I'd probably make a chassis with a wood base and a flat sheet metal top. I have drill press(es) and I can sharpen. I could make a 1" hole in sheet metal by "ruining" a cheap wood bit, and resharpen it for multiple uses. I am open to comments on this plan.
My hobby is "messing around with technical stuff." I have built musical instruments that are actually useful, added wires to our existing home (without burning it down) and repaired a variety of yard equipment and home appliances. I understand the value of short, neat wiring. Like I said my limited soldering experience has produced ugly joints, but I would buy $15 worth of resistors and terminal strips and practice before I launched into an amp. If I get another old tube amp from a big name maker I would probably just write the check and get it rebuilt, I was very happy with the man who rebuilt the last one.
@StepheK - If your home brew amp costs $600 what cheaper options are out there? I do not need earth shattering power. My Magnavox is 10-ish watts per channel theoretically, into a 4-ohm load, my speakers are higher, more like 8. I am happy with that.
Saying "I built it" might be nice.
Just have a strong premium high ampere drill and hold on tight with both hands. Like a Milwaukee 1/2" Hole-Shooter drill. Clamp down your work part firmly. With the larger sizes, 1'+ especially. A lot of torque envolved. In steel and stainless is very tough going.
Do want to Caution anyone who hasn't used a Step bit. I've drilled a few holes with larger step bits, for drill life testing. They can injure your wrists and drills, if not careful!!!
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The iron/transformers are a fixed cost and even decent ones new aren't cheap. Cheaper options are visiting yard sales and thrift stores looking for old tube amp consoles and pulling them out in hopes of finding something good or again yard sale and hope someone has some old tube gear for sale that has good iron. It's hard to build a nice tube amp with new parts for under $500.
Thanks!
JohnT123
Super Member
For the home brew crowd I strongly recommend using Aluminum for the chassis or top plate.
It's easy to cut and will yield great results using hand tools, hole saws and stepper bits. It's also a very low noise metal for electrical projects. I have to build my stuff in my living room, so no heavy machinery is available.
If you use steel, it all becomes more difficult
Is there an advantage to bent aluminum versus flat top on wooden sides? I am not arguing - I have done neither, I have seen both online. My problem with bending aluminum is 1) I have to measure very precisely, wood can be shaved down until it is a perfect rectangle and 2) I am afraid the bends will not be straight, causing me to scrap a lot of metal. I am pretty confident with wood.
Aluminum plate tops on a wood base are a very common and accepted method of home-brew tube amplifier construction.
One recent build that I liked a lot was this one:
https://audiokarma.org/forums/index.php?threads/my-je-labs-el34-build.981321/
It used aluminum plates on the base so you could mount the connections/swiches on the front/back like a traditional chassis, but didn't require any bends.
Another alternative is to use a pre-fabbed Hammond (or other) aluminum box, but they tend to be on the thinner side and may require bracing if heavier transformers are utilized.
One recent build that I liked a lot was this one:
https://audiokarma.org/forums/index.php?threads/my-je-labs-el34-build.981321/
It used aluminum plates on the base so you could mount the connections/swiches on the front/back like a traditional chassis, but didn't require any bends.
Another alternative is to use a pre-fabbed Hammond (or other) aluminum box, but they tend to be on the thinner side and may require bracing if heavier transformers are utilized.
That is a nice one!
trainbuftony
Electron Herder
I tend to like to have aluminum surrounding the entire circuit.
When I order a hammond box I do so with the full knowledge that I'll need reinforcement struts under the chassis.
I have a policy of always trying to spin a problem into a solution...
Put the mounting screws of your transformer into the strut, and use the strut to mount the components under the chassis so the screws dont come through the top.
If you use 2" angle for the reinforcement, you can mount the can caps, chokes, and ss regulators to the strut.
Makes a great heat sink too.
Wrapping the whole circuit protects it from radiated fields from all directions, which is preferred for sensitive circuits, but not a huge deal on power amps.
When I order a hammond box I do so with the full knowledge that I'll need reinforcement struts under the chassis.
I have a policy of always trying to spin a problem into a solution...
Put the mounting screws of your transformer into the strut, and use the strut to mount the components under the chassis so the screws dont come through the top.
If you use 2" angle for the reinforcement, you can mount the can caps, chokes, and ss regulators to the strut.
Makes a great heat sink too.
Wrapping the whole circuit protects it from radiated fields from all directions, which is preferred for sensitive circuits, but not a huge deal on power amps.
@JohnT123 @Squidward @trainbuftony
Do aluminum chassis cause more care to be given to grounding with dissimilar metals?
Do aluminum chassis cause more care to be given to grounding with dissimilar metals?
Thanks for pointing this out, when I looked at the linked build I didn't pay any attention to the chassis, just looked at the circuit.
I don't believe so. Just make sure that all the metal bits are well connected and you should have very low resistance between each piece, for safety.
To be clear, I am not of the school of thought that you should use the chassis as a ground bus (this construction technique is used sometimes in vintage pieces, especially). I think for the actual circuit part of the build you should use a discreet bus.
