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THE AM BULLETIN BOARD => QSO => Topic started by: K1JJ on August 31, 2020, 03:30:24 PM

Title: Check out this guy's generator "Quiet Box" technique- Rig fan noise reduction?
Post by: K1JJ on August 31, 2020, 03:30:24 PM

Tremendous decrease in noise dB using baffling that lets air flow but not sound.  This could be adapted to a big linear or any amp. Someone was telling me about doing this recently.

This 14 minute video is worth watching beginning to end...


Title: Re: Check out this guy's generator "Quiet Box" technique- Rig fan noise reduction?
Post by: w8khk on August 31, 2020, 05:12:49 PM
Tom, I think you and I were discussing tube cooling in earlier threads.

I know the 3CX3000F3 has a very noisy plate heat exchanger and requires a great deal of air flow even with just the filament dissipating in excess of 400 watts.  I plan to use this tube for a series-modulated legal-limit transmitter, and the goal is to make it quieter than a desktop linear amplifier.  Due to the inefficiency of series modulation in legal-limit rigs, it is necessary to have an excess of dissipation capability in the modulation stage, and the availability of the 3CX3000 finally makes this journey seem feasible.  As soon as the series modulation prototype activities with the 810 modulator and 8000 final are completed, I will embark upon the cooling system for this new rig.

The enclosure will be a Hewlett Packard Minicomputer rack cabinet that previously housed an HP-3000 Series III vintage 1977 system.  It is all aluminum, with cast base, top, and verticals, and removable sheet aluminum side panels.  Dimensions are 22" wide, 33" deep, and 64" tall.
Slots in the verticals hold stainless steel strips tapped 10-32 and are adjustable for attaching standard 19" rack panels to the front and rear, and the same feature is available for mounting hardware within the sides of the enclosure.

The bottom and top castings have large rectangular openings, and the top cover has a series of small rectangular louvers front to back on either side for quiet air exit.

I plan to park the rig in a carpeted area, on 3" casters.  A furnace filter will slide in under a 3/4" plywood shelf on the bottom, which will support the power transformer, filter chokes, and oil capacitors.  Bleeder resistors will be in the top of the enclosure under the air exit louvers.

I will drill 3/4" or 1" holes around the power supply components to allow plenty of air to flow at low velocity from the filter area.  I will line the entire side and back area of the cabinet walls with 2" thick large-cell foam composite material.  

Above the power supply will be a two-stage labyrinth of 2" thick fire-safe fiberglass insulation, supporting the automotive air conditioning blower assembly, with the ductwork of the blower constructed of layered 3/4 inch thick styrofoam.  The blower has a variable-speed DC motor and a composite squirrel cage with many closely spaced fins, very wide, and it produces a great deal of air flow at relatively low RPM.  Speed may be adjusted automatically between transmit and standby periods based upon air and tube temperature.

The 3CX3000F3 will be sitting in a fiberglass shelf within the styrofoam plenum, supporting the plate cooler on a hole with a ridge and clamp, with cooling air flowing directly on the base of the tube and filament seals.  A sheet metal enclosure will be external to the styrofoam plenum, and connections will be made through large ceramic feed-through insulators.  While most of the air will flow through the tube radiator, a limited amount of air will be allowed to pass around the outside of the tube radiator, thus protecting the insulating material from excessive radiant heat.

Above the tube enclosure will be another ductwork labyrinth to quiet the exit air, and the whine from the blower and tube radiator fins. This ductwork will be in the back of the cabinet, and the metering and other circuitry will be in the front, with insulation behind the meters and other components to abate any noise from the front of the plenum.

Above the modulator stage will reside the final RF amplifier, and it may either be a pair of 304-TLs, 250-THs, or 450-THs.  The 304-TLs, will probably be the preferred choice due to the enormous filament emission and their ability to provide high power output at relatively low plate voltages.  With the 5000 volt 1 amp power supply from a Collins 5KW FM broadcast transmitter, 3000 volts will appear across the series modulator tube with no modulation, providing 2000 volts at 1/2 amp to the final.  This configuration should easily provide reasonable, but not excessive, positive peaks somewhat greater than 100 percent.

I believe it will be possible to create a legal-limit rig that is quieter than the kitchen refrigerator, with judicious use of fiberglass, styrofoam, and large open-cell foam, providing many small paths for large amounts of air to pass slowly in a circuitous route into and out of the enclosure.  In addition to insulation, I believe it is very important to minimize the use of metal or other hard structures that would telegraph the mechanical blower noise in a manner that would be very difficult to subdue.  With sufficient air flow, the temperatures encountered should not degrade any of the insulation materials significantly.  Once this noise abatement solution is verified, the interconnections to complete the rig may be accomplished rather quickly and easily.

Many performance measurements must be taken, such that the modulator may be  run within the linear range of the plate characteristic curve, so that IMD products from the modulation circuits are minimized.  This is the reason the power supply and modulator are over-built, to allow operation at conservative power ratings with reliable and clean performance.  The modulator will be tested with a dummy load equivalent to the modulating impedance of the final amplifier before the RF sections are added to the mix.

I realize this is a very large enclosure for a legal-limit transmitter, but the space is available, and I really prefer having the rig local to the operating position instead of in a garage or storage room.  If it is not quieter than a standard 1KW broadcast transmitter with blower-cooked 4-400 tubes, then my project will be considered a failure.

Tom, I noticed you are embarking on a 20 to 35 watt PW AM rig, class B plate modulated.  Instead of adding mod iron (transformer and reactor) this would be an ideal opportunity to try hi-fi series modulation.  The power supply should be easy, and you might come out as efficient as class B, considering all the overhead of driving and heating the two modulator tubes and drivers.  You could also test various tubes on the bench, pine-board style, to perfect your modulator before committing it to the new chassis and enclosure.  GL on your new project.  What color paint will you use?  She might look good in contrasting pink!

