The AM Forum

I'm looking for a big Rotron CX-33 blower. In my search, I've tested two blowers I have in the cellar. It seems they both produce an annoying whine between 1000 to 2000 hz.  This can create havoc as background whistle on the air if the blower is in the same room. I don't mind rushing air, however.
Smaller blowers seem to generate only air rush, for the most part.

A Variac tends to diminish the sound and even "tune" it to a minimum, but it's not enough.

The blower I use now is in a sound-proof box, but I can still hear some whine coming thru with the airflow.

Those of you running or familiar with BC transmitters - do you usually hear this whining sound on bigger blowers and is it bearing trouble or is it just the design?

How does one test to know a used blower has a noise problem that is excessive compared to a new unit? (without a new unit available)

Thanks.

T

The big mechanical blowers have a lot of moving parts that add to the whine, Tom. Bearings, blader, armatures all add to it. Whirring is usually the blades cutting through the air. Whine tends to be the mechanical parts but shouldn't be terribly objectionable. If it's loud, you could have worn bearings. Check the play side-to-side or up and down, see how well things fit.

The 'whisper' style fans get away from this by minimizing the moving parts. First one I ever had 30+ years ago was a 3 blade Rotron which I finally managed to ruin by messing around with it too much. The only down side to them is once they're spent, that's it. No rebuilding with new bearings, etc. And the big ones can be pricey.

Gee, I thought it just added some background audio class to your transmitted signal. Probably better then chirping birds or barking dogs in the background.

On AM I am OK.   With the 4X1 AM rig, the blower is smaller and all I hear is air white noise. That is FB with me.    The Class E rig has just a tiny fan and is silent.

But on ssb, I run a larger AB1 final and it needs more air. The 1000 hz whine just loves to dominate the narrow 150 - 3Kc  ssb audio system.  I've gotten reports from Eu about having a constant "tone," on the signal, even when using a noise gate.

I sometimes hear it on others' signals and it seems so much like worn bearings to me, but can't be sure.

It's been a long time since I've heard a big BC rig running, but don't remember if a GOOD blower is dominated by a whining tone or just washed out by air noise.


T

Get a different mike, something that requires close talking and is very directional.

I use a Heil headset on SSB.

Tubes with handles shouldnt be that close anyway, they need their air. Or find one that runs on water.

Yeah, I've already tried everything I can think of other than moving the amp into another room.  I have the RE-20's back to the noise and close talk it. I even located the blower outside the house, but got too cold in winter.

I already use a noise gate.

Water cooling would be slick, I agree.  But I already have a supply of good linear air tubes, so don't want to fool with water tubes.  I wonder if an air-cooled external anode structure could be outfitted with a water jacket?

Moving the rig into the other room is just too detached and too hard to tune using manual controls.

If I could drop the whine and noise only another 10db, I'd be OK.  Since moving the blower back inside the room, the noise has gone over the limit of concealing... sigh.   I consider a  30 db signal to blower noise ratio the absolute minimum. Otherwise it sounds like a machine shop.  I actually prefer 40 db if possible.

T

Tom..Do you monitor anode temperature(s) ?? I'm wondering if the noise levels can't be reduced
by reduced airflow ..Amateur levels and key down times etc....  Steve

Hi Steve,

Yes, I do monitor anode temp using a probe and digital readout unit.    I have a Variac on the blower and have the airflow at about the level it should be for long life.

I had some bad experiences running the tube with too little air and learned my lesson.   (popped filament seal)

T

Need a cheap digital thermometer that will go to 500 Deg F ...or a mechanical probe type between the fins?
Barbeque digital with remote?

Any way of using one of them thar cube refrigerators and wrapping the coils round the tub??

klc

Find a good motor rebuild shop in your area and take the blower down for testing/rebuild/opinion.

I messed around with one and could never get that high pitched noise out. I found a shop local and a week later, I got the blower back rewound, painted, new bearings, new wire and a new Tag on it. It looked better then new and cost $80! 

I rubber mounted the blower and made a rubber gasket.  Its silent.  I cant even hear the damn thing unless its spooling up.

C

some older equipment uses a cloth sleeve between the blower and other things like a filter or the tube anode. With the blowers on rubber mounts and the cloth ducts, the noises are not amplified by the mechanical arrangement. A pair of old jeans can be used for pressure duct. Dryer vent or stripped a/c duct can be used for suction side.

A large, slow blower with big blades is also quieter. If you can find one from a 6000 series VAX computer, 12VDC is silent, 24VDC is a 'hush' sound. PN 12-27848-01. cost varies widely depending on from whom.

