Multimeters (the one that can measure capacitance and inductance), the intermittent switch in question is a latching push-on push-off type, and I think it's not latching reliably due to it being a bit loose due to manufacturing tolerances, I'm thinking of trying some judiciously applied super glue to some strategic places on the switch to try and make it more reliable without getting any super glue inside the switch, but knowing Murphy's Law something will go wrong like my fingers being glued together, etc...lol.


Getting back on track, when I get round to soldering in all the parts I might first do a drawing and plan it all out so that I can get the layout all neat and tidy, I've always wanted to try using tagstrips to build a guitar amp so this amp build project is a good excuse for it, so that's that decided, tagstrips it is.

Good luck with the super glue... you'll need it. (the luck that is with that super runny stuff that loves to everywhere except where you want it.. ...)

Are you sure it isn't a cracked solder joint causing the intermittent fault? If it doesn't work maybe as a backup plan you could replace the whole power switch arrangement with a C&K toggle?

Quote:

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Originally Posted by Marcel

Good luck with the super glue... you'll need it. (the luck that is with that super runny stuff that loves to everywhere except where you want it.. ...)

Are you sure it isn't a cracked solder joint causing the intermittent fault? If it doesn't work maybe as a backup plan you could replace the whole power switch arrangement with a C&K toggle?

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Yep, it wasn't a cracked solder-joint that caused the switch to be intermittent, turns out I was right, the switch has two plastic parts that clip together and keep the contacts, spring, etc inside it, however the two plastic parts were a bit of a loose fit, that caused the latching mechanism to not latch properly all the time with each push of the power button, the super glue did the trick and now the multimeter powers up reliably, so I'm calling it fixed until it decides to play-up again....touchwood.


And the best thing about that is, I didn't end up gluing my fingers together by mistake.....hahaha.

Update:

Made a bit more progress on my Moonlight Amp build, I was just having a look through one of the cupboards in my bedroom and found a small white cardboard box with some valves in it, in the box were some assorted valves including all the ones I will be needing to build the Moonlight Amp, here's a pic of them:

Attachment 21516


Sorry for the image-quality, it's the best I can do at the moment, anyway, on the far left is the EF86 Valve, in the middle is the 12AX7, and on the right is the 6SN7GT (I happen to have two of these, from memory they were originally Trigon branded).


Next thing I need to do is work out what power and output transformers I need to buy, since the dimensions of them are going to dictate the dimensions of the diecast aluminium box I will be using as the chassis, fortunately, I happen to know of a website where I can order both from, Evatco, I might even go ahead and order the chassis from Evatco too, as well as anything else I may need.

Now, I've decided that I'm going to make some changes to the Moonlight Amp schematic, and replace the original single tone-control with a 3-band Marshall-style Tone-Stack (more control over the tone that the amp produces), the volume control will then act as a master-volume, for control-knobs, I think I'll go with some chicken-head style ones, and I've got a choice of going with an illuminated rocker-switch for the power switch, or a toggle-switch, but I'm still debating that in my head at the moment.

For those of you interested, here's a link to the Evatco website where you can order parts and etc for guitar amps online:

http://www.evatco.com.au/


I've ordered stuff from them before, and so far, I have had no issues whatsoever.


Note: Click on "Product List" to see all the parts and etc that you can order from them.

Note 2: Be sure to bookmark the link if it does prove useful to you.

Looks like you have it all planned out... looking forward to seeing the bits come together.

Can you put a link up for the Evacto site please Doc. You have pinged my curiosity....

Yeah, and the chicken head knobs are the Bee's knees..... can see them at a distance and you know exactly where you're at... not like Marshall knobs at all....

Quote:

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Originally Posted by Marcel

Looks like you have it all planned out... looking forward to seeing the bits come together.

Can you put a link up for the Evacto site please Doc. You have pinged my curiosity....

Yeah, and the chicken head knobs are the Bee's knees..... can see them at a distance and you know exactly where you're at... not like Marshall knobs at all....

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Can do, I'll post the link to the Evatco website in my last post for you mate, hope it proves to be of some use to you in your amp build.


After thinking about it some more, I've come to the realization that if I change the stock tone control-circuit to a Marshall tone-stack, I'll need to replace the single EF86 valve with either one 12AX7, or two 12AX7 valves so that I can better drive the Marshall tone-stack with a Cathode-Follower stage, then I had a thought "What if I replaced the stock preamp circuit with the Marshall Plexi circuit and mod the amp so that it is essentially a 1 Watt Marshall Plexi?", wouldn't that be cool?, then I could crank it up and not have it ear-splittingly loud.

Yep... certainly is useful.... One part that I had to temporarily substitute is available on the Evacto site... I'll peruse the rest of the site before I put an order in just in case I find something else I need/desire....