Thanks for pointing this out, when I looked at the linked build I didn't pay any attention to the chassis, just looked at the circuit.
That's one of my favorite parts about this hobby, especially the DIY aspect: I'm constantly learning something. I really enjoy looking at everyone's different builds. Each person brings a different aspect of craftsmanship to the table, and I find that I always have something I can learn from just about anyone.
I might be worried about brass and aluminum. Even then put some grease on the connection to keep the oxygen out and it will be fine.
trainbuftony
Electron Herder
I havent had any issues yet.
I have all of my audio grounds soldered to a Romex bus wire.
Same on the power supply side.
The chassis will be the higher impedance ground vs the fat copper wire soldered between the capacitors negative terminal and my "house earth"
Any stray electrical signals which touch the chassis must have not only a low impedance path to earth ground, there must be no path for signal (including power supply ripple current) to back feed onto our audio signal ground. Same for case ground it should pass those signals directly o the house earth terminal and never on a conductor that has signal or power grounds.
Electrolysis between parts, I havent seen yet, that's more of a problem in dirty environments.
I remember working on a Porsche transmission made of magnesium, the bolts all had a little plastic seal to keep crud out, crud leads to electrolysis, and eventually your bolts corrode into your case and the whole thing is scrapped.
I'm not building magnesium sand rails, so horses for courses.
Btw, the "cage" on that "rail" was made of black iron plumbing pipe.
Not chromoly, not DOM, plumbing pipe, threads still attached.
Waste of a perfectly good Porsche motor if you ask me...
Back on topic...
You shouldn't rely on screws and contact for ground, unless you have to.
For an aluminum chassis, you can't exactly solder to, use a star washer ground lug, and screw it into clean threads, and dont pass signal current through it.
Back to drive lines for a minute.
In learning how they work one of our tasks was to list power flow.
Starting at the torque converter and ending at the tail shaft, we were expected to list every part in order that the power moved through in each gear. At one point I could do it too.
Same goes for currents in your amplifier.
You know your design much better when you have a running list in your head of the components that each individual circuit passes through and the order, and direction in which it does so..
All for a better understanding of everything.
I have all of my audio grounds soldered to a Romex bus wire.
Same on the power supply side.
The chassis will be the higher impedance ground vs the fat copper wire soldered between the capacitors negative terminal and my "house earth"
Any stray electrical signals which touch the chassis must have not only a low impedance path to earth ground, there must be no path for signal (including power supply ripple current) to back feed onto our audio signal ground. Same for case ground it should pass those signals directly o the house earth terminal and never on a conductor that has signal or power grounds.
Electrolysis between parts, I havent seen yet, that's more of a problem in dirty environments.
I remember working on a Porsche transmission made of magnesium, the bolts all had a little plastic seal to keep crud out, crud leads to electrolysis, and eventually your bolts corrode into your case and the whole thing is scrapped.
I'm not building magnesium sand rails, so horses for courses.
Btw, the "cage" on that "rail" was made of black iron plumbing pipe.
Not chromoly, not DOM, plumbing pipe, threads still attached.
Waste of a perfectly good Porsche motor if you ask me...
Back on topic...
You shouldn't rely on screws and contact for ground, unless you have to.
For an aluminum chassis, you can't exactly solder to, use a star washer ground lug, and screw it into clean threads, and dont pass signal current through it.
Back to drive lines for a minute.
In learning how they work one of our tasks was to list power flow.
Starting at the torque converter and ending at the tail shaft, we were expected to list every part in order that the power moved through in each gear. At one point I could do it too.
Same goes for currents in your amplifier.
You know your design much better when you have a running list in your head of the components that each individual circuit passes through and the order, and direction in which it does so..
All for a better understanding of everything.
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BMC
Active Member
So following up on doing Stephe’s mods…I added the plate to plate feedback. I think I will keep it this way. The bass is sounding better than it ever did. Unmodded, acoustic bass sounded nasally. Now it has the right timbre. Overall a more linear response.
Stephe - Thanks again for the videos, schematic and BOM. Made it really easy.
I recently lucked into a pair of Forte II’s with the Crites mods, so I don’t even think I need a sub now.
Stephe - Thanks again for the videos, schematic and BOM. Made it really easy.
I recently lucked into a pair of Forte II’s with the Crites mods, so I don’t even think I need a sub now.
FlaCharlie
Super Member
The socket rewire is very simple. On both tubes, Pins 1,2,3,6,7 and 8 are the same. The only thing that changes is the heater wiring.
The 6n2 has the 6.3v heater wires hooked up to pins 4 and 5. To use a 12AX7 at 6.3v you just disconnect the wire from pin 5 and move it to Pin 9. If the 6N2 has Pin 9 (which is an internal shield) connected somewhere, remove that connection. Then just connect pins 4 and 5 together.
So for the 12AX7 one wire goes to Pins 4 and 5 and the other goes to Pin 9.
Remember, pins are numbered clockwise when viewed from the bottom.