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Finally - Some 4-1000A Grounded Grid Linear IMD Specs




 
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Author Topic: Finally - Some 4-1000A Grounded Grid Linear IMD Specs  (Read 28702 times)
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ke7trp
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« Reply #25 on: December 11, 2012, 01:46:43 PM »

S9 into PHX Shane on 10 watts.. I was on 18 watts from the FT101EE.

C
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« Reply #26 on: December 11, 2012, 02:21:31 PM »

Below are a few comments from Frank/ WA1GFZ. He's been busy lately but was nice enough to send me an email with the best explanation I've heard of why two-tone tests are used to measure IMD vs: other methods like a pulse pecker, etc...

T
----------------------


Frank:  BTW the ARRL two  tone test gives you an automatic 6dB better number



Tom:  
OK on the glowbar .  I did the test again and see no difference this time. It's tough to get readings that close (3db) I guess.


Frank:
There is no @ucking  feedback loop with a resistor on the input

 
Tom:
Why doesnít a 10% avg pecker pulse, with wide band crud, work as well as a 50% two-tone?   Does the avg power have an effect on IMD?

Frank:
The whole point of the two tone test is to mix the signals which generate mixing products when the stage goes nonlinear. Remember you canít plate modulate a class A final because you need the non linear final to generate the upper and lower sidebands.

When testing a linear amp you donít want it to go nonlinear so this is a method of measuring how nonlinear the stage is.  As you increase the power input of the two tone test at some point you hit zero which generates IMD and then you flat top again generates IMD.

When you have no IMD the two signals go through the amp clean with no mixing products.

A pulse is just gating the amp on and off, it will generate distortion when saturated but also generate imd when pulse turns off. A non linear input so it is useless for IMD. A good test if you are building a radar system though. There is a test method for that in the spec I sent you.

-----------




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« Reply #27 on: December 11, 2012, 08:47:38 PM »

Makes a lot of sense to use "two tone" vs square pulse for a distortion test.

Gating is good to quickly tune an amp for peak power into a dummy load.

A nicer test keyer or pulser for tuning a linear amp would shape the drive pulse somewhat Gaussian and avoid the sharp amplitude edges that can generate a plate current mess well outside the most efficient pi network bandwidth. - It's still only good for tuning, not distortion measurements.
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« Reply #28 on: December 11, 2012, 09:20:21 PM »

Yes, you have to have TWO tones to intermodulate.

Just remember that the two-tone test is a static test. It shows the IMD at a given power, so it is FB for characterizing tubes. But it says nothing about the dynamic regulation of the power supply which can have some impact on the real world IMD of an amp (the input/output doesn't remain constant). Dynamic or transient intermodulation distortion measurements are more common in the audio amp world than the RF amp world.

Throw in the ALC effects in some transceivers and the actual on the air performance is probably far worse in many cases than the lab IMD tests.
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« Reply #29 on: December 11, 2012, 10:20:35 PM »


Just remember that the two-tone test is a static test. It shows the IMD at a given power, so it is FB for characterizing tubes. But it says nothing about the dynamic regulation of the power supply which can have some impact on the real world IMD of an amp (the input/output doesn't remain constant). Dynamic or transient intermodulation distortion measurements are more common in the audio amp world than the RF amp world.

Throw in the ALC effects in some transceivers and the actual on the air performance is probably far worse in many cases than the lab IMD tests.

Good point. Yep, I agree about the dynamic test as well as testing the WHOLE station infrastructure.  That's why in the past I like to play through audio voice tapes and tune the band up and down looking for crud levels 3kc, 5kc and 8kc away.  It's amazing how much trash can be generated by a raspy "Yallo."   It's all relative, but this is a good way to check a transceiver and then add on the linear and see how much it changes the crud level. (Assuming the signal is normalized and no overload)    If there is a sagging power supply, ALC, audio parasitics, or other dynamic problems, it will show up with a voice program acid test.  

Actually the pecker pulse test is not a bad test to check relative side channel crud levels cuz it is a constant level and makes measurements easy. Also, we can run the amp full peak power at only 10% duty cycle - less heat.  I can easily see the effects of loading, overdriving and other parameters using the pecker side trash.  I'll bet a white noise generator would work well too.

