[AMRadio] Re: How can we rid the HF bands of AM ?


Geoff w5omr at satx.rr.com
Fri Aug 12 07:17:34 EDT 2005


>> You like fiddlin', and that's fine.  But, for me (and I think some 
>> others here agree) there's nothing finer than saying "rig here is 
>> homebrew" and getting that "man, that's a good sounding rig" report 
>> in return.  The feeling is indescribable.
>
>
> I can relate to that. First time we cathode-modulated Dennis W5FRS's 
> homebrew 250TH amp with an Altec 1570B tube amp, we were ecstatic over 
> the reports. I think the one that got me was "sounds just like a 
> broadcast station". We did tweek the amp quite a bit to get the 
> modulation linear, It was originally running AB2, and we had to 
> increase the bias and drive, and load it a little less, but it sure 
> did sound sweet. Some pice are here:
> http://rawfire.torche.com/~opcom/dbrady1/index.html


Fire is best served in it's natural state?  ;-)

Say HI to Dennis for me.  I've worked him a few times on 80m CW down at 
3.546Mc and once or twice on 7.160Mc AM and just the other day, we 
-almost- had a QSO on 7.050Mc CW, from my Viking II at around 50w.  It's 
the rig that serves as the exciter for the 250THx250TH rig.

>> I tried to sell a 450TH, appearing as if it was unused, no scratches 
>> on the base - asked in here for $50 for it, in the metal 
>> spring-loaded shipping frame, in a box.  No one bit.  Put it on ebay 
>> for 7 days, and started it off at 9.95 and it sold for some 3X (plus 
>> shipping) what I was asking for it originally, in here.
>
>
> A 450TH is a very specialized tube today. Sometimes it takes just the 
> right buyer. $50's cheep cheep for a new one, I think.


I've got SEVERAL 450TL's that are eating up a lot of space in my shack.  
I could use the money, right now.  They all have filament glow.

> I did go through a few of 5R4's in the modulator bias supply before I 
> figured out that one of the 304TH's had an intermittent short from 
> filament to grid, but only with HV applied. Oddly, whatever happened 
> 'blew up' the filament in the 5R4 before any fuses or overloads could 
> trip, and then the bias supply fuse blew.. This was in a supposedly 
> good 304TH I bought from ebay. Due to the scarcity and high costs of 
> 304TH's, I have changed to 3-500Z's now, and I need to re-do the bias 
> supply for 0-30V instead of 0-300V! I think a 10:1 voltage 
> divider/bleeder drawing about 100mA at 300V should do (The modulator 
> bias power supply is rated 200mA). That way the rig stays more 
> original, and the grid supply now has a 300 ohm resistance!


I need to see about locating a 304TH rig I know of.  I don't know if 
it'll ever be put back on the air, by the present caretaker of that 
rig... and it's just the final section.
That thing used be be a pair of 304TH's (or was it TL's, John?) 
modulated by (4) 813's in push-pull, parallel.  I full kilowatt DC input 
to the final, with something like 1100 or 1200v at 750 or 800mA.  
Somewhere in that area.  As a pulse Radar tube, the 304's will 
-certainly- handle the current!

>> Just for grins and giggles, try putting 2 50 ohm resistors in series 
>> to ground, and then the grid to the center of the two 50 ohm 
>> resistors.  It's helped other rigs.
>
>
> I will try that. I am not sure of the effect it will have with the 
> grid tank feeding a low resistance load of 100 ohms, but it will point 
> me in the right direction.


It provides a voltage divider network.  Anything stray, goes to ground.

> I'm looking at these conditions (they represent much more power than I 
> will legally use, but should be good for figuring. There were no 
> figures for 2500V or less.)
>
> Plate Modulated Class C Amplifier [CCS]
> Plate Voltage ................................. 3000 V
> Grid No. 2 Voltage ............................ 500 V
> Grid No. 1 Voltage ............................ -200 V
> Peak Grid No. 1 Voltage........................ 340 V
> Grid No. 1 Current ............................ 12 mA
> Plate Current ................................. 600 mA
> Grid No. 2 Current ............................ 145 mA
> Driving Power ................................. 12 W
> Power Output (approx) ......................... 1390 W
> Maximum Frequency ............................. 110 MHz
>
> My question, is from these, how do I figure the impedance of the load 
> presented by the grid circuit to the grid tank?


