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I am having a hard time deciding what to do for bias control

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Old 09-14-2011, 07:11 PM
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Default I am having a hard time deciding what to do for bias control


I will not build an HF amp that is not all modes simply will not nueter an amp like that. Plus you never know when the day will come like emergency, WWIII or something unpredictable might happen where you might want AM. Granted even when I talked on CB I never spent much time on AM 98% of my time was on SSB. On those rare occasion though when I wanted to talk on AM or every time I travel I75 from Ga to Mi or MI to Ga. having AM has saved me tons of tickets. So I just can not see building a fantastic amp and not having the oldest mode of operation. You guys do realize that AM was around before they figured out how to chop one side off to make a SSB rig and then they discovered FM......Wait technicly CW came first didn't then AM....????? Please correct me I do not want to lie to anyone.

So I have 2 different ways I can do this I can use multiple Zener Diodes, mutliple resistor values and a common capacitor in the chain and just use a switch to select the right diode resistor combo to get the right bias that or close enough.....I like the simplicity of that idea and if something goes wrong you can trouble shoot it in about 30 seconds time. The other would be to use a transistor and associated partsto regulate the bias. THis method would provide the perfect bias at any power level so you would get slightly better power output, slightly longer tube life and less heat in the box. I am just not sure how reliable or durable this method would be.

I already gave up my desire to use tube 1/2 or full wave rectifer's in series to do the ac-dc regulation. Basicly to get the current output I needed I would have to have used not less then 10-20 rectifer tubes in a combo of series and paralleled network to get tot he proper current and voltage. I would have had to power them up 1-2 or more minute at least before putting any high voltage through them and they would take up a lot of space and generate a lot of heat. I had questions that no one seemed to be able to answer about them as well.....I do not think you should parallel tube rectifiers but no one I asked could tell me for sure. So I changed my mind and now am going with modern silicone diode 1000V 6a, 450V 100uf Teflon Caps and balancing resistors mostly to bleed them down because I do not believe you need to balance this type of circuit with them to be honest.It is a rectifier doubler circuit. I have 23 of the above 100uf 450V caps to play with. I am using two of them to 2 of them int he case of 25 to repair a 10meter Linear. I know a lot of guys use 240uf 450V caps in their rectifier/doubler designs but I got a good price on the above.

I hate changing my mind about design I am usually a very decisive and do not change my mind often but this is all knew stuff for me. I have repaired plenty of stuff but this is a combo of my idea's and other peoples idea and I find that my lack of experience creeps in and causes some doubt. So what do you guys and gals think should I stick to the simple idiot proof Zener bias control and use a switch to select my bias or should I go solid state? If I go solid state I will not need to babysit it at all it will be out of sight and out of mind.


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Old 09-15-2011, 08:10 AM
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A linear amp uses the same bias voltage on AM as it does on SSB and should produce almost exactly the same peak output.

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Old 09-15-2011, 07:02 PM
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Quote:
Originally Posted by Kamikaze View Post
A linear amp uses the same bias voltage on AM as it does on SSB and should produce almost exactly the same peak output.

Exactly. Make the amp class AB1 and leave it at that.As for what method to use, what's wrong with the old tried and true variable potentiometer? Set it and forget it.
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Old 09-15-2011, 11:31 PM
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So just make it AB1 and not bother switching between class C and AB1? Ok that does make the most sense. Ok.........That makes sense!! See this is why I like being able to ask questions. I appreciate it!

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Old 09-16-2011, 03:56 AM
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Leaving the amp in class AB will mean a bit lower efficiency in CW and FM modes. I don't know off hand of any commercial amps that switch bias for different modes.I think there was at one time an amp or couple amps that did have provisions to switch bias to allow a bit better efficiency on CW but that was a long time ago.
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Old 09-17-2011, 04:13 AM
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Quote:
Originally Posted by Captain Kilowatt View Post
Leaving the amp in class AB will mean a bit lower efficiency in CW and FM modes. I don't know off hand of any commercial amps that switch bias for different modes.I think there was at one time an amp or couple amps that did have provisions to switch bias to allow a bit better efficiency on CW but that was a long time ago.
30S-1 and it is only a 52 year old design. That was just yesterday

Before the 1500 watt P.E.P. rule went into effect legal limit amps had a CW and SSB position which usually changed plate voltage via taps on the plate transformer. Most did not actually change the zener used for bias but they DID change the conduction angle due to the lower plate V.

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