But to get a standardized measurement that can be compared to the real whirl, we need to run the two-tone to compare apples to apples.


* Pulled out the 4X1  and got the new 8877 mounted in the chassis and wiring it up now...

T
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« Reply #30 on: December 12, 2012, 05:14:35 PM »

Tom, et al,

***I'm probably in over my head on this one but I'll give it a try.***

I believe there is a relationship between IMD performance and the 1dB compression point of amplifiers. Researching this is giving me a headache as all the papers that I can find about it are full of difficult (for me) mathmatical equations so I can't put a number on it here.

Regardless, I was wondering if it may be useful to do 1dB compression point tests in evaluating an amplifier as the test method is easy and not subject to the IMD performance of the driving source. 1dBCP measurements are simple, require less test equipment, and reliable. This may be an aid during amplifier tuning/adjusting with a two-tone test done afterward to confirm the results.

For those without the equipment to do two-tone testing it may be the only way to guestimate an amplifiers IMD or, at the least, to know how hard they should (or shouldn't) drive it.

Just throwing it out there.

Don

~~~
Reference sources for those with a willingness to subject themselves to unpleasant experiences.
(Take 2 aspirin and get out your slide rule.)

"A Simple Technique for IIP3 prediction from the Gain Compression Curve"
http://wami.eng.usf.edu/Conferences/WAMICON/2005/Electronic-Materials/posters/tp-1.pdf

"Gain Compression or Expansion, Distortion and Other Large Signal Power Amplifier Related Phenomena"
http://www.av.it.pt/nbcarvalho/docs/cn4.pdf

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« Reply #31 on: December 12, 2012, 06:43:08 PM »

Which is another reason to lease or borrow some lab equipment.

All this work could go on for a while. Later on Mistla Vu find out his existing test method not so fb.
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« Reply #32 on: December 12, 2012, 08:19:05 PM »

Which is another reason to lease or borrow some lab equipment.

All this work could go on for a while. Later on Mistla Vu find out his existing test method not so fb.

Hehehheh.....   Well, at least there's progress of some sort.  Before I used to call the Huzman and axe him to listen and see if he hears any splatter.  At least now I have an idea what is going on in a relative way.


Interesting on the compression tests, Don. Yes, I can see how that could work because if both a driver and amplifier are perfectly linear, there would be no compression throughout the full range - until it ran out of headroom - and so forth.

As I sit here typing, the new 8877 is idling for a few hours burn in before I throw the HV switch and test it out. I have high hopes for better IMD with this tube.   But I think going from a glowing 4X1/ 3X1  to a "no soul" external anus 8877 is a lean to the dark side on this BB....  Grin  (pun intended)

I did like the 4X1 cuz it's partly radiation cooling and let me run the blower way down. The 8877 needs all air and is louder.

T
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« Reply #33 on: December 12, 2012, 11:25:07 PM »

Well, I fired up the 8877 (3CX-1500A7) and ran some tests. This is a 1500 watt dissipation tube.  It is in the same sub-chassis as the previous 4X1, but I redesigned the tank values, etc. plate  Q=12 with input cathode tuning.  

This tube is definately a cut above the 4X1 in GG for linear service. At 3500V it put out a robust 2500w pep.  It is very stable with not a hint of instability. At about 16db gain, it takes just about 35 watts to make 1500w out.  This means the driver can run at a lower level to improve cleanliness.

I ran it at around 1500w output for these IMD tests.   I took notes on the various IMD levels compared to  the 4X1 GG  and also the transceiver barefoot, so I have relative numbers.

Using voice, I see at least 5-8db better/lower  crud levels above 3kc than the 4X1.  The average noise floor is lower when speaking.  The pecker pulse side crud measures out to be about 10db better at times. I was seeing the side crud above 3.1kc down about 45db in many cases.

The two tone test showed about 38db 3rd. My 1000D at 200w and the 4X1 GG at 1500w were much worse than that with the two-tone at about -31  to -33db.  