I have several. I love them over noisy and vibrating rackmounted dual squirrel cage units. They are meant to operate with the shaft horizontal, but I have had no trouble (yet) setting them on racks to suck out the hot air, shaft vertical. They make good pressure too, but I have not measured it to know if it is enough for a TX tube.

Hmmmmm.....  that's not a bad idea, especially for only $80. Rebuilding might be the way to go.  My existing blower does run well and blows enuff air, despite its whine. I can hear it change pitch and intensity as it warms up, so figger it has a problem.   I'll call around tmw and see what can be done.

I know Goodrich sells a rebuilt CX-33, but they want like $600+ plus a core... yikes.

Interesting that yours is now silent, Clark. That gives me some hope.


Opcom: 

OK on the bigger, slower blower. Yes, that seems to be the ticket for low noise. If I had one I'd use it.    The propellor-fan units - I've seen a few guys use them and if they are designed to handle back pressure, they do work for bigger tubes.  On any blower, it's all about the design, whether they can handle back pressure or fall apart.  Many are designed just for ventilation and massive air flow.


Maybe Carl knows if a water jacket of some kind wud work with a 3CX-3000A7.  I'll bet a combination of filament air for the seals, some air thru the fins and an outer water jacket wud work. At least the level of air could be reduced 10-15db to do the job quietly.

T

For 3CX3000A7, adding a water cooling device outside of the fin structure around the anode will not be very effective, as it would rely on conduction through the relatively thin vertical fins. Water cooled anodes have the water jacket in intimate contact with the copper anode cylinder itself in all cases. If it is tested, be sure that the outside temperature of the tube remains below 200 deg C at all times. Usually the hottest point is the ceramic just below the anode. To measure this requires temperature sensitive paint such as Tempilaq, with only a very tiny dot (such as using a toothpick to apply it).

The whining sound in large blowers is often from the tips of the fan impeller. Manufacturers have made significant progress in designing the tips to reduce wind noise, such as having backwards curved edges and low speeds. However, the air performance changes, usually for the worst. I have two 2 HP blowers on the big tube amplifier at work, and one of them was extremely obnoxious, almost painful. It was a Cincinnati PB14 size, 14 inch impeller diameter, blowing just on the filament through the socket in the cavity amp. Even though it fed through 5 inch hoses to the PA, it could be heard through the inlet filter. We buillt a box around it, with some improvement, but not enough.  Eventually I was able to change it to a different fan, get the required cooling performance, while dropping the sound over 10 dB in the 400-800 Hz region. The manufacturers curves provided insight into the acoustic performance in dBa. Now we don't need hearing protection around the beast.

If it's a single pitch tone or even a few pitches, a very sharp op amp audio filter/s might be worth a try right after or close to the mike preamp in the audio chain.  Be interesting to see your noise on an audio spectrum analyzer.  Wouldn't be hi fi but might be acceptable.

Bill Orr wrote a nice study on blower performance regards to speed and impeller diameter, as the signs on the highways say "Speed Kills"..... the ears in this case.

EBM Papst has a wide range of products and are known for high quality. I use one of their muffin fans in converted LK-800's that work just fine at 5 out and thats with three 3CPX800A7's which have a pretty restricted anode. The back pressure is easily overcome unlike a 3X3 which might benefit from one of their blowers
http://www.ebmpapst.us/Documents/ccm/Compact_Centrifugal.pdf

The "smallest" water cooled triode for GG service is the 3CW20000A7 which is the one I showed on here in the past and we talked about at the AM dinner. Its readily available plus a rebuilt is very reasonable once you find a core.

http://frank.pocnet.net/sheets/140/3/3CW20000A7.pdf

Pretty much all medium to large air-cooled external anode tubes require a high pressure blower as opposed to a fan.  I had a BE FM-30 running a 3 HP centrifugal blower that was so loud I had to take the (wired) phone outside to hear.  Pretty much had to shout to converse inside the room.

On the other hand, I once replaced the blower in an old RCA 5 kw FM with a single-phase 1/2 hp unit obtained from Grainger that was remarkably quiet.   I used rubber mounts and a canvas/velcro air coupler to keep the vibration from the chassis.

In a properly designed system, the primary noise will be the air rushing sound.  Gotta isolate the vibration from the chassis, and it's best to have the intake side enclosed as well.

Chris

Tom, are you sure it's the blower causing the whine (mechanical) and not a whistle effect within the blower or piping? Sort of like a pipe organ effect.  If you vary the speed using a variac the frequency would change indicating a mechanical issue I would think.  If it's a whistle effect the frequency wouldn't change but intensity would.

Lots of good replies, thanks.

