Hi,

Random question. Internal anode tubes, such as the 3-500z, show plate color when running at full power. Do external anode tubes, such as the 4cx250, show anode color in the same way??

Bob

I don’t know but my guess would be no, since I think if in the presence of oxygen running that hot they would destroy themselves

Just a guess while awaiting a chemical analysis of the plate material

Check the data sheet. They usually say.

Greetings to Bob and the Forum:

No external anode tube should "show color" when operating within its designed parameters including adequate air flow.

Take a look at my avatar. This is an 8807 transmitting tube.... used back in the days of analog TV transmitters. Back when I used to maintain such transmitters, we once had an airflow problem.... and pulled a tube showing burnt black flakes of plating at the base of the fins closest to the anode. Needless to say, the reason we got into that particular PA cavity was because the tube failed.... and after looking at the physical appearance, it became obvious why.

Regards,

Hi,

Thanks to all who replied.

I am working on a Hallicrafters SR-2000 which uses a pair of 8122s in the final. These are an external anode, ceramic tube with a rated 350 watt plate dissipation. While idly pondering the meaning of life, the universe, and everything, I got thinking about how small these tubes were compared to the 3-500z tubes in my old Heathkit amp and how the plates would show color when tuning up at full power.

I didn't see anything on the 8122s to indicate that they ever got anywhere near that hot which struck me as pretty amazing given their diminutive size. I guess it could be viewed as an intermediate step to the modern high-power semiconductor devices which can handle comparable power subject to the ability to carry off all that heat efficiently.

Back to more mundane matters such as the two 2.7 meg ohm carbon composition resistors in the HV metering circuit - one of which measures 4.3 megs and the other 3.5 megs.

Thanks again,

Bob K7DYB (ex AB1MN)

it's 42. And thanks for all the fish.

Greetings to Bob and the Forum:




The last part of the above quote is the whole thing in a nutshell. For tubes with an internal anode in a glass envelope, there are only two methods of removing the heat due to plate dissipation... the principal one is by radiation and the other is by conduction to a plate cap with cooling fins. Since radiation is the principal means of cooling, the plates run a lot hotter.

In an external anode tube, the plate has a lot more surface area (fins) together with direct impingement of cooling air. This is a much more efficient method of heat removal and so the plate can (and should) run much cooler. Excessive heat can cause enough thermal expansion in the external anode structure that it will fail the metal to ceramic seal where the anode joins the rest of the tube. The moral of the story is be sure the airflow is adequate and ideally, have a means of detecting an interruption in said flow that will automatically shut the transmitter down.

Collins does this in their 30S-1 linear amplifier with a bi-metalic switch that is in the exhaust airflow from the 4CX1000. This will detect either lost airflow or excessive heat from the tube.

The Harris-Gates BT-18 TV transmitters that use the 8807 tube (my avatar) in the final use a small plastic tube inserted into the side of the main cooling air duct that comes right off the blower. The other end of this tube goes to a diaphragm pressure switch that will shut down the transmitter if pressure is lost. There's plenty of pressure to operate the diaphragm switch; the blower is a fairly large squirrel-cage blower with about a 1 hp 3-phase motor driving it.

Regards,

Some external anode tubes came in multiple versions, with different numbers of course. There were the air cooled versions but there were also conduction cooled types (i.e. bolted to a heat sink), and water or steam cooled ones.

Greetings to Jim and the Forum:

Yes, there were lots of variations on the theme. There were also internal anode tubes that were water cooled; check out the 8D21 used in the RCA TT-5 television transmitter.



RCA came up with a modification kit to convert these transmitters to an air cooled tube and most of them were... thank goodness.

I've changed out a number of these... I'd guess you'd call this an external anode tube except for the fact that anode was the inner wall of the lower water jacket and was not visible.... and it was referred to as a "collector".



Anyway.... it doesn't change the fact that none of these tubes were designed to run red.... and if they did, they were in trouble.

Regards,

Hi, Jim. What is the plate dissipation of that thing? With nothing to show scale except what I would guess are "two man lift" handles, I am guessing at least 50 kw., but probably more, judging by the relative size of the base.

John

Greetings to John and the Forum:

Are you asking about the IOT D2100B (second picture) or the RCA 8D21 (first picture)? Or, since you mentioned lift handles, are you asking about the 8807 (my avatar)?

If you are asking about the 8807, it has a plate dissipation of 15 KW. It has interesting filament specifications: 9.5 volts at 135 amperes.

Here's the data sheet:



If you are asking about the IOT D2100 B (second picture), it doesn't really have a plate dissipation rating because it doesn't really have a plate. In typical operation, it runs at 35 KV collector potential at about 1.4 amperes for an input power of 49 KW and it makes about 22 KW RF output... about 44% efficiency... which is the IOT's claim to fame. Incidentally, it is about 3 feet tall and is removed and replaced with a chain hoist.

Here's what it looks like when it is installed in its "trolley" and in place in the transmitter:



If you want to know about the 8D21, you'll have to look it up on the internet. I never had anything to do with the RCA TT-5. I went over to KCAL channel 9 when they were removing a TT-5 from service as their back-up transmitter in the 1990's and got a brief tour from the late Frank Grill, their transmitter chief engineer at the time, so I know a little bit about them, but KCAL's transmitter had been modified to use air-cooled finals and I've never even seen an 8D21. Unfortunately, I don't remember what the air-cooled tubes were either. That was a long time ago.

Regards,