So, bottom line is I will probably never change back to a glass tube again for linear use. A similar tube, the 3CPX-1500A7, is on ebay for under $250 these days and these inexpensive tubes are supposedly good according to web posts. No excuse now. The 8877 I am using is an old MRI pull I got for $300 ten years ago. It puts out 2500w in my 6M amp, so it's got good emission.


These are preliminary tests, but I am a happy camper so far.



More later.

T

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« Reply #34 on: December 13, 2012, 10:09:13 AM »

Pro Tek ?? Built in a hot rod shop?

I wouldnt be too concerned about any IMD that is at least in the mid 30's as its usually buried in the band noise at least for 20M and below.
On 6M it can make a big difference.
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« Reply #35 on: December 13, 2012, 11:19:00 AM »

Pro Tek ?? Built in a hot rod shop?

I wouldnt be too concerned about any IMD that is at least in the mid 30's as its usually buried in the band noise at least for 20M and below.
On 6M it can make a big difference.


Yes, 6M is a bear. Even "clean" signals can be heard far up the band when the noise floor is S1.


Frank brought up the point that it may not be a fair test between the 8877 and 4X1 cuz the exciter needs more drive for the 4X1 -  so the 1000D is not as clean.  This is true.  However, I did run a number of tests with the 1000D at 10 watts drive for both amplifiers. Even though the resultant output power was higher with the 8877, it was cleaner than the 4X1 GG amp.

Heck, today I thought that using a Collins 32S3 as an IPA - pair of 6146 finals with heavy NFB could be an interesting way to get a good 100Mw to 100 watt  -45db 3rd driver.  I could use the low level output of the 1000D and use the 32S3 as a linear.   Increase the NFB a little and we're there.


T


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« Reply #36 on: December 13, 2012, 04:02:47 PM »

Ive never heard of Pro Tek and Id be suspicious of ones labled Made in USA since Eimac is the only source here.
I just noticed they have 3-500Z's marked PRC Grin
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« Reply #37 on: December 13, 2012, 05:49:02 PM »

Ive never heard of Pro Tek and Id be suspicious of ones labled Made in USA since Eimac is the only source here.
I just noticed they have 3-500Z's marked PRC Grin


http://www.dandcelectronics.com/protek.html

Pro-Tekģ  is a wholly owned and operated subsidiary of D & C Electronics Co. in Brooksville, FL. Our products are designed, engineered, and produced in the United States and other locations around the world.  Final testing and quality control is done at our facility in Brooksville, FL.  Our products are being used in almost every industry including but not limited to broadcast, military, industrial heating, medical, and scientific research. Many OEM's and end users around the world have come to rely on our products outstanding performance as well as our unparalleled customer service.  We build a full line of high performance triodes, tetrodes, and pentodes along with the smaller audio and ham radio tubes. We also build sockets and socket repair kits, and can custom built to suite your application.   

Jim
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« Reply #38 on: December 13, 2012, 07:14:45 PM »

Take a look at the IMD transmitted signals from a 32S3, regular ricebox and a class A FT-1000MP Mk IV driving an 8877.

Looks like the 32S3 with NFB adds about 6db over an FT-1000D.  The regular ricebox looks nasty.   But that class A and 8877 combination is quite outstanding.   The MkV, class A, barefoot, is a dream. (5th order down -70db!)
 Why don't any of MY solid state amps work that well in class A?

T

Go down about 1/3 of the document: (starting at page 17)

http://www.sherweng.com/documents/NC0B-Contest-U-2008-9.pdf
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« Reply #39 on: December 13, 2012, 08:08:00 PM »

WOW..  Nice Data Tom.  That thing sounds clean with the Class A radio!   

C
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« Reply #40 on: December 13, 2012, 09:04:15 PM »

Quote
Pro-Tekģ  is a wholly owned and operated subsidiary of D & C Electronics Co. in Brooksville, FL. Our products are designed, engineered, and produced in the United States and other locations around the world

Translation: A rebrander using NOS or low use Eimac tubes that have been washed and relabled. Otherwise they are from the PRC with our own brand painted on.

