The AM Forum
March 28, 2024, 10:37:16 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: 1 2 [3]   Go Down
  Print  
Author Topic: need ideas for a low power rig.  (Read 28154 times)
0 Members and 1 Guest are viewing this topic.
KA2DZT
Member

Offline Offline

Posts: 2192


« Reply #50 on: January 25, 2015, 10:44:01 PM »

On the small chassis you pictured , where do you plan to get 1500 volts?  It won't be from that Triad xfmr you pictured.  The R-118A  xfmr is 750 volts CT. at 175ma.

I have built 1400 volt supplies using two xfmrs like the R-118A.  Put a FWB around each xfmr.  Connect the diode rectifier output (of the first xfmr) directly to the negative terminal of the second xfmr FWB and the two voltages will add together.  Second xfmr has to be able to withstand the higher voltage on its secondary.  You can do this with any two xfmrs and get whatever voltage levels you need.  You can use any combination of FWB or FWct rectifiers.

Fred
Logged
N2DTS
Contributing
Member
*
Offline Offline

Posts: 2307


« Reply #51 on: January 25, 2015, 11:21:05 PM »

Full wave bridge, should give 1500+ volts cap input, the regulation is not important because screen mod is steady average.
The cap is a problem though, I need a 2000 volt cap and dont like series electrolytic caps that much, it would take 4 in series.
The tube modulator will do a nice job on a single 4x150, 100 watts out if you push it some, 50 watts is easy, you just need some voltage on the tube. I do not think they do much at 800 volts, but I should test that again.
I have about 15 old 4cx250b's, good used ones plus some new ones.

They are small,
They take almost no drive,
They do not need much air at lower power levels,
They screen modulate very well and easy,
I have a crap load of them.
Logged
KA2DZT
Member

Offline Offline

Posts: 2192


« Reply #52 on: January 25, 2015, 11:35:59 PM »

Cap input, no load, 750 volts x 1.4, about 1100 volts peak.  Add any load and the voltage drops from that.  Plus it will put a heavy VA load on that xfmr.

You need a bigger xfmr or a second xfmr.

Do you have any other similar xfmr like the R-118A??
Logged
N2DTS
Contributing
Member
*
Offline Offline

Posts: 2307


« Reply #53 on: January 26, 2015, 07:55:01 AM »

I have some others, but they get bigger.
The trans is 750-0-750, 175 ma,  2100 volts no load cap input, 1350 choke input.
A single 4x150 would pull about 150 mills, 202 watts in, 50 to 60 out, 150 watts plate dissipation out of 250.
The high voltage is a pain though...

Logged
KA2DZT
Member

Offline Offline

Posts: 2192


« Reply #54 on: January 26, 2015, 11:47:51 AM »

I have some others, but they get bigger.
The trans is 750-0-750, 175 ma,  2100 volts no load cap input, 1350 choke input.
A single 4x150 would pull about 150 mills, 202 watts in, 50 to 60 out, 150 watts plate dissipation out of 250.
The high voltage is a pain though...



My catalogs show the R-118A as 750v CT.  To me that means 375v-0-375v.
Logged
N2DTS
Contributing
Member
*
Offline Offline

Posts: 2307


« Reply #55 on: January 26, 2015, 12:34:58 PM »

I better check that out then!
Logged
N2DTS
Contributing
Member
*
Offline Offline

Posts: 2307


« Reply #56 on: January 26, 2015, 08:01:10 PM »

Yes, that trans is 750 volts center tapped.
I tried tests with one 4x150 with the tube modulator, at 700 volts you can get about 10 or 15 watts out.
They really want 1500 volts and up for screen modulation, so that is out.
I have transformers that will do it, but things tend to get large with high voltage.
Logged
KD6VXI
Contributing
Member
*
Offline Offline

Posts: 2648


Making AM GREAT Again!


« Reply #57 on: January 26, 2015, 08:54:08 PM »

Voltage doubler.

--Shane
KD6VXI
Logged
KA2DZT
Member

Offline Offline

Posts: 2192


« Reply #58 on: January 26, 2015, 09:13:59 PM »

Yes, that trans is 750 volts center tapped.
I tried tests with one 4x150 with the tube modulator, at 700 volts you can get about 10 or 15 watts out.
They really want 1500 volts and up for screen modulation, so that is out.
I have transformers that will do it, but things tend to get large with high voltage.


