Some time ago I built my first tube amplifier, a 6A5G single-ended design. The results were great for a first project, but I decided to revisit it with more experience and make some circuit / aesthetic changes. The original had an anodized aluminum finish in gold made by Landfall Systems, but over time I decided it was a bit much and wanted more flexibility with color choices in the future. I put together a at-home powder coating setup. I ordered a bare, raw aluminum chassis from Landfall and drilled according to my planned layout. Here is some of the equipment.
Makita air compressor with in-line air filter
Eastwood dual-voltage powder coating gun
The largest toaster oven money can buy, can coat up to a 16.5 inch wide chassis with this oven.
Here is the top plate after masking and coating, silicon plugs were used to fill tight tolerance holes.
Some powder wrapped around to the interior of the top plate, affecting the metal-to-metal chassis ground connection. I sanded it down with 80 grit to remove the coating, then 180 grit to smooth things out.
Here is the interior after the build was completed, I can provide a schematic if someone is very curious, but it is a pretty straightforward SET design with a CCS loaded common cathode gain stage. Here are the highlights:
-GEC U18/20 rectifier (SS hybrid), CLCLCRC passive supply with Lundahl chokes, Clarity Caps on the 6A5G PS tap
-Cascode CCS loaded gain stage with mu output, MH4 / REN904 / AC/HL drivers, unbypassed Riken carbon film cathode resistor, Miflex copper coupling caps
-6A5G output with Mills MRA10 cathode resistor, Clarity Cap bypass caps
-OPT Lundahl LL1620 60mA
And some pictures of the completed amplifier. The color is called Stage Pass Bronze from Prismatic Powders.
Rear panel with WBT binding posts, RCA inputs, Goldpoint speaker / headphone output switch.
Here is the amp with a pair of Mazda AC/HL drivers, Philco (Sylvania) 6A5G outputs, GEC U18/20 rectifier.
And in my stereo with French Visseaux 6A5G.
The sound is fantastic, paired with my recently restored Snell J/II, this is roughly a 3.75W into 8ohm amplifier, here are some measurements.
1W into 8ohms FFT
Frequency response, down about 1dB at 20Hz and 20kHz
So the powder coating was a success, very happy with the result, going to use this approach on future amplifiers, thanks for checking it out!
Makita air compressor with in-line air filter
Eastwood dual-voltage powder coating gun
The largest toaster oven money can buy, can coat up to a 16.5 inch wide chassis with this oven.
Here is the top plate after masking and coating, silicon plugs were used to fill tight tolerance holes.
Some powder wrapped around to the interior of the top plate, affecting the metal-to-metal chassis ground connection. I sanded it down with 80 grit to remove the coating, then 180 grit to smooth things out.
Here is the interior after the build was completed, I can provide a schematic if someone is very curious, but it is a pretty straightforward SET design with a CCS loaded common cathode gain stage. Here are the highlights:
-GEC U18/20 rectifier (SS hybrid), CLCLCRC passive supply with Lundahl chokes, Clarity Caps on the 6A5G PS tap
-Cascode CCS loaded gain stage with mu output, MH4 / REN904 / AC/HL drivers, unbypassed Riken carbon film cathode resistor, Miflex copper coupling caps
-6A5G output with Mills MRA10 cathode resistor, Clarity Cap bypass caps
-OPT Lundahl LL1620 60mA
And some pictures of the completed amplifier. The color is called Stage Pass Bronze from Prismatic Powders.
Rear panel with WBT binding posts, RCA inputs, Goldpoint speaker / headphone output switch.
Here is the amp with a pair of Mazda AC/HL drivers, Philco (Sylvania) 6A5G outputs, GEC U18/20 rectifier.
And in my stereo with French Visseaux 6A5G.
The sound is fantastic, paired with my recently restored Snell J/II, this is roughly a 3.75W into 8ohm amplifier, here are some measurements.
1W into 8ohms FFT
Frequency response, down about 1dB at 20Hz and 20kHz
So the powder coating was a success, very happy with the result, going to use this approach on future amplifiers, thanks for checking it out!
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Now that is a nice looking amp. I have been looking at refinishing metal amp cases recently, from 70's models mostly. I had come to the conclusion that powder coating just wouldn't look good, this has certainly changed my mind. How much would you estimate the equipment cost to be? I have that same compressor already, so minus that.
Very nice! I love seeing project posts. I have some questions if you dont mind...
1) How is the adhesion on aluminum? I assume it will easily pass a fingernail scratch test. Any other way to gauge the durability you got in the end?
2) To what grit did you sand the surface before application? Any other surface prep?
3) What size sprayer nozzle do you need for powder coat?