Wasnt there something in the news recently about a US outfit selling Chinese junk to the US military rebranded as Eimac, etc ?
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« Reply #41 on: December 14, 2012, 07:52:08 AM »


Tom,

    I have been playing with the 8877 tube for a number of years with a Dentron MLA-2500 conversion from two 8875's to a single 8877. In the process I've learned a few things on what not to do, and as a consequence I have had two Eimac 8877's fail on me.

    Mistake #1: I used a filament dropping resistor to drop the 6.3v (for 8875's) down to 5v, but it was a bit high, at 5.3 volts (Standby). It would be better to be closer to 5V standby, and then maybe 4.8v at full strap. Lots of debate on proper filament voltage, but for sure too high is as bad as too low.

    Mistake #2: Dentron in the MLA-2500 only drove the 8875 cathodes. The filament had one side grounded to chassis ground. It is better to use a trifiliar filament choke and drive the cathode and filament pins common mode. Put a .01 uf cap between the three pins at the tube socket. Don't ground the filament supply, let it float at cathode potential instead.

Why did my 8877's fail? Each time the amp was in standby, and I heard a flurry of sparks inside the amplifier. 

My theory: Dentron uses a 47K cathode bias resistor in standby to provide cutoff bias to the tube. So, in standby mode, that might be 200v bias on the 8877 cathode. With the filament grounded, and with the filament running HOT (5.3v), a heater to cathode short will result in zero bias, turning the AMP ON when the PTT relay is not energized. The AMP takes off in a parasitic. This even bypasses the zener diode for setting 8v bias.

So what happened to my tubes? In both cases the filament opened. On the 2nd tube, it took out the 0-1 amp cathode current meter. I found that I can still use the tubes by first making a fixture using a 12V charged 100,000 mfd capacitor across the filament pins, and then tapping the tube with a ball peen hammer until the filament connection at the base makes contact. At that point it spot welds itself, and the tube works fine so long as I bring up the AMP with a variac. Once the tube is cold, if I turn on the amp suddenly, the filament opens again, requiring me to pull the tube and repeat the spot weld process. So far I've done this many times, and both tubes can be restored to 100% functionality.

   Also you might look at the Rich Measures low Q parasitic suppressors:

   http://www.somis.org/Price-Info.html

Jim
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« Reply #42 on: December 14, 2012, 08:55:33 AM »


Tom,

    I have been playing with the 8877 tube for a number of years with a Dentron MLA-2500 conversion from two 8875's to a single 8877. In the process I've learned a few things on what not to do, and as a consequence I have had two Eimac 8877's fail on me.

    Mistake #1: I used a filament dropping resistor to drop the 6.3v (for 8875's) down to 5v, but it was a bit high, at 5.3 volts (Standby). It would be better to be closer to 5V standby, and then maybe 4.8v at full strap. Lots of debate on proper filament voltage, but for sure too high is as bad as too low.

    Mistake #2: Dentron in the MLA-2500 only drove the 8875 cathodes. The filament had one side grounded to chassis ground. It is better to use a trifiliar filament choke and drive the cathode and filament pins common mode. Put a .01 uf cap between the three pins at the tube socket. Don't ground the filament supply, let it float at cathode potential instead.

Why did my 8877's fail? Each time the amp was in standby, and I heard a flurry of sparks inside the amplifier. 

My theory: Dentron uses a 47K cathode bias resistor in standby to provide cutoff bias to the tube. So, in standby mode, that might be 200v bias on the 8877 cathode. With the filament grounded, and with the filament running HOT (5.3v), a heater to cathode short will result in zero bias, turning the AMP ON when the PTT relay is not energized. The AMP takes off in a parasitic. This even bypasses the zener diode for setting 8v bias.

So what happened to my tubes? In both cases the filament opened. On the 2nd tube, it took out the 0-1 amp cathode current meter. I found that I can still use the tubes by first making a fixture using a 12V charged 100,000 mfd capacitor across the filament pins, and then tapping the tube with a ball peen hammer until the filament connection at the base makes contact. At that point it spot welds itself, and the tube works fine so long as I bring up the AMP with a variac. Once the tube is cold, if I turn on the amp suddenly, the filament opens again, requiring me to pull the tube and repeat the spot weld process. So far I've done this many times, and both tubes can be restored to 100% functionality.