Bob, the mechanical bearing whine varies in pitch as the blower speeds up.  When shut off, I can hear the grinding sound slowly come to a stop, so pretty sure it is worn bearings or mechcanical in nature,  in this case.


John, yes, a water jacket is probably a bad idea. I can see how there's little heat on the perimeter of the tube and most cooling is accomplished nearer the anode, deeper into the fins.


I agree that a blower is the choice for a big tube. The fans have less force into back pressure. Though, I know of someone running a few 8877's using a pair of stacked rotary fans mounted back-to-back. They seem to work well for him.  Maybe certain rotary fan designs will work with the required back pressure.

So, it looks like there ARE blowers that are quieter than others.   It's probably a matter of having quiet bearings, running it as slowly as possible and having an efficient air transfer ducting system.   I have mine in a styrofoam-lined box now and maybe by having the bearings rebuilt will solve the noise problem.  I will look into the rebuilding next.  Even if it cost $200 to rebuild, that's cheaper than the current prices on CX-33 blowers.

Thanks again for the suggestions. I now have a plan.

T

My T-368 was the first time I heard a mechanical whining sound as you describe. And yes, it was the bearings, not air rush.

Have you considered a "pull" rather than "push" for air flow?

With your blower in a soundproof box you perhaps have already done a "remote" blower configuration. At a radio station I once saw what looked like a range hood, custom fabricated, OVER a 20V2.  Their idea, specifically for noise mitigation, was to pull air up and through the ceiling. The blower was remote-mounted in the station attic and vented outward to points unknown.

The transmitter was placed right in the air studio, during the days when the FCC required hourly measurements, and the "Third Phone Endorsed" ticket holder was usually the on-air DJ.  So, all the op had to do was look beyond the mixing console and eye the meter array.

My own noise mitigation was a remote-mounted blower like yours on the homebrew rig I now have up on 10M.  Stock, in the original design, there was a small, internally mounted and fairly quiet squirrel-cage blower that pulled ambient air from the back panel of the RF tank, pressurizing the module through the socket holes for a pair of 4-250A.  I upgraded the final tubes and felt more air flow was needed, so I placed in series a more powerful squirrel cage blower with a nozzle, to which I could mount flex tubing. 

I suspended the thing as a tripod and tucked it in the corner of the radio room where the rack sat, connected to a receptacle mount on the back of the RF tank that I epoxied to the inlet of the original internal blower. Worked well, and the isolation prevents the external blower from using the entire rack as a sounding board, had I used brackets and hard pipe to configure.

Yes, a remote blower is a great way to go to greatly reduce noise. It's important in order to run any level of audio processing.  I think the stations that have a totally silent background have a good start to FB audio.   The other way is to use a glass tube final with a small muffin fan blowing air on the filament and seals.

For remote blowers, the air ducting hose is also critical. I use a smooth-inside 4" fiberglass hose I found on eBay. That dryer duct found in hardware stores has too many ribs inside and causes turbulance. This decreases flow and also creates more noise when moving a lot of air.

Of course, the best method - and I use it for smaller rigs, is to build a small cub-chassis for the tube and attach the blower directly. No better efficiency than that.

I did try the air PULL/suck method, but  that's how I blew the filament seal. It seems that pulling hot air DOWN from the anode and then over the filament structure is a poor way to go. It was definately much quieter, however.  I went back to the standard PUSH Fil to plate path.

OK on your remote blower system. Yes, mechanically isolating it from vibration is important. I have mine sitting in syrofoam, so it is dead. But that darn bearing noise sounds like a jet engine at times.

I located an AC motor rebuilder about 20 miles away. Looks like a small shop that has done it since the 50's.  Should get a good deal there. That's one idea I never really considered until Clark mentioned he got his done for only $80.  Heck they even rewound it too. Maybe just replacing the bearings wud be a breeze.     I have a couple more blowers here that need bearings, so WTF, might do them too.  

I would say that at least 50% of older, BIG blowers I find at flea markets have this problem to some degree.


T

You don't have to go legal limit with water cooled. There are some qrp ones.

http://item.mobileweb.ebay.com/viewitem?itemId=400321466364&cmd=VIDESC

Interesting find, Brandon.


They have the filament specs and look a little like a 3CX-3000A7.    I'll bet they are not a bad tube for linear service, but did not see any IMD specs.
If I wasn't already committed to a tube, I might try one out.


