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[Amps] Dummy Load

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Subject: [Amps] Dummy Load
From: "Stein Roar, LA6FJA/KI4KJP" <>
Date: Wed, 1 Feb 2006 01:23:21 +0100
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Do anyone which have tip for a home made oil cooled dummy load

Stein Roar Brobakken   (aka Rag)
NO-2830 Raufoss
Skype(IP Phone): la6fja
GSM#1: +47 47 34 38 72
GSM#2: +47 48 22 44 21 (WORK)
GSM#3  +47 91 72 3872
Radio Amateur Call: LA6FJA - KI4KJP -DU1/KI4KJP
Locator: JP50IR N 60.4 - E 10.4
Member of: NRRL #LA-M 11387, WWYC#129, CTC#540, WE4YL SCDC DX Club,
LA5G Gjövik- og Toten Radio Group
Yahoo: stanley_vik_24

-----Opprinnelig melding-----
Fra: [] På
vegne av John T. M. Lyles
Sendt: 1. februar 2006 01:01
Emne: [Amps] Cooling base of tetrodes - an odd example tspa

When we developed the Broadcast Electronics FM30 and 30A transmitter 
in the early 1980s, we found that customers were calling to tell us 
that the little knob on the bottom of the Eimac 4CX20,000A/8990 
tetrode was very oxidized and looked overheated when their tubes were 
replaced. We looked into the problem and found that we had a dead 
spot of stagnant air just there in the little pocket around the inner 
filament contact area - an area we called the filament 'button'. 
Despite that we had ~ 5 hp Cincinnatti PB series blower, we missed 
that one spot. The lower input compartment was pressurized from this 
large fan, and air routed through both the socket and several holes 
in the deck (with EMI attenuating louviers) to pressurize the upper 
anode area. Like the scheme mentioned in an earlier posting 
(attributed to K2RIW), we fitted the upper cavity transmission line 
resonator tightly around the anode cooler, so that it ducted the hot 
exhaust out of the top of the cabinet. An insulating sheet of 
Rexolite blocked the air from leaking out of the cavity above the 
tube, so that it all had to leave through the anode. [The general 
layout can be seen in the late Jim Aurand's US Patent 4,363,000 
issued Dec. 7, 1982. There may be a way to view this patentent at the 
USPO free online. Back to my story though.]

The solution was found by our clever mechanical engineer, in that we 
installed a small PTFE pipe from beneath the amplifier enclosure, 
which stuck up into this area under the tetrode base. It bled a small 
amount of pressured are out of the amplifier (which was under 
pressure, remember) to the atmosphere through the pipe, and that air 
flow was enough to cool the filament 'button' to lower its operating 
temperature and give acceptable long life. The heating was just due 
to the filament power alone. So in this design, we achieved proper 
cooling of the filament base with reversed airflow to the outside.

By the way, we fully instrumented the filament 'button' with a 
thermocouple and floated the meter at the filament voltage, while we 
energized and deenergized the filament and blower. What we found was 
astounding, that the heat buildup under there would really creep up 
even after the cooling fan was shut down. I think we might have 
extended the filament cooling timer after that too, but my memory 
escapes me on that. We later modified the transmitter to have a 
larger blower with only half speed, to cut the noise level. Our 
product had been accused (rightly so) of sounding like a jet airplane 
starting up. Yet, we never had overheating problems at even high 
elevation transmitter sites at full 30 kW output at 108 MHz.

I just looked at a new 4CX15,000A on my shelf here and it has a 
series of small holes which are around the circumference of the inner 
filament contact, to make sure some air gets moved around at the 
ceramic to metal seals there.


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