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Author Topic: To Tom - W8JI - from Brian K7ZRZ w/clipperton L  (Read 14615 times)
W8JI
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« Reply #15 on: February 23, 2010, 12:15:01 PM »

One of its original little problems was too much bias - holding the idle plate current down way too low (about 50-60 ma. on HV/SSB position. It seemed to cause a fuzzyness on low audio components (breath) and a bit of a ripple on a very low-power carrier. Increasing the idle plate current to about 120 mils has taken that dirtyness away completely. I hear the very same thing going on in the other amp that my friend Jeff K7ZSA has now - which also has the same low idle plate current.

Just thought I'd place these comments here in case someone else might run across this and be interested in my experience.

Brian K7ZRZ


Brian,

The ripple is actually caused by Dentron not using a center tap on the filament winding. That's a very bad design. Changing the bias changes the % of hum modulation and distortion caused by that, but does not actually make it go completely away.

There are fixes for that.

You can add a small 6-12 volt filament transformer with a CT, like a 2 to 5 ampere cheap filament transformer, across the filament winding and get the dc CT from it (even though voltage and current is supplied by the original winding) or use a resistor "artificial center tap".

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K7ZRZ
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« Reply #16 on: February 24, 2010, 08:17:26 AM »

Tom,

Thanks for the tip. With the replacement power supply component board having much smaller filter caps, there is now ample room in that area for such an addition (small filament xfmr), I believe. Do I understand you correctly that such an added transformer's secondary would be wired directly across the existing filament supply and the CT bonded directly to ground - and that's it?

And can you please elaborate a bit on the resistor artificial CT method, or post a link to such a discussion.  Thanks. You have two of us watching you again with interest.

Brian K7ZRZ
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Brian K7ZRZ
W8JI
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« Reply #17 on: February 24, 2010, 02:06:59 PM »

Tom,

Thanks for the tip. With the replacement power supply component board having much smaller filter caps, there is now ample room in that area for such an addition (small filament xfmr), I believe. Do I understand you correctly that such an added transformer's secondary would be wired directly across the existing filament supply and the CT bonded directly to ground - and that's it?

And can you please elaborate a bit on the resistor artificial CT method, or post a link to such a discussion.  Thanks. You have two of us watching you again with interest.

Brian K7ZRZ

Instead of the tube cathode current return picking off one side of the existing filament transformer, that return wire goes to the CT of the new transformer. The new xfmr is across the secondary of the original. The new transformer  can be any voltage reasonably over 6 volts.

The resistors can be two 5.6 ohm 5 watt resistors in series. That would be 11.2 ohms across 6.3 volts, or 0.6 amps. The cathode return would go to the junction of those two series resistors. The equivalent cathode return resistance would be 2.3 ohms, and balanced for AC. The resistors would dissipate about 3 watts maximum each worse case.

You could use the resistors in lieu of a transformer.

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K7ZRZ
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« Reply #18 on: March 16, 2010, 08:49:47 AM »

Tom,

Both Jeff K7ZSA and I (each with a Clipperton L), are interested in making (one of) the filament winding CT modification(s) you have suggested here.  Just to make sure we completely understand what you have instructed in your reply above, could you please fetch the amplifier-section schematic from here, http://bingoldsby.com/P09.gif - and either make SPECIFIC details about what you mean by what you wrote, or just modify the schematic in the ways you have in mind, and post it back to me somehow.

The point I'm not sure about is your reference to the "cathode current return picking off one side of the filament transformer."  I'm sure this is elementary to you, but I have to be absolutely sure of what I'm doing before rewiring the thing... and just now, I'm not.

Thank you very much for helping us through this little detail.

Brian K7ZRZ
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Brian K7ZRZ
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« Reply #19 on: March 16, 2010, 05:12:45 PM »

Can't use that schematic.  The change in wiring is at the filament winding of the transformer Brian, not in the RF section.

I have a schematic...QRX.
« Last Edit: March 16, 2010, 05:16:44 PM by Tom Rauch » Logged
W8JI
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« Reply #20 on: March 16, 2010, 07:17:07 PM »

Can't use that schematic.  The change in wiring is at the filament winding of the transformer Brian, not in the RF section.

