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[K0JKJ]

 A Swan Mark 1 modified to use 3-500's currently on Ebay got me to look at the Mark 1 that I have that has the original 3-400Z's. Despite what the listing  and the lister says there is not enough head room to just install 3-500's in place of 3-400's without some sort of modification. There is only 0.420” of clearance from a straight edge between the front panel and the fan bracket.  Since the 3-500 is physically just a 3-400 with 0.500" more plate in height and the tube is 0.500" taller there must be a way to upgrade without major modes.

The modified unit looks to have HR-6 plate caps which I believe are 0.375" tall.  Does any one know the actual height and diameter of the HR-6 plate caps?  The plate caps on my Mark 1 are 0.675" tall . One is 0.855" in diameter and the other is 0.770".   Replacing the tube socket standoffs with ones that are 0.250" shorter should give the same  headroom clearance.

Since the 3-500s would run warmer I am not sure about how  the change in plate cap area or air flow  due to less socket gap that would impact the tube cooling. But it looks like the combination of a shorter tube cap and standoffs would give enough clearance.

Does this sound like a doable reliable mod to the unit. I don't want to invest in a pair of 3-500's and then have problems with heat dissipation or melting filament leads.

[W2LO]

 Buying any piece of used equipment is a process that requires careful examination of the item, the seller's reputation, etc. Many of these things are difficult to ascertain in a web-based purchase.
 When you add to this the issue of modifications to a piece of equipment, that, for me, is where I would draw the line and not get involved. There are just too many things that can be wrong. I'd pass on this item and look for something else.

    W2LO

[WB2WIK]

You should always type a SUBJECT in the "subject" line...

Anyway, the Mark I should be modifiable for 3-500Zs, as I did this many (many!) years ago on an old Mark-VI, which is the six meter version of the same amp.  It originally used 3-400Zs also, and of course within about one year, Eimac discontinued those (like in about 1969 or something).

My mod is a hazy memory now, but I recall mounting the tube sockets down lower in the chassis, using standoffs to drop the sockets down, and that created enough clearance for the 500Zs to fit just fine without changing anything else.

The original cooling fan seemed to blow enough air in the right places that it didn't require any changes.

I added a few volts of cathode bias with a 50W or 100W stud zener, as the idling current with the 500Zs was a bit high until a bit of positive cathode bias was added to bring it down some.  I think in the end, I had about 100mA idling current for the pair of tubes.

This was not a major modification, and I'm pretty sure the Mark I sheet metal is the same design, maybe even the same metal, as the Mark-VI.

Obviously, a sloppy job can be done anytime one wishes to be sloppy!  But mine came out fine, and I sold the amp for a very premium price years later.

WB2WIK/6

[K0JKJ]

I am not looking to bid on the one on Ebay. I have one that I picked up at a fair price for what it is.
I am looking to get the one I have on the air.  
I had a QSO while I was  HF mobile witht a guy that had a couple of Mark 1's and  several 3-400's. But they are approaching unobtainium status.  And I lost his call. So I am looking for a way to use 3-500's which are fairly common.

 It seems that the shorter socket standoffs and a shorter plate cap is the way to go. I would like to keep the external appearance as close to stock as possible. If fact I am thinking about re-stuffing the original caps instead of going the axial caps mounted on plexi-glass that the one of Ebay has.

If it is possible to add an extra contact to the band switch or find one with the same detent angle I'll consider adding 160 meters to mine with an additional coil.  I had priced a new cover for the unit but that was too rich for my blood.

As for the lack of a subject, I tried to correct a typo on my first try on this posting and didn't realize the less than friendly software would not only erase my posting but also the subject. Live and learn

[WB2WIK]

K0JKJ, if you need a new cover you might consider making a drawing of what you need and bringing it around to a few local sheet metal shops for bid.

Around here, when a shop isn't busy they'll run single-piece odd jobs for almost nothing, just to keep the machines and workers busy.  I've had covers custom fabricated, including vent hole punch patterns and paint or powder-coat baking, very inexpensively.  

Good luck!

WB2WIK/6