[R-390] CU-872A/U RX multicoupler

Thu Jul 17 18:51:53 EDT 2014

Roger,

That was a very interesting rundown on the use of the
multicouplers.

But, one error....the 6922 is NOT a hot 12AU7 or 12AX7.
It's most common replacement is the 6DJ8....go here
for the specs on the 6922.

http://www.audiomatica.com/tubes/6dj8.htm

thanks, Phil

----- Original Message ----- 
From: "Roger Ruszkowski" <flowertime01 at wmconnect.com>
To: <R-390 at mailman.qth.net>
Sent: Thursday, July 17, 2014 5:08 PM
Subject: Re: [R-390] CU-872A/U RX multicoupler

>
> Chris,
>
> Does any one have a CU872 they would part with?
> I would like to own one for my R390's.
>
> Once upon a time 68 -75 I use to take care of whole bunches of these 
> items.
>
> Today we have op amps and digital displays.
> We care not about sensitive meter movements.
> The CU872 has a 50 micro amp like 1 x 10 -6 meter for self test.
>
> It has 20 6922 tubes dual triodes 10 deep cascode string.
> It is 1 RF in 8 RF out. Zero gain. But you spread out to 8 so the gain is 
> 8.
> But port to port is zero gain.
>
> Range is 500 KHz to 30 plus mega hertz.
> They were used world wide with lots of big antenna farms and many hundred 
> R390 receivers.
>
> We would run then 3 deep to fan one antenna out to 512 receivers.
> You could do it. We just never got that many receivers and operators all 
> interested
> in the same geographic area a big rhombic antenna covered.
> We did run three deep to get out to eight different rooms and then more 
> than 8 receivers in a room.
> Maybe 14 receivers on 7 positions in one room all looking at the same 
> region of the world.
>
> The point is we though the reception out of the units for all receivers 
> was as good as a receiver would
> get if it was just coupled direct to the antenna. And these test were run 
> often to show the problem
> was not the antenna, receivers cable or the CU872 couplers.
>
> The noise floor was excellent.
>
> Inside the unit has a filter that cuts out every thing below 2 MHz.
> This keep the AM broad cast band and cross modulation products out of 
> everyone's receiver.
>
> You can re cable the BNC connectors and by pass this filter if you want to 
> listen to the AM band.
>
> The unit is two identical amplifier pairs. The one input is split and 
> drives two set of 10 tubes.
> to four outputs. So 1/2 input yields 1 output for a real gain of 2.
>
> You can save half of your tubes and just run a 4 output device if you 
> want.
>
> You can re cable the amps into a series for a gain of 4 to four receivers.
>
> You have to think about the impedance matches as you are only driving one 
> amp from the
> antenna and not two amps in parallel. And you are taking one output back 
> to the second amp
> where you again have the same mismatch ratio. If you are only work a mega 
> hertz of interest,
> the transformers should be doable.
>
> The 6922 are just low noise 12 au7 or 12ax7 with a gain of just 33.
> The tube is 12 volts only it doe snot have a center tap filament to run on 
> 6.3 volts.
> 12 AT 7 have a gain of 100.
>
> You can repopulate your CU872 with "hot" 12AU7 12AX7 12AT7 type dual 
> triodes.
> I do not know what will happen to the noise floor or the gain.
>
> We run 6922 24 x 7 for about 2 years and then just changes a whole batch 
> of 20.
>
> We would grab 4 units on a semi annual twice a year basis and just give 
> one of them 20 news tubes and put it back in the rack.
>
> We would then run 80 tubes through the tube tester and select the best 60 
> to re populate
> the other 3 units. We sort of chose the top 20 next best 20 and third best 
> 20.
>
> The top 40 would all look good. The last 20 would be the best of 40 and 
> you could tell
> they were not as conductive as the best tubes.
>
> It was a method to keep every thing running and it worked.
>
> Hundreds of operators had CU872's inserted between their antennas and 
> their receivers
> and could not hear the difference as you swapped them from one to two or 
> three deep
> or from port to port or direct to the antenna.
>
> Cu872's were in the world from before my time in 68 and were still going 
> strong
> through the 70's into the 80's.
>
> For every 4 or 5 receiver in the Julian Creek Massacre there should have 
> been one CU872.
>
> No one knows what they were or why anyone would want one.
> Those of who do know have no reason to speak of such things.
>
> If you have one I would like to discuss acquiring one from  someone.
> One would be a life time supply.
>
> Antenna to one receiver gain is nil.
> Antenna to one receiver loss is nil.
> Antenna to one receiver noise floor change is nil.
> Antenna to one receiver sensitivity  change is nil.
> Antenna to 8 receivers is like 8 receivers on 8 antennas.
>
> Roger AI4NI
>
>
> Do any of you have one of these that is in active duty?    I'm curious 
> about
> its performance, etc.
> Since Goggle didn't turn up anything in the way of conversations about 
> this
> unit, it may be that the going price of 6922's caused these to be
> sacrificed.
> Many of you frequent the same forums and lists so please excuse the
> multi-posts.  I am casting a wide net hoping to catch a bonafide user to
> talk with.
> Thanks es 73,
> Chris
> W7JPG
>
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