Another one:

http://www.cafr.ebay.ca/itm/CPI-Eimac-YU-191-191B-High-Mu-Power-Triode-Tube-7000W-/400323202273?pt=LH_DefaultDomain_0&hash=item5d351f4ce1




(1) CPI / Eimac YU-191B high-mu power triode tube
 
Specifications:

• Manufacturer: CPI
• Model: Eimac YU-191A
• Type: High-mu water cooled power triode
• Electrical characteristics: ◦ Filament: ■ Material: Thoriated tungsten
■ Voltage: 7.5 +/- 0.37 V
■ Current @ 7.5 V: 51.5 A

◦ Amplification factor, average: 160
◦ Direct inter-electrode capacitances (grounded filament): ■ Cin: 38.0 pF
■ Cout: 0.6 pF
■ Cgp: 24.0 pF

◦ Direct inter-electrode capacitances (grounded grid): ■ Cin: 38.0 pF
■ Cout: 24.0 pF
■ Cpk: 0.6 pF

◦ Frequency of maximum ratings (CW): 110MHz

• Mechanical characteristics: ◦ Maximum overall dimensions: 9.8" L x 7.1" W (248mm x 180mm)
◦ Net weight: 5.5 lbs (2.5 kg)
◦ Operating position: Vertical, base up or down
◦ Maximum operating temperature: ■ Envelope and ceramic/metal seals: 250 °C

◦ Cooling: Water
◦ Base: Special coaxial

There will always be some speciality tubes around and those happen to be used.

Ive even built water jackets for 7289's for 1296 and 2304 MHz use.

Just think of Tom using this on 40M with his stack ;D

JJ, I would go for the rebuild... but it should be pretty easy to check the blower assembly itself for noise... fire it up and listen to see where the sound is radiating from. Is it the final amplifier assembly or back at the box??

if it is coming down the hose and out, then you can install what would be an acoustical LPF in the line... which is essentially a muffler...

Of course if it is the bearings, it should scream back at the blower itself. A stethoscope would be one way to listen to the bearings, but another is a wood stick placed on the bearing point and the end in your ear... if that's blazing loud then it's ur bearings...

if I had the problem and the time, I'd take the blower apart, and get at the bearing housing, and try to shoot some liquid lube in there and see if it quiets... if it did, then the rebuild is mandatory, not to mention taking the impeller off and running the motor without a load at different speeds and having a listen... the noise would be worse with the load of course. If ur bold you can fake a load while it runs...

the grind on spool down shows bad bearings anyhow...

so it's one of three possibilites:
- cavity resonance (organ pipe)
- impeller noise (related to "tip speed" - like a whistle)
- bearing noise

            _-_-bear

If the blower still takes time to wind down there is a good chance there is no damage and a few drops of 0W-5 synthetic does wonders for long term silence.

Bear, yes, a stethoscope or listening stick is a good idea.  I will give it a listen.


Carl, I did try to oil it one time but it appears difficult to get at the bearings and requires special tools.  I may look it over again before bringing it in for a rebuild.

In its present state I can hear it all the way across the house when running - all bearing whine.  It's a really FB blower, otherwise.

T

Thats how mine was.  High pitched whine.  IT will be silent after rebuild.  Most of the time as hams, We get ahold of blowers second hand and they have alot more time on them then we think.  RF generators, BC rigs ect, are somtimes on 24/7 365 so wear is heavy. 

C

So with water cooled you still have to cool the water unless you leave the tap running.  ;D

Need Flow meter as well as temperature shutdown control.  Water taken outside with pumps, radiators, more fans. ?  Water is not compressable so R.V. Required too.

Water treatment for magnesium and calcium precipitates?  - additives monitoring?

All is not simple in water world.

Liquid cool, closed loop with inert heat transfer liquid, then to a heat exchanger.

What was it they put the Cray computer in a bath of? Fluorinert? Or??

                 _-_-bear

Pure distilled water and leakage current monitoring is all that is needed besides tubing, pump and a large enough container. After all this is not a 24/7 system. Making it smaller requires a transmission cooler (aka heat exchanger in that other world) and muffin fan. Temperature/flow monitoring are optional depending how close to the edge you want to live.

After XX hours the water will become sufficiently contaminated by metal ions to require changing. In the ham world that could be several years. Belmont Springs has 5 gal jugs for $6.49. You can ask about double distilling and deionization (aka 3D) if pushing the HV envelope.

Hmmm, we're getting close to a more fun project... Tubing, fans, large enough container....?
Need a little corn squeezin's added to that "de-ionized" water.
Your "xx" hours would magically be transformed to "xxx" Fine with three passes.  ;D

Now a simpler fire water world.

Love it.  Works for me.  ;D

* * * * * * * * * *

Seriously, I've followed this discussion with interest.  Many good points made that I'll be filing away in memory.  Bravo Zulu, gentlemen. ;)

Don't forget the shop vac.