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Class AB baising help needed!

ExitThirteen

ExitThirteen

Grumpy and Cranky
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Apr 18, 2008
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Cornpatch of Iowa
Heya all!

I have an Xforce 2x4 base amplifier, and I want to be able to run SSB with it. It was originally biased class B (biased about .35V to the bases of the transistors) but that isn't clean enough for me for SSB use. I want to be able to bias this AB1. What's a good way to bias this amp for AB1? I'd need a parts list of what to get. Is .6V at the bases about what I'd need to make it at least closer to AB1? Thoughts and suggestions?

I'm currently using a switching supply to run the amp, so it has good regulated DC going to it, and the bias voltage is switched thru a relay. Any help would be much appreciated. Thanks!


~Cheers~
 

Two thoughts, it's a 2x4 which will never be clean unless maybe you drive it with 8-10 watts peak. Even at that a 2x4 is not practical and would be my last ever choice in amplifiers.

That said if you want true regulated AB send it to Peakaboo and see if he will work on it for you. If not sell it and buy an AB amplifier.
 
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Well, perhaps Bob85 or Mack will see this and shed some light on what I need to do.

9C1Driver, you're right, not the best kind of amp to be working on, but if I've got it, I might as well make the best of it! Maybe if Peakaboo sees this, he might know something also. I'd send it to him, if he'd put it in!


Thanks for the posts so far!

~Cheers~
 
Its not so much about the voltage, when u bias amp, you need to bias it so it draws around 100mA per transistor. Generally i bias slightly higher to about 125ma +/-. Could be closer to around .65-.68V. Also bias circuit shouldbe regulated so that amp can maintain same voltage while at idle or during talk time. Many of the cheaper circuits like the simple diode are class B and voltage drops down while talking so that amp is very close to if not class C.
 
AudioShockwav said:
I think if Bob85 or Mack see this...
I know that Bob had a very neat set-up, I think that is what Mack used in the Amp he built.

look about 6 or 7 posts from the top....Bob
http://www.worldwidedx.com/amplifiers/37454-bias-design-amplifier-design-filtering-etc-3.html

It goes on from there, good stuff.


73
Jeff
Click to expand...

That topic was one of the best i have read on forum. Forgot all about it. Thanks. I still use the Motorola design but with a regulator. works great.
 
Might be a good idea to put a hi limit temp switch in amp as well. Ao when it gets too hot it will shut the keying circuit off. Might save ya some $$$$ down the road.
 
ExitThirteen said:
Heya all!

I have an Xforce 2x4 base amplifier, and I want to be able to run SSB with it. It was originally biased class B (biased about .35V to the bases of the transistors) but that isn't clean enough for me for SSB use. I want to be able to bias this AB1. What's a good way to bias this amp for AB1? I'd need a parts list of what to get. Is .6V at the bases about what I'd need to make it at least closer to AB1? Thoughts and suggestions?

I'm currently using a switching supply to run the amp, so it has good regulated DC going to it, and the bias voltage is switched thru a relay. Any help would be much appreciated. Thanks!


~Cheers~
Click to expand...
Yup .6 or a tad more good, the circuit bob came up with has only been used to run 2 transistors in a stage as far as I know so you may need to build 2 of them for the output stage, I would not use the drive section in ssb mode, you will regret it.

Make sure you have tracking diodes to prevent thermal runaway, I used some transistors as the diodes for this, I don't remember the numbers on those but I can look it up if you decide to go that route.
 
You can use a tip31c. I bought a bunch of diodes in the TO220 case. Just looked it up in mouser catalog. That way no confusion on pin outs. In a tight pinch, I just use a small diode and mount it on top of pill. Most say not too, but if you look at Kenwood radios and the like, that's what they do. I agree with Mack. On Ssb. It would be safest to disable driver section. It will save you money later on.
 
Yeah, I knew that I needed to run them about .6V or so, but what specifically should I be using for diodes and resistors?

Currently it runs 1N5820 diodes which drop .35V. Those are ran in series with a 10 ohm, 20W resistor. This setup is run on each set of transistors for a total of 3 sets.

I tried 1N5401 diodes, which drop .7V with the same 10 ohm 20W resistors, and promptly cooked all 4 finals. The drivers were fine and didn't sustain any damage. Soooo... it's back to the drawing board. What would I need to do to effectively drop the voltage down to .6V or so?

I am thinking of bypassing the driver section and running it as a straight 4.

Thanks for all the info so far :)

~Cheers~
 
You have no regulation, the game changes by raising the voltage at the bases from .3 to .7.

I smoked the finals more times than I care to remember in my magnaforce 900 back in the days because the drive stage was too much and would create thermal runaway.

One time it actually blew the caps of 2 of the finals.
 

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