The AM Forum

Do the old buzzard millen / national type plate caps offer any type of cooling effect on plate seals, or is it only a matter of insulation and mechanical and electrical fit?

I'd bet they do, but for old buzzard tubes, they aren't needed.

well, ceramic does handle heat well obviously, but I look at a 813 plate cap and think how spindly it is compared to say a 4-125 with the HR plate cap on it.

The 813 has a wire coming through the top of the tube but the 4-125 has a thick metal post that will transfer heat to the cap. A radiator will help keep the seal cool.
The little wire on the 813 will not transfer much heat so a ceramic cap will be fine.
the plate of the 4-125 is supported from the top of the tube to reduce output C so it needed to be stronger. That extra metal needs to be kept cool.

true. unless the tube is made for it internally, I guess HR-style plate caps would not be of much value.

the "old" H&K 4e27 has a thinner post but it's not what I would call a wire. Its a post all right. Just a thin one.

the Eimac 5-125B has both a ventilated base and a thick post at the top. For that, you get 50 more watts of plate rating.

thanks Frank, you clarified my thinking a good bit.  ;)

Then next phase of tube design was external anode where the seal is moved down near the base allowing higher operating temperatures at the top of the tube. also the support issue goes away with a solid metal plate sitting on a ceramic base. Heat likes to rise so it is easier to protect the seal. This also makes the tube smaller since there is no glass to protect near the plate connection.

Another thing to consider is that the plate cap on something like an 811a is a thin bit of formed sheet metal, soldered to a wire and held in place by "cement" - they frequently break loose.

If you use a metal plate cap with a set screw you stand a very good chance of crushing the metal part and so breaking the cement bond...

Otoh, I have seen where the ceramic type clips so strongly that it too breaks the cement bond on older tubes... a problem.

                    _-_-bear

same breakage of cement seen here. It can be helped by working with the metal contact inside the cap so that it is not too tight. The Old Buzzard phenolic caps with the wire straight up do not seem to have this issue as much. Different metal thickness and construction. I've changed all mine to these except 4 of them.

I prefer the National or Millen metal grips made of springy sheet metal that slips over the cap. If they get stuck, you can easily loosen them with a small thin screwdriver.  The ceramic or phenolic ones can only be removed by forcing them off (perhaps with the aid of a lubricating solvent such as WD-40 or PB Blaster), which often breaks the bonding material on the cap or even worse, breaks the seal on the glass envelope.

The National caps are optimum for buzzardly appearance, since that is what was most often used pre-WW2.  I have a beautiful set of plate caps for my HF-300's.  They were originally part of the E.F. Johnson plate connector assembly designed for tubes like the 849 or 204-A.  I removed them from the ceramic base and attached them to 3/4" copper strap to make plate cap connectors for the jumbo size plate caps.

I have a bunch of heat dissipating 833-A connectors salvaged from some military surplus equipment, that are turned out with a lathe, split into two sections, and held together with springs that also cause them to grip the plates.  I would be reluctant to use anything with a set screw.

I have some 450TH/TL tubes planned for duty, and I see in my old Eimac guide they just used plain old spring clips, the kind that are one piece of springy flat ribbon stuff looped back on itself.  Give 'em a pinch to go on/off.  No heat-radiating ones shown.  The older large series just had a lot more distance between the active bits and the connections maybe.

For those porcelain jobs, I always open them up a bit 9same as Opcom) so as not to make them grab too tight, for as Don said, there goes the seal....

For those black 'beehive' things (Opcom again), I've seen those on all sorts of old RCA broadcast gear.  Think BTA-1L & 250

In my ppp 100TH mod deck, I never came up with the grid ends for the tubes, and just used little alligator clips till I found some metal bits that would allow me to use plate caps.
That was at least 20 years ago and the alligator clips are still there....

I expected them to fail, but thay never did....

Brett

Fahnstock clips were the choice of many and Ive used them from 829B's t0 100TH's and 4E27A's. The ceramic caps look nice on 866's, 872's, HF-200/300's, 810's and any other tube that has a ceramic ring on top or side.

I use the full grip brass spring caps on 811, 813, 572B and similar with a metal cap cemented directly to the glass that has the plate lead soldered to it.

All others get the heat dissipating type and especially if you are runing close to maximum ratings or above. 813's at 3KV+ for instance.

Carl
KM1H

How long were the phenolic ones on to get stuck? I've never had any of them in my picture  stick, I believe they have much less tension but make as good a contact. Many ceramics that the wire exits at 90 degrees use a flat strip of metal bent to an inverted U shape, and they need to be tight to make good contact. The ones I use have semicircular contact areas so the connection is very good with minimal pressure.