Title: Re: Check out this guy's generator "Quiet Box" technique- Rig fan noise reduction?
Post by: W1ITT on August 31, 2020, 05:38:17 PM
That enclosure is bulky, but he does a good job with it.  I have found that blower noise sound level is important, but the pitch of the noise has a large effect on the annoyance factor, and the long term tiresome factor.  For many years I worked in broadcast transmitter sites, back when it was legally required to have a licensed engineer at the control.  Blower noise was something that slowly ate away at one's body and sense of well being.  We did some sound deadening stuff at one of the sites but it never changed the pitch, just the amplitude.
For ham amplifiers, I'm a fan (no pun intended) of lower RPM blowers.  For ham use on typical amplifiers I like the Dayton 2C647 blower, a 1500 RPM slow roller.  It's easier on the ears than the 3600 RPM blowers, more of a whoosh and less whine.  I suppose conduction cooling with the pump and heat exchanger would be optimum.  My friend Joe K1RQG (sk) built a ring of six 7289s for 1296 moonbounce and the anodes were  water cooled with flow through plate caps and quarter inch water tubes hopping from one tube  to the next and back to the reservoir.  It was silent at the operating position.
Perhaps part of building quieter air cooled amplifiers could involve methodology that attenuated the higher frequency whiny noise components and let most of the rest of the spectrum whoosh on through carrying the BTUs with it.  It'll take a smarter lad than I to figure out how to manage that.

73 de Norm W1ITT

Title: Re: Check out this guy's generator "Quiet Box" technique- Rig fan noise reduction?
Post by: K1JJ on August 31, 2020, 10:13:33 PM
Walt:  Looks like you have really thought it out and have a good plan.  The series mod circuit is defiantly appealing, no doubt.

Yes, the power generator guy did quite a job dropping the noise down so far. We have been losing electrical power for up to a week sometimes and I moved the generator out 90' away from the house. That made a huge noise difference, but even bigger when the wind blows towards the house and the carbon monoxide fumes take effect.  Even 20' away from the house was too close with the wrong wind, even with windows closed.

One thing about this guy's project is there is a very loud machine inside. In contrast, a big amplifier has the air whoosh and smaller blower sound to contend with. So dropping 5-10 dB will be quite a lot.

I also noticed the pitch makes a big difference. In fact, I like the air whoosh much better because on SSB sometimes the air noise is above the ssb bandwidth and not too bad. But a 1KHz whine is too hard to get away from even with a notched EQ.   My big amp uses two blowers side by side backed way off in speed. That is about as quiet as I can get things.

But bottom line is I think parts of this guy's ideas can be used to make amplifiers quieter. The idea that air can easily go around corners and sound cannot is important.


Title: Re: Check out this guy's generator "Quiet Box" technique- Rig fan noise reduction?
Post by: WBear2GCR on September 03, 2020, 08:39:05 PM
Back in the early 70s, I was at some farm in some place in rural NY... music.
Anyhow they had a band, and generators. Two, iirc.

VERY loud.
So loud the band really could not be heard.

I noticed they had two key components in ready supply:
- hay bales
- black water pipe.

Dunno why they had 20-30ft lengths of water pipe...

So, somehow I got the people to make a 3 bale high enclosure around the generators -
at some distance from them.

And then I proceeded to supervise the removal of the muffler and the extension of the
exhaust out about 50 ft or so, with the muffler placed at the end.

Very quiet.

Nothing to do with blower noise.  :-\


Title: Re: Check out this guy's generator "Quiet Box" technique- Rig fan noise reduction?
Post by: Opcom on September 13, 2020, 01:24:02 AM
I made an anti-vibration mount for the blower from a 3/4" thick piece of rubber foam sandwiched between two 3/32" or 1/16" thick rigid aluminum plates I forget which thickness. Weatherstrip adhesive "yellow monkey boogers", 3M automotive type, holds the sandwich together. The blower mounts to vertical studs sticking through the top plate, and the mount attaches to the wooden floor of the rack via lag bolts through holes in the lower plate. Almost no vibration is transmitted to the chassis/rack where otherwise a 'hum' would be caused. It is effective despite the blower being originally sized for cooling a 4CX5000 and 4-1000A together. Maybe that kind of mount would help you with your automotive blower.

Title: Re: Check out this guy's generator "Quiet Box" technique- Rig fan noise reduction?
Post by: WU2D on September 13, 2020, 07:00:14 AM
The Dynamotor in my Collins ARC-2 Transmitter-Receiver sounds like a Jet Engine in the shack. I stuck two of those pipe foam tubes into the inlet and outlet in the back and bent them down below the bench. This reduced the noise to tolerable for a while... To be fair, they mounted this thing in a B-29 somewhat remotely. The dynamotor itself is loud, but they added a fan to the end of the dyno to move air at the same time, and that is the culprit. No wonder folks built power supplied to replace so many dynamotors. To be fair, my BC-348 and BC-652 Dynamotors are almost silent.

Title: Re: Check out this guy's generator "Quiet Box" technique- Rig fan noise reduction?
Post by: WBear2GCR on September 22, 2020, 03:52:58 PM
There is a lot of info on noise reduction, mostly for HVAC stuff, ducts & fans.
(Car mufflers are just an application of the same stuff.)

It revolves around Helmholtz resonators and other methods of turning the path
into a LPF. Generally, that's the aim - think of it in those terms, and the idea
gets simpler. High frequency sound likes a direct path... and hard surfaces to
bounce off.

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands