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Author Topic: HF Amp Drive  (Read 24632 times)
W3RSW
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Posts: 606




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« Reply #15 on: August 09, 2014, 12:04:36 PM »


heathkit sb 230 schematic
Components and circuitry seem adequate although I was never a fan of grids lifted slightly from ground for application of bias.

Fwiw, some amps are optimum on 20 meters or another middle band as far as amplification and efficiency goes including some Alphas.  Meeting the power output spec. with reasonable, but not same drive over all the bands seemed ok with designers.

Another thought although unlikely on the lowest band and not showing up on others, particularly higher in freq. is that circuit constants for 80 are producing parasitics that would show at much higher freqs. than your LP100a can read.  I've read that the grid ring looses some contact over time and coupled with no. Parasitic suppressor going from plate to coupling cap may be inducing some parasitics.

But stuff like that is a long shot compared with simple component design trade off making 40 hotter than 80, ( e.g. That single 1000 pf plate coupling cap. , etc.) as others have mentioned.
-But Amp Still within spec.

The 8873/4/5 series are high gain (200) ceramic triodes that need high conductance connections to the grid, plate and cathode to be stable.  Carefully pull the tube, (see the caveats in the eham.net  reviews) and clean those tube and connecting rings. Add a parasitic suppressor too, not just the two beads on the B+ supply.
« Last Edit: August 09, 2014, 12:06:44 PM by W3RSW » Logged

Rick, W3RSW
N6AJR
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Posts: 9930




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« Reply #16 on: August 09, 2014, 01:22:07 PM »

yup
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XRAY99
Member

Posts: 110




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« Reply #17 on: August 10, 2014, 12:59:28 AM »


heathkit sb 230 schematic
Components and circuitry seem adequate although I was never a fan of grids lifted slightly from ground for application of bias.

Fwiw, some amps are optimum on 20 meters or another middle band as far as amplification and efficiency goes including some Alphas.  Meeting the power output spec. with reasonable, but not same drive over all the bands seemed ok with designers.

Another thought although unlikely on the lowest band and not showing up on others, particularly higher in freq. is that circuit constants for 80 are producing parasitics that would show at much higher freqs. than your LP100a can read.  I've read that the grid ring looses some contact over time and coupled with no. Parasitic suppressor going from plate to coupling cap may be inducing some parasitics.

But stuff like that is a long shot compared with simple component design trade off making 40 hotter than 80, ( e.g. That single 1000 pf plate coupling cap. , etc.) as others have mentioned.
-But Amp Still within spec.

The 8873/4/5 series are high gain (200) ceramic triodes that need high conductance connections to the grid, plate and cathode to be stable.  Carefully pull the tube, (see the caveats in the eham.net  reviews) and clean those tube and connecting rings. Add a parasitic suppressor too, not just the two beads on the B+ supply.

Would parasitics on 80 meters be visible on a 50 mhz oscilloscope? 
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W3RSW
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Posts: 606




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« Reply #18 on: August 10, 2014, 05:29:19 AM »

Possibly, but if they were that low (50mhz or so) your LP 100 a would have already rolled them into its total power seen.  I was thinking of VHF parasitics and that was a long shot thought anyway since if you really had a bad case of parasitics, that would probably affect every band.

But hook up an attenuator to a pickoff, say from your dummy load and see. Your scope should show some freqs higher than 50, just not calibrated.

But I'd look at what follows first:   Grin
Rereading your symptoms, sounds like something else is going on. One Reviewer says it's very easy to tune and load. Is the tuning tricky only on 80 but ok elsewhere?  The load control not doing much, but only on 80, could mean the parallel, fixed tune cap, c13 (100pf 5kv), or the fixed load cap, c14 (500pf 2kv) may be bad as others have mentioned but more specific as to where to look. The caps may be ok, but the solder joints not.

 Oh and check the band switch wafer contacts to these two caps.  -Carbon arc burns,  bad solder joints, lifted switch contacts, etc.
« Last Edit: August 10, 2014, 05:57:59 AM by W3RSW » Logged

Rick, W3RSW
KD1I
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Posts: 466




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« Reply #19 on: August 20, 2014, 02:51:37 PM »

Actually, the power levels you mention are rather low for an SB200.     I had a similar situation where all bands showed low power out.  Changing the tubes did not resolve the issue. Changing the filter caps did. If yours are original as mine were, they can be a bit leaky. This is a voltage doubler circuit so they are more then just filter caps. The values I used were a bit higher than original but if you decide to try it, don't raise the capacitance too much over stock. Also, I changed the rectifier diodes as well because with the higher value caps the current specs of the original diodes are marginal. Modern replacements are very cheap.    Best 73, Jim
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N4CR
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Posts: 1757




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« Reply #20 on: August 20, 2014, 04:29:53 PM »

I.  On 80 meters with 45 watts in I get about 350 - 380 out.  On 40 it's 35 in with approx. 480 out.

2.  This is with a LP100A set to read avg. power.

The LP-100A is calibrated per band and at one time was a kit.. Was it a kit? Professionally calibrated? Each band has a linearity, a high calibrate and thermal correction if I recall. Been a while since I calibrated mine. But for sure it's per band calibration.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
KK5DR
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Posts: 631


WWW

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« Reply #21 on: September 06, 2014, 04:30:59 PM »


Would parasitics on 80 meters be visible on a 50 mhz oscilloscope? 
[/quote]
Perhaps, depends on the frequency of the parasitic. Most of them are in the 90-250MHz range. It happens that the top end of the oscillation seems tied to the upper limit of the tube in use.
There is such a thing as low frequency parasitic as well. These are oscillations in the MF, LF, and VLF ranges. These tend to be less destructive than the VHF type. They usually manifest themselves as an amp that continues to transmit an unstable carrier like signal, even after it has returned to receive mode.
Parasitic oscillations tend to be extremely broad-band and unstable. It's unlikely you would ever see anything on a scope that would look like a carrier waveform. It would look most like a hysteris or "ringing" type broad-band noise pulse that covers several megahertz of range in the VHF spectrum.
Hope this info helps you in your experimentations.

Matt KK5DR "I favor the EIMC based antenna tuner."
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