4) Can I use my HVLP (high volume low pressure) sprayer? I have a 5 stage turbo sprayer for fine automotive/motorcycle work. I see you're using a compressor sprayer.
5) Toaster oven mmm! We've had one of those unused in the basement for years, Christmas gift.
6) How many coats before it went in the oven?
7) How long in the oven and how hot?
You have some high end parts there too, its beautiful, something to be very proud of. The tranny covers are very cool too.
I'm tooling up for making chassis now too, easily in numbers, so this is timely. I'm going with .125 aluminum U bent to form the front top and rear panels from one sheet, then sides with wood. .125 is too thick to bend without surface stretching/splitting and getting it square is impossible even with my hydraulic brake. So will be cutting a backside V groove to bend only 1/32 inch, then braze a bead on the backside of the bend at exactly 90 degrees for permanent strength. the groove method also gives a much smaller radius bend, looks nicer.
1) How is the adhesion on aluminum? I assume it will easily pass a fingernail scratch test. Any other way to gauge the durability you got in the end?
2) To what grit did you sand the surface before application? Any other surface prep?
3) What size sprayer nozzle do you need for powder coat?
4) Can I use my HVLP (high volume low pressure) sprayer? I have a 5 stage turbo sprayer for fine automotive/motorcycle work. I see you're using a compressor sprayer.
5) Toaster oven mmm! We've had one of those unused in the basement for years, Christmas gift.
6) How many coats before it went in the oven?
7) How long in the oven and how hot?
You have some high end parts there too, its beautiful, something to be very proud of. The tranny covers are very cool too.
I'm tooling up for making chassis now too, easily in numbers, so this is timely. I'm going with .125 aluminum U bent to form the front top and rear panels from one sheet, then sides with wood. .125 is too thick to bend without surface stretching/splitting and getting it square is impossible even with my hydraulic brake. So will be cutting a backside V groove to bend only 1/32 inch, then braze a bead on the backside of the bend at exactly 90 degrees for permanent strength. the groove method also gives a much smaller radius bend, looks nicer.
I would open up the tolerances on those holes ever so slightly so you don't have to deal with those obnoxious plugs.
I struggled also with the wire racks that the toaster ovens come with, and instead I switched to a small cookie sheet and I use 9 pin PCB sockets upside down to space parts off the cookie sheet.
Did you set this one up to also be able to run 6B4Gs?
I struggled also with the wire racks that the toaster ovens come with, and instead I switched to a small cookie sheet and I use 9 pin PCB sockets upside down to space parts off the cookie sheet.
Did you set this one up to also be able to run 6B4Gs?
Thank you all for the compliments!
Linearojg1976 - as far as the cost goes, here is a breakdown, minus the air compressor.
Toaster oven - $120 (determines the size of the chassis that can be baked)
Eastwood painting stand - $40
Silicon plug kit - $40
High temperature polyester masking tape - $9 per roll (I bought 1/8" and 1/2")
In-line air filter - $10
Air pressure regulator - $15
Eastwood pre-painting prep aerosol - $10
Powder - roughly $15 / lbs from Prismatic Powders, depending on the type
So that comes out to somewhere around $250-275.
Windcrest77 - I am new to powder coating myself, but I'll do my best to answer your questions.
1) adhesion on aluminum is good, I have not done a specific durability test, but I know that powder coating is often used on aluminum car wheels, so obviously durability is key in that case.
2) the raw aluminum chassis was machined by Landfall Systems, the top plate is sheet metal from the look of it and the side panels are extruded, all very smooth so no sanding necessary. If I were sanding, I would think 220 grit would get you a nice smooth finish with the powder applied, it melts, flows, and cures so any minor imperfections on the metal surface do not show through, assuming they are not significant gashes, scratches, etc. For prep, I cleaned with a Eastwood's pre painting spray, masked the portions I did not want powder coated on the interior with the high temperature tape, filled tight tolerance holes with silicon plugs, went over the part with a tack rag to remove any dust, then went over it once more with my air compressor blow gun. I did a close visual inspection before spraying to be sure no fibers or dust were visible before spraying, as that will create speckling / spots on the finish. The great part about powder coating is if you screw up the coating, you can blow it right off and respray.
3/4) To be honest, I'm not familiar with that equipment, so you might have to do some Googling to find out. There are several different powder coat guns available on the market, the one I purchased from Eastwood is very entry level, whereas a more professional model for high-volume work could cost as much as $1000 from what I have gathered.
5) Yup, the bigger the better! Unless you are making a very small chassis. Powder coating car wheels is a very common practice, in which case shops use full size ovens.
