New Acrosound project

[Brice]

Hi all,

Mid 2019 I had an idea in mind, but never actually made it as a formal project.

It was a simple idea: creating an Acrosound 20, the monobloc version of the famous 20-20, but not simply cloning it, but rather adding a new amplifier to the Acrosound family, 60 years later. I know... crazy isn't it?

I wanted to make it look like the Acrosound ULII, but smaller and with the exact Acrosound 20-20 circuit topology. In other words, I wanted to create a ULII little brother, called the "UL 20".


On Nov 19th 2019, I sent this picture of my beautiful skilled art talented handwritten sketch to my partner in crime, MultiChamp12 and OneBean, representing the chassis I had in mind:




Beautiful isn't it ?

They both were anthousiat about the idea and both of them decided to embark with me on this journey.

The end result is this, with the ULII behind to have an idea of the scale:





So what did we have to start with?

- The original Acrosound 20-20 schematic

- The amplifier PCB I made before, with some slight modifications from the work
we did, X3workshop, Master Dave Gillespie and I in Nov 2016. These PCBs could be reused as is for this project.

- Half of a quart of a perfectly matched automotive paint I've had from my ULII restoration
project a few years back.


So from 10,000 feet we needed a power transformer, an output transformer, a power supply,
a chassis and all the parts which make an amplifier. That's all.

For the power transformer, I knew Heyboer was able to create one.

For the output transformer, my work with the 20-20 involved the cloning of its transformer by
Transcendar, so we could have some made for that project.

We also needed a solid power supply, a grounding scheme, safety feature, design choice, etc...which we did in a collaborative and iterative manner.

So let's start this project.

For the chassis design, this task was for Master OneBean...

 

[multichamp12]

Well here we go!

Brice got me into acrosound a few years ago, and I have not looked back since. I purchased from him a Acro 20-20, UL-II, Allen 75 with Acro Transformers, and a Acrosound S1001 Preamp. We spent many months planning and executing this project. My main job was to pot the output and power transformers for the 3 pairs of amps. I used regular tar to do this job, and I was able to do this in my garage by melting the tar in a pot on a portable stove-top.



When designing this amp, myself, Brice, and Justin all pooled our ideas together. We used banana jacks on the back of the amps for easy "hands free" bias, and also incorporated an 8 pin octal socket that can be easily wired to power the acrosound S1001 preamp.

One beauty of this amp is the PCB boards (designed by Brice) that we used, which includes the newly designed power supply and Power Transformer, something the original 20-20 was lacking. The power supply board is a tight, but perfect fit. I am a fan of point to point wiring, but using the PS board turned out beautifully. Also, discrete LED's are mounted on the front of the amp next to the power switch.

It just so happened that I was the first one to assemble a pair of the amps, which was definitely fun.

Completed PS board ready for installation:

















I have rolled many of my favorite preamps with our Acro-20's including:

Brook 4B:



Scott 121a:

 

[trainbuftony]

My kind of crazy.
Is transcendar still operating?
Can I order the transformers?

 

[Brice]

Absolutely: Reference # TRANSFORMER BH-040-OT (TO610)

 

[trainbuftony]

You are a god amongst men.
A gentleman and a scholar.
Both of you.

 

[Onebean]

I had the good fortune of meeting Brice through a thread I started when I was searching for a McIntosh tube amp. Through discussions on that thread and in PM’s, he introduced me to Acrosound. I ended up purchasing a pair of ULII’s, and quickly fell in love with their sound. When Brice suggested we team up with Ethan to re-create a mono amp version of the Stereo 20-20 amp using EL84 tubes I was instantly on board. I really like the EL84, and could hardly wait to hear an EL84 based Acrosound amp. My roll in the build was chassis and tube cage design and fabricate. I started with a rough sketch Brice provided, and the general goal was to keep the proportions of the ULII amp, but shrink it. As you have seen, it worked out fantastic. Here’s a general overview of the chassis and tube cage design and build.

Starting from this sketch, I modeled the chassis and transformer cans up to get an idea of how it would look. Then Brice sent me a scaled view of the power supply boards, and all the part numbers for the components, so I could model the power supply board up completely. Once we thought we had the design nailed down, I built the first prototype chassis on the laser and press brake at my workplace. This is also how I have access to nice 3D software to model up the parts.



Once I had the first chassis cut and bent, I purchased all the hardware, powder coated it, and built a mock up off the amp.





I thought it looked fantastic, but it became obvious very quickly we needed to resize the chassis slightly. It was a bit to narrow for the PS board design, and a bit too short for the PS caps. Overall the concept was solid, so I designed a revision 2 stretching it just a bit. Brice also asked for some ventilation holes on one end cap near the PS, and we discussed a few changes on fasteners and assembly to keep the faceplate and top cleaner looking. The result of these changes gave us our rev 2 chassis, and my first attempt at building the tube cage.

 

[Onebean]

Revision 2 of the chassis allowed room for the PS board and components, the switch, a newly identified choke that was a bit smaller than revision 1, the IEC jack, and LED’s. It’s not the easiest to assemble, but it fit. My next task was to complete the cage design, and figure out a way to build it. We really wanted it to match the style of the ULII cages, and I feel like we nailed it. I was able to source the perforated material in the same thickness, hole size, and hole spacing as the originals 20-20. I designed the cage frame, and we cut it out on the laser.



From here, it got more hands on, and less machine made. I had some scrap hickory wood that I made forms from. I cut and sanded these to paper templates made from the CAD model. The idea was to bend the steel around these forms, then use them to hold everything in place while welding the cages.





Once I had the forms built, I started hand bending the frame and perforated material.

