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:

---

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.... ;-) ...

---

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:

---

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...

---

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:

---

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?

---

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:

---

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....

---

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.