6/7) It is a single coat, the oven temperature and baking time are depending on the specific powder being used. Prismatic Powders (and other powder suppliers) have instructions on how long and how hot to bake. This particular powder I used was baked for 12min at 400F with the temperature of the parts checked with an IR thermometer. Note that the time starts when the part reaches 400F, not when placed in the oven, which can take several minutes.
Hope that helps 🙂
Linearojg1976 - as far as the cost goes, here is a breakdown, minus the air compressor.
Toaster oven - $120 (determines the size of the chassis that can be baked)
Eastwood painting stand - $40
Silicon plug kit - $40
High temperature polyester masking tape - $9 per roll (I bought 1/8" and 1/2")
In-line air filter - $10
Air pressure regulator - $15
Eastwood pre-painting prep aerosol - $10
Powder - roughly $15 / lbs from Prismatic Powders, depending on the type
So that comes out to somewhere around $250-275.
Windcrest77 - I am new to powder coating myself, but I'll do my best to answer your questions.
1) adhesion on aluminum is good, I have not done a specific durability test, but I know that powder coating is often used on aluminum car wheels, so obviously durability is key in that case.
2) the raw aluminum chassis was machined by Landfall Systems, the top plate is sheet metal from the look of it and the side panels are extruded, all very smooth so no sanding necessary. If I were sanding, I would think 220 grit would get you a nice smooth finish with the powder applied, it melts, flows, and cures so any minor imperfections on the metal surface do not show through, assuming they are not significant gashes, scratches, etc. For prep, I cleaned with a Eastwood's pre painting spray, masked the portions I did not want powder coated on the interior with the high temperature tape, filled tight tolerance holes with silicon plugs, went over the part with a tack rag to remove any dust, then went over it once more with my air compressor blow gun. I did a close visual inspection before spraying to be sure no fibers or dust were visible before spraying, as that will create speckling / spots on the finish. The great part about powder coating is if you screw up the coating, you can blow it right off and respray.
3/4) To be honest, I'm not familiar with that equipment, so you might have to do some Googling to find out. There are several different powder coat guns available on the market, the one I purchased from Eastwood is very entry level, whereas a more professional model for high-volume work could cost as much as $1000 from what I have gathered.
5) Yup, the bigger the better! Unless you are making a very small chassis. Powder coating car wheels is a very common practice, in which case shops use full size ovens.
6/7) It is a single coat, the oven temperature and baking time are depending on the specific powder being used. Prismatic Powders (and other powder suppliers) have instructions on how long and how hot to bake. This particular powder I used was baked for 12min at 400F with the temperature of the parts checked with an IR thermometer. Note that the time starts when the part reaches 400F, not when placed in the oven, which can take several minutes.
Hope that helps 🙂
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Hey audiowize - yes, the plugs are a bit of a pain, I'll probably leave them out in the future.
The wire racks are a bit of a struggle too - the spraying stand I purchased is made for thicker full-sized oven racks and is not adjustable, so I had to wedge a piece of metal between the clamp and the thinner toaster oven rack to get it to sit level, not ideal, I like your cookie sheet idea.
No this amp was made for 6A5G only, I could run the 6B4G on AC, but the issue is the 6A5G heaters are center-tapped and ground-referenced through the cathode, as such the heaters are not center-tapped by themselves.
The wire racks are a bit of a struggle too - the spraying stand I purchased is made for thicker full-sized oven racks and is not adjustable, so I had to wedge a piece of metal between the clamp and the thinner toaster oven rack to get it to sit level, not ideal, I like your cookie sheet idea.
No this amp was made for 6A5G only, I could run the 6B4G on AC, but the issue is the 6A5G heaters are center-tapped and ground-referenced through the cathode, as such the heaters are not center-tapped by themselves.
Well, you could add a resistor from pin 7 to pin 8 and a resistor from pin 2 to pin 8, say 22 ohms, then the 6B4G would work. AC heating is indeed an issue, and there isn't a whole ton of room left in there for a pair of DC supplies!
True, I could do a parallel center-tap, but I think I will just stick to the 6A5G. I have quite a collection so no worries on backups. Get this though, I have a pair of French Visseaux 6A5G, they manufactured them under Sylvania license, the only other tube manufacturer to make them. BUT, for whtever reason, they didn't include cathode sleeves! Only discovered this after purchasing, they are essentially a 6B4G-6A5G hybrid, or a 6B4G with the heaters center-tapped, directly-heated.
So if I ever get urge to listen to that sweet AC hum on the background, can always pop those in 🙂
So if I ever get urge to listen to that sweet AC hum on the background, can always pop those in 🙂