• You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

Modulation in Reverse and PEP drops SWR?

Alabama Buckeye

Dogma Heretic
Jul 29, 2022
136
87
28
Magic City Alabama
I've got 3 issues with my CB radio performance.
All which occur when I put my heavy wool socks on. Actually, I rarely stress the Fatboy 8X above 1000W. I've never ventured to see it's full potential.

I drive the 8X with a variable DaveMade, 2 pill, M80. I DK into the M80 with 1.5-2W which results in a DK ~35. Could go without the driver, with full open DK on the Stryker 955, 30 swinging ~75W PEP. But I apply the throttle more readily with the variable of the M80. But if I push the M80 to approaching 200 PEP then the FB8X will cause one or another or all three symptoms to occur.

1. DK swings to 800-900 with no modulation (PEP) reflection.
2. DK swings backwards with modulation,
3. Modulation drops SWR remarkably.

Any positive insight will be appreciated.
 

brandon7861

Well-Known Member
Nov 28, 2018
258
278
93
Ringsted, Denmark
The attached one page document talks about downward modulation, but written for tubes. Still interesting though.

DK too high. If the finals are already operating near the voltage rails to achieve that 900w DK, there is no way to get positive modulation. That leaves you with more downward modulation than upward.

I'd bet that the reduction in SWR with modulation is a result of the downward modulated periods bringing the amplifier into a more linear region that generates less harmonic content, which then suggests 900w DK is way too much for that amp. I've never had a fatboy and have no clue if 8x means it has 8 pills, but aren't these types of transistors good for about 100w each? What wattage do you say this thing is rated to put out?
 

Attachments

  • Downward Modulation.pdf
    528.2 KB · Views: 4

Alabama Buckeye

Dogma Heretic
Jul 29, 2022
136
87
28
Magic City Alabama
I think you would find your audio is a lot louder with that radio driving the 8-transistor box directly. The driver amp is being driven far too hard by that radio. Pretty sure that's the root of what you're seeing.

73
Perhaps I wasn't clear. I drive the M80, never above, 1.5-2W which will DK ~30-35W and PEP, with variable peaked, upwards to 200W; I usually keep it 100-125 PEP.
I can use the Srtyker 955 or 655 alone at full power will also produce 30-35W, but only 75W PEP, to drive the FB8X. Maybe pushing 10W to each FB8X pill.

It's when I push the M80 to 200W PEP into the FB8X that I get the symptoms.
That's 25W to each SC2879 pill. A Pill rated for 100W+.

I've seen stories, YouTube, that these 8X boxes produce upwards of 2000W.
No interest in testing it to such levels; though seeing 1500 will bring on a smile.
 

Alabama Buckeye

Dogma Heretic
Jul 29, 2022
136
87
28
Magic City Alabama
The attached one page document talks about downward modulation, but written for tubes. Still interesting though.

DK too high. If the finals are already operating near the voltage rails to achieve that 900w DK, there is no way to get positive modulation. That leaves you with more downward modulation than upward.

I'd bet that the reduction in SWR with modulation is a result of the downward modulated periods bringing the amplifier into a more linear region that generates less harmonic content, which then suggests 900w DK is way too much for that amp. I've never had a fatboy and have no clue if 8x means it has 8 pills, but aren't these types of transistors good for about 100w each? What wattage do you say this thing is rated to put out?
Yes, FB8X is a 8 Pill 2SC2879 RMS 100W. I know of nothing "published" other than "FB8X for sale".
Absolutely 900W DK is unhealthy. Perhaps smoking dangerous. I dial it back when this happens but why is it happening. As sed just moments ago, the Driver is swing to 200PEP on modulation, then adding in the FB8X the DK goes bonkers with 800-900W DK. Thats a mere 25W per FB pill when modulating. Off the top of my head I don't recall the DK on the Driver when it's swinging 200 PEP. 2X SC2290s (60W RMS) with variable.

Awh, haw! I just again looked at the data sheet: 2SC2290 60W PEP not RMS.

I'm dumb founded. I swear I read output as Watts RMS. Checked 2SC2879 and again 100W PEP not RMS.

Thanks 7861, Thanks Nomad.

Case Closed.
 

dxChat
Help Users
  • No one is chatting at the moment.
  • @ BJ radionut:
    G8DX WIDE 20kc wide ..7L at 80ft and limit on 28.491 20 over here in Ind
  • @ BJ radionut:
    band wide open to Euro's on 10m
  • @ Joseph23:
    can anybody help me with a cobra 139 ssb 23ch. no modulation indicated when keyed up. any help would be appreciated also looking for a NEC1307 1K64C.
  • dxBot:
    AmishLincoln142142 has left the room.