The same way you would for the plate tank. 
R=E/I
R=340v/12mA
R=28.3k Ohms
The power dissapated is a tad over 4w, so (4) 100kohm, 1-watt resistors 
with a parasitic choke wired around 'em would be plenty, I would think.  
Run that in series with the grid drive to the pin. 

Do the same for G2.

>> Seperate 120v circuits?
>
>
> Yes, indeed COL Tucker apparently did not want to install a big ugly 
> 240V outlet, so he apparently used a pair of 20-amp 120V outlets 
> connected to 240V in the breaker box. Of note in this rig, is that 
> neutral is bonded to ground. period. That is, there is no ground prong 
> on the plugs and it is not possible to separate neutral from ground in 
> this rig without a major rewiring job. So, I will have to take care 
> that I do use a 4-wire grounding 240V plug, and see that I really do 
> have a good ground back to the breaker panel. Should present no 
> problem if done correctly.


Many rigs were built with such circuits as "bias supplies from recified 
AC line voltage, with respect to chassis ground".  Shoot, you could even 
rectify AC line voltage, and make sure it's well filters (clean) and add 
it, in series with a DC supply, for an extra 150VDC, or so... "with 
respect to chassis ground".  I think you're gonna probably have to keep 
it on seperate 120v circuits.   A single 240v circuit would work, but 
break it out into two 120v circuits and make sure the plug is properly 
polarized.  Yes... AC polarity.  makes a difference.

>> But.. one works hand in hand with the other.  In fact, John/WA5BXO 
>> and I have been discussing (at length, and credit to John for 
>> sticking with me and my dumb questions ;-)) audio, it's effects on 
>> the transmitted carrier, how much voltage is neeed to fully modulate 
>> the carrier and what is needed in the way of a reactor to sucessfully 
>> couple the audio to the B+ line to the final amplifier.  This 
>> conversation encompassed power supplies, through the modulator and 
>> the final.  All components are cruicial.
>
>
> I picked up a modulation reactor some time ago. There might be room 
> for it if I get rid of the filament transformers for the 673 rectifers 
> and put in solid state. I kind of dread building two 15000PIV 1A 
> bridge rectifiers, but I need to do it anyway.


Find you a pair of 872A's.  Mercury Vapor Rectifiers.  They'll work, 
just fine.  I need to do the same, here.  I have the rectifiers.

>> John and I also talk, from time to time, at length about asymetrical 
>> audio.  I've achieved it, somewhat.
>>
>> Sometimes technology is overwhelming.
>>
>> For less than $100, I bought not only a digital camera that takes 
>> amazing 3.2megapixel pictures, but it also has a 'movie' mode.
>>
>> No audio, but the movie mode is pretty awesome.
>>
>> "Hel-l-l-l-l-o test. one, two, three, four."
>> http://w5omr.shacknet.nu:81/~w5omr/pictures/2005-08-08/000_0049.MOV
>>
>> The 'noise' you see is a box-fan at full speed.  It was over 100° 
>> here, that day.
>
>
> That is very impressive, I always strive to be rid of power supply noise.


The rig is clean - I mean a box-fan, from about 10ft away, blowing 
-hard- on the back of the rig.  I mean, it was H-O-T in the shack.

I guess you know, we're generally up and on the air around 4am or so, 
when the QSO starts, and probably runs till around 6am when folks get 
out - then another group cranks up on 3.890 in North Texas, Oklahoma & 
Arkansas and in South Texas, we're on around 7am or so, on 3.880.

We'll have to talk off-list about your choice of  Linux distros ;-)


---
73 = Best Regards,
-Geoff/W5OMR





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