I think what I will do for this amp build is order a chassis and cover from Evatco to build the amp on, instead of going with my original plan of using a diecast aluminium box bought from Jaycar, Evatco stock some really nice tagboards that I could use to build the amp circuitry on, also, I've figured out that if I add one 12AX7 valve to the valve line-up, I will be able to create something like a 1 channel Marshall Plexi preamp, and still keep the EF86 valve, should be a pretty gainy amp once it's finished.


These are the tagboards/tagstrips I'm thinking of going with:

https://www.evatco.com.au/tag-strip-28-x-2


I'll see if I can create a hand-drawn schematic of the Moonlight amp with the "Plexi" mods to the stock preamp circuit tomorrow, so stay tuned.

Those tagstrips look very similar to what I used in my AC15 build.

I could get up to 3 leads into the holes on each side of each lug/tag which made things a lot easier on real estate consumption... 6 component wires per tag and then the huge hole in the middle for big multistranded cables from underneath came in very handy ...

My only concern was voltage rating between lugs wasn't specified by my supplier, so I ended up leaving lugs vacant when next to any lug with any sort of anode voltage or higher HT on it... Low voltage lugs like cathodes or decoupled tone components I felt were fine when next to earth lugs.

Quote:

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Originally Posted by Marcel

Those tagstrips look very similar to what I used in my AC15 build.

I could get up to 3 leads into the holes on each side of each lug/tag which made things a lot easier on real estate consumption... 6 component wires per tag and then the huge hole in the middle for big multistranded cables from underneath came in very handy ...

My only concern was voltage rating between lugs wasn't specified by my supplier, so I ended up leaving lugs vacant when next to any lug with any sort of anode voltage or higher HT on it... Low voltage lugs like cathodes or decoupled tone components I felt were fine when next to earth lugs.

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Yep, that would be a concern, you definitely don't want the voltage between two adjacent tags to end up exceeding the dielectric-strength of the insulating material and causing a leak, especially if one tag happens to be the one with the negative grid-bias supply for one of the power valves, I'm not sure what the dielectric-strength of Phenolic is, I think it's something like 300V DC or so per so many milimeters, or I could be wrong, on the other hand FR4 Fibreglass will definitely have a higher dielectric-strength.

From looking at the pic of the tagstrips from Evatco, it looks like they're made using FR4 Fibreglass.

Edit (04/05/2023):

Seeing as I just recently ordered a set of 10 of these 28 X 2 tagboards from Evatco, I can confirm that they do indeed use 1.5mm thick (roughly) FR4 Fiberglass as the insulating material, the tags themselves looked to be tin-plated and fastened to the insulating material via brass eyelets with an inside diameter of about 3 or 4mm, this is actually a good and useful thing since brass is solderable so you could use the brass eyelets as well as the tags for mounting components and wires, this gives you more layout options.

One of my first tube amps was the Moonlight, it seemed fair as Simcha Delft (the original designer) had first printed the DIY thing in our NZ musician magazine. I even got one of Simcha's output trannies that he winds himself as he lives here in NZ so why not. I haven't fired it up for years but when I do it's through a cab with 4 x 12's which are alnico magnets, paper cones with a few ribs... found a guy in Christchurch in the South Island had a whole bunch of NOS stuff like this. The only thing I changed was using a valve rectifier and a choke as the old radio power transformer I used had the wrong voltage but by using the voltage drop in the rectifier and the choke I got into the ballpark required.

If this is still a goer, Marcel, I can get Simchas email, and you might even be able to get a output tranny from him.

The first time I tried building a Moonlight amp I had a go at winding my own output transformer, using the laminations and bobbin from an Audio Transformer that was intended for use in 100V PA speaker line applications, winding each layer of the primary winding and separating them with plastic adhesive tape was the most time-consuming part of it, if I remember things right, the primary winding from the centre-tap to each end measured about 27 ohms on my Digital Multimeter, I wasn't sure if it would actually work, but I tried it anyway, and it actually worked first go, I was surprised at how loud my Moonlight amp was, when I overdrove the amp to get the 6SN7 tube to distort, the sound was really smooth.

I don't remember how many turns there were that made up the primary winding, my memory's a bit rusty.

Update:


Seeing as I'm really in the mood for doing some diy amp builds this year, I thought I would pop-by the AX84 website and gather together as much info as possible on the AX84 Moonlight amp, this will be a real fun build for this year, well, at least it'll give me something "constructive" to do anyway, so yes, this build is scheduled for this year.



And, as luck would have it, I managed to scrounge a spare EF86, 12AX7, and 6SN7GT that I can use in my AX84 Moonlight build, next fortnight I'm going to pop-by my local Jaycar Electronics store and buy a diecast aluminium box I can use as a chassis for my build, and maybe some other bits and pieces that I'll need, will also order some other bits from Evatco too.