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Bias design, amplifier design, filtering etc.

Moleculo

Ham Radio Nerd
Apr 14, 2002
9,034
1,335
283
That is just the bias circuit in the picture, it is 6x2SC2879 amplifier

Yep, I knew it was the bias circuit and my guess from the transformers was that it was a 6 transistor amp. Very clean layout. I want a 6 meter amp like that :) :)
 

eagle1911

Active Member
Jan 21, 2011
327
19
28
Once again, old thread.. but a really good one for sure.

But reading all of this and researching in many different places on bias and how to implement it I'm left with a resounding question: Why isn't at least some sort of AB bias just standard equipment in all SSB capable amplifiers, even CB amps?

After all, the circuits are really never all that complex, and the cost in parts is quite low.. A decent thermally tracked bias circuit would add all of $10 in parts in the extreme case when you factor in the discounts afforded when buying in bulk plus a little extra time to build it in.. You could just have a standard bias board made in China for $5.00 or less apiece and stick one in your amp for every transistor pair..

It just seems to me such a comparatively cheap and simple thing for all the fuss that surround it.
 
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Captain Kilowatt

Professional Amateur
Staff member
Apr 6, 2005
16,833
10,962
823
58
Nova Scotia,Canada
But reading all of this and researching in many different places on bias and how to implement it I'm left with a resounding question: Why isn't at least some sort of AB bias just standard equipment in all SSB capable amplifiers, even CB amps?


Because class AB amps just don't give that warm and fuzzy feeling of big numbers on the watt meter like a class C amp does and since probably 90% of CB'ers have no idea what TRUELY good and distortion free audio is, it doesn't matter in the slightest. Take a listen sometime to the wonderfull audio reports given to stations that are obviously distorted and you will see what I mean. The same thing happens on the ham bands as well but no where near as often as at least the amps are not at fault by design but rather in how they are operated.
 

eagle1911

Active Member
Jan 21, 2011
327
19
28
I've noticed that very phenomenon myself on the ham bands. In fact, not long ago I posted here about how I couldn't figure out why I kept getting very good signal reports even though I know I was using a class C amp. I try very hard to never overdrive an amp, so I figured that had something to do with it, plus I use low-pass filters on the output. Knowing that class C amps cause splatter and distortion, it didn't make sense to me at the time, until I realized that most of the distortion produced by class C RF amps is found in the RF portion of the signal, not so much in the AF portion. In other words, it's mostly the RF carrier that is distorted more than the AF voice. This causes IMD, splatter, harmoncs etc, and even does in extreme cases audibly distort the AF as well.

I'm kind of an amplifier snob in that I value a clean, low-distortion signal that doesn't splatter out of band. For me, even more than audio cleanliness it's about consideration of other operators and radio services. Using unfiltered class C amplifiers causes interference on many levels and on many frequencies other than the primary xmit frequency, plus the audio can pay a price in clarity, so given these facts it would seem like the world would be a better place with fewer unfiltered class C amps.. but alas there is always room for improvement in life, so this is no exception. I'm sure with the vast majority of CB amp builders it's about impressing your buyers with the most brute power possible for the cheapest price though..
 
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Captain Kilowatt

Professional Amateur
Staff member
Apr 6, 2005
16,833
10,962
823
58
Nova Scotia,Canada
I'm kind of an amplifier snob in that I value a clean, low-distortion signal that doesn't splatter out of band. For me, even more than audio cleanliness it's about consideration of other operators and radio services.

I and many others thank you for being that snob. :D I am the same way.Having a clean signal is worth more to me than an extra 1/2 an S-unit in signal strength and clean audio is always better to understand on the other end than distorted audio is. I come from a broadcast engineering background and it was my job to operate transmitters in the cleanest possible way both for sound quality as well as RF bandwidth and that way of thinking/operating just continued into the hobby.
 

eagle1911

Active Member
Jan 21, 2011
327
19
28
Figured I'd post this link: Converting class C amplifiers to linear operation

It seems like a sweetly simple and straightforward design, it's what I'll be trying next, but I thought I'd see what people here think of it first..

For thermal tracking the clamping diode can be mounted on one of the RF transistors, and the relay contacts shown can be replaced with a transistor switch.
 

VA3ES

Old Buzzard
Oct 30, 2010
645
79
38
Ottawa, Ontario
I love this stuff! I eat this stuff up! Thank you dudmuck for that link to an an excellent treatise! Very educational.
Finally, I can modify my Palomar Elite 900HD for proper, temperature tracking bias. (y)
 
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eagle1911

Active Member
Jan 21, 2011
327
19
28
Keep us posted on your progress! I've spent a bit of time reading that paper, but I didn't find many circuits that seemed right (read: simple and easy to implement) for modifying existing amps. It seems like most of the info in that paper is geared toward being integrated into a completely new design, but I'm not an expert in converting amps by any means. I've mostly been leaning towards a straightforward voltage regulator/diode-clamp combo circuit, but so far I've only modified one amp (KL300p) and it seemed easiest to go with just the simple diode-clamp by itself mostly due to size constraints.. So far that mod has worked perfectly, so my next move will be attempt to build the circuit in post 128 with as many SMD components as I can, but I'm mostly a thru-hole solderer, not much experience with surface mount so we'll see how it turns out.

EDIT: LOL.. somehow previously I missed the stripped down 723 circuit on page 13.. That's a good one from what I've read in this thread and a few others, but I'm a bit curious what advantages it has over just a simple LM317 or 78XX regulator circuit with a diode clamp..

Plus there are some really good bits of info towards the end of the paper. The more I've read tonight the more I'm realizing I should have been more thorough in my reading before.. Very good stuff there!
 
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ExitThirteen

Grumpy and Cranky
Apr 18, 2008
1,965
1,633
173
Cornpatch of Iowa
I would be interested in making a bias circuit that is regulated with thermal tracking. My bias design is rather crude, but it gets the job done... if anyone has a schematic of a bias circuit that is regulated, let me know. Thanks!


~Cheers~
 
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