   Also you might look at the Rich Measures low Q parasitic suppressors:

   http://www.somis.org/Price-Info.html

Jim
WD5JKO

Jim,

Great info on the care and feeding of this tube series.  The trifilar choke is something I would not have initially considered.  Most of my experience has been on grid driven class C amps.

Joe, GMS
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« Reply #43 on: December 14, 2012, 11:23:36 AM »

Jim,

Thanks for your experiences using the 8877.

I just went thru this whole thing about cathode and filament feeding with Chuck, K1KW.  He recommended the same thing - instead of a DC connection between one side of the fil and cathode at the tube pins, to capacitively couple both fils to the cathode with .01's.  And that's what I did.  

BUT, I grounded the center tap of the 5V fil xfmr.  I was worried about the DC difference between the two as a result of the 47K keying resistor and bias diodes in the cathode. Without a DC connection at the pins, how do they equalize for DC?  I understand that there is a 150 V rating from cathode to fils and the "dreaded" fil to cathode short is a real thing.

If we do not grouund the fil CT,  the cathode will equalize to the fils when driven with RF, but how will it do so for DC?  Thinking more, maybe the cathode shud float for DC. I'm gonna take the fil CT off ground.

But, I also wonder why connecting the cathode to the fil pin and running it thru a bifilar choke is not a good idea? I also wonder if tying the one fil directly to the cathode could induce a small amount of hum/buzzies to the signal. I plan to test this today. Check the sample schematics below.

There are at least five schematics out there for 8877's showing five different fil/cathode configs. It can be confusing.

BTW, I did some more IMD measurements on the 8877 and it is by far my cleanest amplifier. Much better than even the pair of Henry 3-500Z's too. If I can just get the fil/cathode issue in a safe mode, it shud be FB. The 8877 can be a fragile tube if mistreated, but run a long time if run correctly.


Here's some 8877 schematic samples - everyone has their own way of doing it... :-)

http://wc6w.50webs.com/wc6wamps/index.html?fr40.html
 
http://www.w8zr.net/images/W8ZR%208877%20Amp%20Schematics.pdf
 
http://www.bing.com/images/search?q=8877+scehmatic&view=detail&id=4C0A4E3A6F498096371F696A39630E829A5A3653&first=1
 
 
This guys ground the fil xfmr CT.
 
http://www.qro.it/amp/schemi/pdf/90hb3046.pdf
 
 
This guys connects the cathode to the fil:
http://www.wv7u.com/yc156amp/skems/RF_DECK.pdf
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« Reply #44 on: December 18, 2012, 12:57:51 PM »



I was curious about Pro-Tec tubes, and whether comments within this thread were true or not, so I contacted D & C Electronics, the folks who market Pro-Tek tubes ( a subsidiary of D &C). I got this response via email today (re-posted with permission):

"From: Dick Gross <sales AT dandcelectronics.com>
Sent: Tuesday, December 18, 2012 8:25 AM
Subject: Re: Pro-Tec 3CPX1500A7

Hello Jim,

    We realize that there are a lot of things floating around the message
boards online, and you can't believe eveything you read. We have been in
business and a US Government contractor for over 30 years in the same
place and do not and have never rebranded or otherwise manipulated an
Eimac tube for something it's not. We are one of the worlds largest
stocking distributors of new Eimac product, and we have no need to damage
our relationship with the factory with any of these shenanigans. Aside
from the fact that it is illegal, it's just plain bad business. If it's a
new tube, we sell it as new or NOS. If it's used, we test, clean, and sell
it as such. Your thread makes reference to a company that has been in
trouble for rebranding Eimac tubes. This is not us. The California based
company referenced has been completely debarred from any government
contracts and the owner of the company spent a year in jail. All of this
is public record from the US District court.

  With respect to the used tubes showing up on Ebay and elsewhere online,
we assume no responsibility for other peoples representations of them
(new, used, etc.). We own the Pro-Tek trademark, and every item that bears
