Water cools far better than oil, but di-hydrogen oxide can not be left
in the unit when it is not in use.
- note - Water has a dielectric constant of 79, so the mechanical
design will be slightly different with an oil dielectric.
On Jan 31, 2006, at 4:23 PM, Stein Roar, LA6FJA/KI4KJP wrote:
> Hi
> Do anyone which have tip for a home made oil cooled dummy load
>
> #-----------------------------------------------------------#
> Stein Roar Brobakken (aka Rag)
> Grønvold
> NO-2830 Raufoss
> Norway
> mailto:s-roabr@online.no
> MSN: s-roabr@hotmail.com
> Skype(IP Phone): la6fja
> Cellphones
> 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,
> LA-DX-GROUP#274,LA IOTA Club, ARRL, PARA, DX1M MAGELLAN DX Club,
> LA5G Gjövik- og Toten Radio Group
>
> www.qsl.net/la6fja
> Yahoo: stanley_vik_24
> AOL: KI4KJP
>
> -----Opprinnelig melding-----
> Fra: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> På
> vegne av John T. M. Lyles
> Sendt: 1. februar 2006 01:01
> Til: amps@contesting.com
> 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.
>
> 73
> John
> K5PRO
>
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>
>
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>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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