Metal choice for 1.5kW vacuum tube amplifier enclosure.

Discussion in 'Amateur Radio Amplifiers' started by SP4IT, Jan 14, 2022 at 11:06 AM.

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  1. SP4IT

    SP4IT Ham Member QRZ Page

    I was planning to make an enclosure with aluminium sheet until I saw latest prices of it locally.

    I do have lots of sheet steel (normal mild steel) in various thicknesses and some stainless.

    So I'm considering all steel construction now, but I read somewhere one is not supposed to have ferrous steel anywhere near the vacuum tube anode, the tank circuit etc and that those sections have to be shielded from steel parts with aluminium or copper. Otherwise steel will magnetically couple to the circuit and it will cause eddy current losses.

    I would consider stainless, but it is horrible to tap for tiny m3 screws without special carbide taps. So normal steel seems the best option for me.

    The eddy currents precaution seems to make sense, but at the same time I saw lots of amp pictures with steel enclosures. So what is the truth? Can I use 2mm thick (80 thou) sheet steel for the chassis floor, the front, the back and dividers? Then I planned to use thinner steel sheet (0.7mm) bent to make a top cover.

    What do you think?
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I think steel works fine. I love stainless steel.

    Rather than tap it, why not drill holes for threaded inserts? Might need a machine to press them in, but they work well and are very nice.

    Don't know what you have available locally, but if you create very well dimensioned 2d drawings or better still a CAD model of what you want and take it to a good sheet metal shop with CNC brakes and punches (and of course pay them something for this) that saves work and creates a professional result. We have so many such shops around here in competition with each other it seems they go from very busy to not busy at all quite often -- when they're not busy they'd rather work on anything than pay workers to clean machines.:p And they all have presses to install PEM inserts in about two seconds each, and have the inserts in stock.:)
    KD2ACO likes this.
  3. SP4IT

    SP4IT Ham Member QRZ Page

    Thanks for letting me know steel works fine :) I wonder if thickness matters. How far would you say the output tank circuit main coil has to be from a 2mm steel wall not to be affected by it?

    I too like stainless. Most local shops that sell sheet stainless can do cutting cheaply, but finding someone to do bending cheap was a problem for me in the past. So I tend to use a mix of tig welding and screws.

    For this design I like the flexibility of being able to just grab a cordless drill. Drill a hole, tap it and attach something. This chassis is probably going to be used temporarily for the amp. Then I'll design a better one for it and I'll repurpose the chassis for something else.

    Unless someone tells me I'm going to have issues with eddy currents, coil inductances etc I'm almost set on using mild steel at this stage.
  4. WB7OKU

    WB7OKU Ham Member QRZ Page

    The cover on my Yaesu FL-2100B amplifier is steel, as is the cover on my Icom IC-746 transceiver. On the tube amp the chassis and PA cage are aluminum.
  5. SP4IT

    SP4IT Ham Member QRZ Page

    Interesting. I saw some DIY and factory made amps that used steel (yellow galvanised) angle iron plus thin sheet steel covers(0.5mm~1mm, 20~40thou). Including without any additional internal boxes(just some dividers).

    The PA stage of my TS-520 is in an aluminium box too. I always assumed this is for safety of anyone doing the servicing,

    I wonder what would be the best way to measure steel's influence on the working of the amp. Perhaps by measuring ferrous elements temperature increase.

    For sure one can take an air coil, move a piece of steel nearby and the coil's inductance changes. However, how much this matters I don't know.
  6. SM0AOM

    SM0AOM Ham Member QRZ Page

    As a former designer of high-power RF circuits, I can say that the influence of steel panels nearby an inductor is governed primarily by how close the panel is, and by the field distribution around the inductor.

    For aluminium panels, a conservative "rule of thumb" for being able to disregard the influence on Q from shielding around inductors is a distance of one coil diameter.

    Steel has very varying magnetic properties, both with frequency and applied field strengths, so it is probably best to actually measure the Q of the inductor intended for use at different frequencies and distances from the steel shielding.

    Inductors intended for kW amplifiers usually have dimensions which place their unloaded Q-values in the 300 to 400 range, so the losses in a typical application are in the "few" percent range. A decrease in unloaded Q from 400 to 200 roughly means an increase of inductor loss from 12/400 or 3 % to 12/200 or 6 %.

    These increased losses will manifest themselves as slight heating of the shielding panels.

    WF9Q and SP4IT like this.
  7. SP4IT

    SP4IT Ham Member QRZ Page

    Thank you for this very precise answer :)
  8. K0IZ

    K0IZ Ham Member QRZ Page

    Here in US there are metal scrap yards, some specialize in iron/steel, others in aluminum, etc. Cost of aluminum is by the pound. If you can find some pieces that work, cost probably 1/3 new. 3003 type is a good choice.
  9. W9WQA

    W9WQA Ham Member QRZ Page

    i should have a hinged lid 19 inch cabinet if u pay shipping !!
    need cleanup and paint

    also prob most parts needed for amp x tube
  10. SP4IT

    SP4IT Ham Member QRZ Page

    Thank you for offering it. I'm trying to source as much as I can locally (I'm in Europe). This is mostly due to time required for shipping things across the pond cost effectively.

    Here, most scrap yards sell in bulk multiple train loads of stuff. Most of them don't want to have anything to do with members of the public buying individual pieces. There is one scrap yard nearby that is different. Unfortunately they only have steel and some stainless in form of flanges, pipes etc. Unfortunately, it is closed now. I was told by the owner they have to close down the business, because they can't afford to deal with all the red tape (new EU environmental rules for scrap dealers). It is a bit ridiculous, those rules are supposed to improve recycling and reuse, but they have precisely an opposite effect. The only scrap yard a person can walk in to had to close down. This is similar to CO2 costs that increased 10-fold this year. Lots of people that installed (green) gas boilers, go back to their dirty coal burners and they burn worst kind of illegal coal, trash etc. It is seriously awful, but I digress.

    There are also car breaking lots, but that's different. A lot of nice parts can be sourced there cheaply, not much metal stock.

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