I have a schematic...QRX.

Try this

http://www.w8ji.com/dentron%20clipperton.htm
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K7ZRZ
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« Reply #21 on: March 16, 2010, 07:42:43 PM »

Thanks, Tom, that makes a foolproof instuction for both of us... and I deeply appreciate it. I have sent the link on to Jeff also.  I'm sure it will be the resistor network.  73

Brian K7ZRZ
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Brian K7ZRZ
K7ZSA
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« Reply #22 on: March 18, 2010, 03:16:36 PM »

This question is for Tom, W8JI.  Regarding the phantom center tap resistor network across the filament secondary to provide a center tap to eliminate hum caused by the AC inbalance Tom's circuit shows using two 10 ohm 2 watt METAL resistors, however; at that voltage and resistance the power dissapation is almost 2 watts, leaving no headroom and likely running pretty hot.  I would like to use 5 watt resistors, which are wire-wound.  Do you see any problem using this type of resistor?  Since they are on the "cold" end of the filament rf choke I doubt that wire-wound would be a problem, but I would like to hear from the pros!
Thanks
Jeff
K7ZSA
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W8JI
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« Reply #23 on: March 18, 2010, 04:42:52 PM »

This question is for Tom, W8JI.  Regarding the phantom center tap resistor network across the filament secondary to provide a center tap to eliminate hum caused by the AC inbalance Tom's circuit shows using two 10 ohm 2 watt METAL resistors, however; at that voltage and resistance the power dissapation is almost 2 watts, leaving no headroom and likely running pretty hot.  I would like to use 5 watt resistors, which are wire-wound.  Do you see any problem using this type of resistor?  Since they are on the "cold" end of the filament rf choke I doubt that wire-wound would be a problem, but I would like to hear from the pros!
Thanks
Jeff
K7ZSA

Jeff,

I understand your point.

6.5 volts filament across 20 ohms. 6.5^2/20 = 2.1 watts total or 1.05 watts per resistor.

While they rest at 1 watt per resistor, they do go a little over 2 watts from maximum plate current. That's why I said to use a OX/OY series metal composition 2 watt resistor. They have exceptional thermal reserve, and are very stable with long life even when hammered hard. As a matter of fact they are designed to be really hammered hard with overloads.

http://www.ohmite.com/cgi-bin/showpage.cgi?product=ox_oy_series

I would never use a carbon like that, but many metal resistors would be fine. The OY, in 70 degree C ambient environment without forced air, can handle 2 watts CCS. I'm sure they would easily handle the Clipperton's environment. Do not use a regular metal unless it is a 3 watt!

If you use a 5 watt resistor, you could use a 6.8 ohm. That would be 13.6 ohms across 6.5 volts, or 3.2 watts across two resistors. 1.6 watts per resistor. That would work really well.

Maybe the OX/OY series 2 watts are hard to get, or expensive??

73 Tom




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K7ZSA
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« Reply #24 on: March 19, 2010, 08:49:04 AM »

You make two very good points.  The total wattage is actually divided among the two resistors, and, the advantage of using metal comp. resistors.  Whether I use 5.6 ohm, 6.8 ohm, or 10 ohms will get the job done with different power consumption levels.  I can order anything, but I have wire-wound available at hand and that was one question that remains un-answered.  Can you see any problem using that type of power resistor?
Thanks and 73
Jeff
K7ZSA
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W8JI
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« Reply #25 on: March 19, 2010, 10:07:56 AM »

You make two very good points.  The total wattage is actually divided among the two resistors, and, the advantage of using metal comp. resistors.  Whether I use 5.6 ohm, 6.8 ohm, or 10 ohms will get the job done with different power consumption levels.  I can order anything, but I have wire-wound available at hand and that was one question that remains un-answered.  Can you see any problem using that type of power resistor?
Thanks and 73
Jeff
K7ZSA

There is no problem with using wire wound. Used the lowest resistance value the dissipation allows that you have handy. Don't go too extreme, but the transformer will take another amp or two filament load without a sweat.
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