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Amp cooling questions

Discussion in 'Amateur Radio Amplifiers' started by N4EYZ, Jan 18, 2016.

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  1. N4EYZ

    N4EYZ Ham Member QRZ Page

    I have a home brew amp that uses a pair of 572-B's. It came from a SK, I've owned the amp about 2 yrs and have used it very little but plan to use it more very shortly. The builder had a good reputation for quality work and that rep seems to be demonstrated in this amp. My question(s): This amp has a 5" muffin fan that is a bit of a howler........lots of air movement and the resulting fan noise. How much cooling does a 572 require? I think this amp is electrically similar to an SB-200, weren't they pretty quiet operating? Thinking of making some fan changes but don't want to damage the tubes. All I do is light to average rag chewing. No high duty cycle modes.

  2. W8JX

    W8JX Ham Member QRZ Page

    There is no such thing as too much air flow in a amp. You could use a small fan if noise is bad but remember that air flow can also cool other parts in amp. Ultimately how much power output you run has a bearing on how much cooling you need.
  3. G3YRO

    G3YRO Ham Member QRZ Page

    I run a Linear Amp with a pair of 572Bs (an FL2000B) . . .

    Under each valve is a fan that keeps the air circulating past each one, from bottom to top.

    HOWEVER . . . . not only do the valves get hot really quickly when running full power, the whole chassis and case gets really hot if used for over an hour.

    SO . . . . I have a couple of nice powerful 5 inch fans (240V), one sits on top of the case, above the PA compartment, sucking the hot air out . . . the other sits against the back, driving cold air in past the valve bases etc (neither have been physically attached, they just sit there)

    This makes a BIG difference . . . it stops the valves getting so hot in the first place . . . but also stops the case (and all the other components inside) getting hotter and hotter!

    Like the previous post, I agree you can't have too much air flow . . . not if you want the valves and all the other components to last . . . so I would leave your "noisy" fan in place!
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd also say an air cooling system really should be a "system" in a self-contained amplifier where the power supply and other power dissipating components reside along with the tube or tubes.

    572Bs themselves don't need much air flow, but other stuff might. Filter capacitors like electrolytics especially can age and fail much more quickly if they get too hot, and their own bleeder resistors are often mounted close to them, which creates more heat.

    I use headphones for all home operating, so don't notice fan noise.:)
  5. KM1H

    KM1H Ham Member QRZ Page

    The 572B doesnt require any fan cooling at 2000V or less with the resultant lower power. They do need to be mounted in the clear for that and not jammed in as in a SB-200, Fl-2100B, Clipperton L, etc.
    Excessive fan speed and noise can be reduced with a 10-20W resistor, Dentron used 330 to 660 Ohms to quiet them down.

  6. W8JX

    W8JX Ham Member QRZ Page

    This is rich. No fan cooling required for a tube with a 25 wat heater and maybe dissipating up to 100 watts too. By this faulty logic, 811 amps need no fan cooling because they are running at lower power yet (most of time anyway) Yes some will argue that heat radiates via inferred energy but problem is here that it can heat air around tube and chassis too a reflect some heat back into tube. There is no magic barrier that only lets heat out of tube and not back in.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Most power tubes except for external anode tubes radiate the heat and don't need much cooling at all.

    I've run my 4-1000A amp at 1.5 kW output for hours with no cooling at all...the cooling is mostly for the seals at the base pins and on top at the anode, and a very small 'phono motor' fan can do that job. Most of the heat generated is radiated.

    And my 4-1000A is a broadcast pull from 1985, it still works fine.
  8. N4EYZ

    N4EYZ Ham Member QRZ Page

    Interesting and varied opinions on cooling. I do understand the importance of proper cooling. And I want to get a long life out of this modest amp. FWIW--the rf deck and power supply are separate units, the rf deck has open sides and a grill top cover. It was originally built to slide into a 19" rack. In such it would probably be a bit quieter.
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If the power supply is separate, and the RF deck has a lot of venting, you won't need much boost cooling from a fan.

    One dual 572B amp, the Heath HA-14 "Compact KW" didn't have any kind of fan for the tubes. Many of those are still on the air, still working, some with the "original tubes" from the 1960s. Its RF section is very compact, and the power supplies (AC or DC input were available) were separate.

    W0BTU likes this.
  10. N4EYZ

    N4EYZ Ham Member QRZ Page

    Here is the amp.... IMG_0707.JPG IMG_0709.JPG

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