Cooling requirements of pair 3-500z Tubes

Discussion in 'Amateur Radio Amplifiers' started by N8FVJ, Nov 28, 2019.

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  1. N8FVJ

    N8FVJ Ham Member QRZ Page

    While I am going thru my LK-500zb amp, I believe I will order as quiet a muffin fan as I can find. Before I order I wonder how much CFM is required. I believe 100 cfm is the minimum requirement for two 3-500z tubes. Thoughts?
     
  2. N6UH

    N6UH Ham Member QRZ Page

    There's a great section on cooling in the Eimac data sheet!
     
  3. N8FVJ

    N8FVJ Ham Member QRZ Page

    Eimac cooling is only for a chimney with a blower and states 16 cfm.

    I am referring to using a muffin fan that most amps use. The only reference I found on the internet is someone replaced a fan in a Kenwood TL-922 with 120 cfm and it worked well.

    What is most interesting is the technical amplifier rebuilds such as the SB-220 talks about fans, but no one has reference to CFM flow for cooling. All they state is to buy a replacement fan from Harbach for the SB-220 and on the Harbach site it only states their fan turns 30% higher RPM with no reference to CFM flow. You would think as important as tube cooling is someone would have a minimum cfm requirement- not.

    I come to the conclusion the minimum flow should be 120 cfm. There is a 120mm square 12 volt DC fan available from Digikey that flows 120 cfm and noise is 42 dBa. Anther fan from Digikey is rated at 158 cfm and noise is 49 dBa. I will use the 158 cfm and my amp has a high/low fan speed switch. I will install a relay to operate the fan at high speed during transmit and low speed with amp in standby. I can by pass this function with the switch and run high speed all the time.

    So note- this post is apparently the first reference on the internet regarding muffin fan cfm flow rates for a pair of 3-500z tubes and the Kenwood TL-922 120 cfm new fan was successful per the owner.
     
  4. N6UH

    N6UH Ham Member QRZ Page

    I wasn't referring the the CFM required, but the temperatures the base and anode seals should be kept below. However your cooling is oriented provided you maintain a lower seal temperatures than specified you should be good to go.
     
  5. VE2GCE

    VE2GCE Ham Member QRZ Page

    My LK500ZC amplifier has a high-low switch for the fan.
    I find this odd, you would assume you want the fan running at high all the time.
     
  6. KN1M

    KN1M Ham Member QRZ Page

    The AL1200 amp I had a few years ago had taps (3 or 4 IIRC) on the xfrmr for fan speeds. I outboarded a switch so I could control the speed without opening the box. Of course it was a squirrel cage blower and not a muffin fan. Audible at all speeds. Kept the shack toasty.
     
  7. N8FVJ

    N8FVJ Ham Member QRZ Page

    I suspect during receive the amp tubes only needs filament cooling thus can operate with less fan noise from the amp during receive in the low fan setting.
     
    VE2GCE likes this.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    CFM ratings under uncontrolled conditions don't mean much. You have to check the fan ratings against various back pressures.

    A 100 CFM axial fan like a "muffin" fan is only 100 CFM with no back pressure, just sitting with nothing impeding air flow or creating recirculation, which is a different condition than in an amplifier. Chances are most 100 CFM axial fans once installed with obtructions in the way (tubes, and everything else) it drops to 15-20 CFM, maybe less, which is likely enough for the tube seals assuming there's air flow across the socket (pins) and the anode pin cooler.

    Remember with glass tubes like 3-500Zs, since the internal elements are in a vaccum, there's no real anode cooling created by a fan no matter how powerful it is. The anode cools by radiation; but the pin seals cool by conduction and air flow there helps a great deal.
     
    WA7PRC likes this.
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    Just play it by ear. A 120 mm should work fine.

     
  10. N8FVJ

    N8FVJ Ham Member QRZ Page

    I believe muffin fans have similar back pressure CFM curves.
     

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