 

[Onebean]

The next step in the cage construction was to hold the frame at the correct width in a wood jig, insert the solid side material, and spot weld it into place. I used an inexpensive spot welder I purchased at Harbor Freight, and it worked really good. Once these were spot welded in, the cage frame was really strong.




The next step was bending the perf material, and spot welding it to the cage frame. I made another hickory wood form to bend the perf material before fitting it to the cage frame. This form was smaller than the main frame form, because the perf material needed to slide inside the frame. It fit really nicely. Once it was set in place, I used the spot welder to hold it to the frame.






Now I just had to repeat the process for enough cages for 6 amps, plus a few extras just in case.

 

[onlysleeping]

This looks awesome!!

 

[Onebean]

Next up was to powder coat the chassis and cages, and get the amp badges made. I color matched the Acrosound brown to my ULII amps, and ordered the powder. We have a small batch manual powder coat oven at my work, so I cycled all the parts through, making everything look a nice shiny Brown.





The shiny new fasteners looked a little out of place on the brown chassis, so I did some aging on the hardware with a combination of vinegar, and Muriatic acid (work in a well vented area!) The fasteners looked much better taking the bright shine off them. Brice had a painter local to him, and he handled the gold on the transformer cans, which turn out amazing. The last piece to tackle was the brass faceplate. I sourced a piece of .04” brass, and polished it up to remove any surface in-perfections. Then I cut them out on our water jet machine at work, and took them to a friend who owns a trophy shop. Brice and his son did the layout work on the logo, and sent it to me. I forwarded it to my friend, and he used a special printing machine to lay the logo down on the brass.




Now all I had to do was transport these chassis damage free to Brice and Ethan, so they could melt some solder. Having the chassis modeled up made it easy to water jet custom foam packing to keep these safe during transit. I wrapped them up, and sent them on their way. Fortunately they made it damage free to Brice and Ethan, and my amps made it back to me in the same packaging damage free.



And the finished amp with its big brother the ULII setting behind it

 

[Brice]

For the power supply, I reused the PT of the Acrosound 20-20 Heyboer made for me (part number HTS-12848 125VAC primary, 155vac@.6amps & 6.3VCT@4 amps secondary).

My requirements were:

- I wanted to add a delay B+ built in, to avoid cold startup issues due to the direct coupling design.

- I had to still use a voltage doubler to generate the B+, but I wanted to have enough capacitance reserve
, and an excellent filtration with electrolytic and film caps, snubber network, etc..

- The PB board should also have a 6.3VAC fuse, which also would shut off the B+ supply if it blew.

- The PS should be the star point for the grounding scheme, with all the starting points away from the PS caps ground.

- The option to have the chassis to PS ground, or the the earth ground from the EIC, with just a jumper on the board.

- Keeping all the PS function under the PT, and have a plate to separate the PS section
from the amplifier section. While a good design idea, this made limited space to fill
all the PS elements under the chassis.


For the time delay, I simply used a NE555 in monostable config with a relay.
To power that IC, I had to create a small voltage doubler from the 6.3VAC to 12VDC.
And if the 6.3VAC fuse blows, this voltage doubler stops, disengaging the
relay which would stop the B+. That little supply powers the red LED too.

The supply is really simple. A full wave integrated SS bridge, the voltage doubler with 2 caps
of 180uF 400V, a choke 1.5H 200mA, 2 x 220uF 500V caps with a 3.3uF 600V film cap in parallel.
The PCB has also additional and decoupling caps onboard.

This B+ supply is not fused on the PS board directly but on the amplifier board.
You're never too careful with power tubes DC design.
Of course we also have a main fuse on the IEC primary.

The PCB is fixed to the chassis with 2 studs from the power transformer. So with a
jumper we can connect the chassis to the PS ground, or from the EIC earth ground.

I did use earth ground for OneBean's amps and mine. I believe MultiChamp12 used the PS ground.
In both cases all the amplifiers were totally silent.

At the end, all my requirements were met.

And I was lucky that only 1 PCB print was necessary: sometimes I don't mess things up


Here is my time delay prototype:


Then the PS:


The supply at play and loaded.



The supply installed: a tied fit but all good and safe at the end.

 

[Brice]

Various P1 views:

 

[Brice]

Various P2 views

 

[Brice]

Final built:

 

[trainbuftony]

@Lavane, if you'd come chill this would be us.
In all seriousness we should give something like this a shot, or entertain it. We live in the heart of ag land and anything buildable can be built here...
I've got skills and so do you, and you're close enough...
I firmly believe we make better decisions in small affinity groups like these guys have formed. We could pull off more together than separately, and do a way better job of it. We both could learn a whole lot too.

Apes together strong...

This is just unbelievable, I can't believe it, Jaw on floor, drool puddling...

Holy mackerel.

 

[tube-a-lou]

What a great project, it looks terrific.

 

[Lavane]

This thread is amazing! I wish I still had contact with all the fabricators I worked with 30 years ago. We has access to really nice metal working equipment. Presses, breaks, sheers, mig and tig welders, punches and pattern tables.

 

[PakProtector]

That looks Factory. Shoulda been a product... would be going for insane $$$ if they did.
cheers,
Douglas

 

[nj pheonix]

 

[rustnrot]

I love replicas. Replica old boats, old cars, old amps. But this is a level above, a replica of an amp that was never built!

Regarding the transformer covers, I like the rounded edges of them similar to the transformer covers on the McIntosh MC30, MC240, MC275, etc. Did you make those covers or did you buy them? I've looked a lot over the past couple of years and have not seen those sold anywhere.