If you have another xfmr like or anything close you can get a 1400 volt supply from two.  I described how in an earlier post.  Two smaller xfmrs can fit nicely on a small chassis than one large xfmr.  A FWB on a 750v ct xfmr can give you the low voltage supply (300V) off the CT and the the high voltage from the diode bridge is then connected to a second 750v ct xfmr that also has a FWB rectifier.  You connect the HV output from the first FWB diodes directly to the negative terminal of the second xfmr FWB.  You end up with a 300+ supply for the low level stages and a 1400 supply for the PA stage.

You can put a PTT relay on the HV line between the two FWB rectifiers and maybe a second relay contacts can be used to PTT the low voltage supply.

Keeping it simple, you can use a RC filter on the low voltage supply and a LC filter on the HV supply.

I've made a number of these supplies and they work perfect.  The only thing is the second xfmr has to be able to withstand the high voltage on its secondary.  Most common xfmrs is this class should be able to handle the high voltage.  On some I've improved some of insulation on the wires coming through the metal end bells.  Also added some additional insulation on the winding core and inside of the end bells.

Fred
Logged
N2DTS
Contributing
Member
*
Offline Offline

Posts: 2307


« Reply #59 on: January 27, 2015, 07:57:50 AM »

Oddly enough I have some smaller transformers that would give 2000 volts out in full wave center tap, I think one is 120ma or its 220 ma, the first number is rubbed off.

I don't think I want to go that way tho, 1500+ volts has space problems, I do not like strings of electrolytic caps at high voltage, plus that and oil filled caps good for the voltage are huge.

I can go 2 ways, a pair of 6146's screen modulated at 25 watts output, or a single one modulated by a pair of 6l6's at 40 watts output.

 
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 8308



WWW
« Reply #60 on: January 27, 2015, 11:05:34 AM »

You should be able to get more than 10W. By way of encouragement, the plate modulated transmitters T-217 and T-282 use a pair of class C 4CX150's at 850VDC to make 100W carrier at 225-430Mhz.

G1=-115V,
G2=180V to 300V maximum,
Eb=860V.

Modulation B+ is also 860V and is by a pair of 811 and the other a pair of 4-65 for modulators. The mod transformer is about 8KCT to 3K (pair of 4X150's).

Manuals are here.
http://www.bunkerofdoom.com/lit/mil_uhf_01/index.html
Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
KA2DZT
Member

Offline Offline

Posts: 2192


« Reply #61 on: January 27, 2015, 11:17:36 AM »

My HB 6146 x 807s used one 730v ct  265ma TV xfmr with an economy supply.  Worked FB, xfmr ran warm to hot after being on for hours.  The xmtr grew larger over time and I now have a second big 730v ct 300ma xfmr TV xfmr for the 650V DC supply and the original supply just handles the 300V DC.  The second big TV xfmr is also used to bring up the 813 x 811s supply from 1200V DC to either 1500V DC or 1800V DC depending on which output from the FWB rectifiers I feed into the CT of the HV plate xfmr.

Very complicated xmtr with a lot of relays and switching.

For your small scale project I like either of the two you mentioned.  You should be able to get by with the one R-118A xfmr,  current rating is a little light but for short duty cycle it may not get too hot.

Fred
Logged
N2DTS
Contributing
Member
*
Offline Offline

Posts: 2307


« Reply #62 on: January 27, 2015, 11:54:22 AM »

Not in screen modulated service if you want clean audio.
Plate modulation is fine at any voltage, but with screen modulation, you only get 100% modulation with 700 volts on the plate at up to about 10 watts, above that, positive modulation drops off a lot.
At 1300 volts and up (more is better) it works great and you can get huge peaks at higher voltages.

Yes, I could plate modulate a 4x150 at 500 to 700 volts on the plate, say 600 volts at 200 ma for 120 watts in and 90 watts out would work fine, but with a pair of 6l6 modulators its pointless and needs air on the tube.

I could likely run it at 600 volts and 100ma and it would be ok and need hardly any air.
Not much advantage over a 6146 though.



You should be able to get more than 10W. By way of encouragement, the plate modulated transmitters T-217 and T-282 use a pair of class C 4CX150's at 850VDC to make 100W carrier at 225-430Mhz.

G1=-115V,
G2=180V to 300V maximum,
Eb=860V.

Modulation B+ is also 860V and is by a pair of 811 and the other a pair of 4-65 for modulators. The mod transformer is about 8KCT to 3K (pair of 4X150's).

Manuals are here.
http://www.bunkerofdoom.com/lit/mil_uhf_01/index.html
Logged
Pages: 1 2 [3]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.073 seconds with 19 queries.