the name was a factory new tube when it left here. We deal with several
different factories around the world, and although a lot of tubes may be
assembled overseas (China, Russia, France, etc.), all final processing,
testing, and QA are done here in the US.

    I give you full permission to repost this, and we may institute legal
actions upon the person(s) who are making these false statements.

Regards,

Rich Gross

Government Contracts (1R4E7)

D & C Electronics"
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« Reply #45 on: December 18, 2012, 06:30:34 PM »

That still doesnt explain the Made In USA on their 3-500G's.

Nor other Chinese tubes labeled and sold as a US brand.

Not much available on Google but this and government actions may not show up.

http://www.bbb.org/west-florida/business-reviews/electronic-equipment-dealers/d-and-c-electronics-company-in-weeki-wachee-fl-90118156/complaints#breakdown

Do you believe this?

http://www.dandcelectronics.com/protek.html

I might have to put my waders on but I'll maintain an open mind for awhile.
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« Reply #46 on: December 18, 2012, 06:52:10 PM »

At least we have experts in consumer complaints available here for comment.
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« Reply #47 on: December 18, 2012, 11:33:02 PM »

Apparently clairvoyant too.   Roll Eyes
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« Reply #48 on: December 19, 2012, 12:53:24 PM »

 Grin   Meanwhile....

Well, after a lot of mods, changes and tests, I ran a full system IMD test on the whole amplifier chain.  Chuck suggested I increase the 8877 Q to over 20. Using about 490pf for C1 and 3,000pf for C2 and about 5uh, I was able to get about Q=25. The power output was about the same as when Q=12.  Being a single ended stage, Q is important for best IMD, just as it is in the input tuning. I'm setting up my other amps for higher Q too.

I used recorded voice audio to best simulate real whirl.  I was amazed.  The spectrum analyzer tests showed upwards of 40-45 db 3rd.  I then listened to a remote receiver located 100 yards away from the shack and found that at S9+30, the side crud fell into the noise when up 3.5 to 4.0khz. This verifies the IMD test in the shack. It is definately cleaner than other amps I've used in the past.

I do not see much difference in IMD when idling the amp at 50 ma or 200 ma or 300 ma.  Some orders of IMD get better while other shift the other way. So I will run it lower for less heat.  I know that W8JI has written this to be true too, which kind of flies in the face of traditional practice.

The next project is building a tube amp to go from 10mW to 100 W at -45db 3rd or better, to drive the 8877.   The bottleneck now is definately the FT-1000D. It should be 10db cleaner than the 8877 to do the system justice.  I'm not sure what the tube lineup will be yet.

In addition, the blower noise MUST be reduced. On AM this is mandatory and on ssb it is important. I am putting mucho air thru since I learned my lesson about failing tube seals. I was thinking of some kind of sound proof barrier in from of the amplifier rack, like soundproof curtains or  panels, or even some styrofoam sheets built into paneling. If I can drop the noise another -20db, a new RF speech clipper can be used on ssb. The RF clipper, common in the 60's 70's,  is a board from Germany that converts audio to 455khz, clips it at the desired level, and then converts back into audio for the rig. On SSB, it produces the classic 6db of average increase in voice density, but all the audio harmonic trash is filtered by the 455 filters, etc.  Right now my average density is low, but it needs to be until I get a quiet shack.

So, there's lots of upgrades planned here.

T

 
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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« Reply #49 on: December 19, 2012, 03:50:15 PM »

Tom,

If you do an audio chain, you can notch filter the blower out.  That's what I did, upon the suggestion of VE7RF.  A simple notch filter placed BEFORE the noise gate did wonders. 

Mine is in software, so I can set it for a VERY steep curve, JUST taking the blower noise out.  It's almost undetectable when it comes to my voice, since the voice is several dB higher than the blower noise regardless.  Just enough notch to kill it, and you're FB again.

Or, get a BIGGER wheel.  Turn it mo slow.


--Shane
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