Go Back   WorldwideDX Amateur Radio Forums > Amateur Radio Related > Amplifiers


Amateur Callsign Lookup
Enter Callsign:

Quick and Dirty Class "AB" mod for KL300p

Like Tree11Likes

Reply
 
LinkBack Thread Tools Display Modes
  #25  
Old 03-10-2012, 01:51 PM
liquidh8's Avatar
Senior Member
iTrader: (0)
 
Join Date: Aug 2011
Posts: 271
liquidh8 is on a distinguished road
Default


Glad I found this. I have a buddy with a freebe working old class c 2 pill, that a bias mod like this would work on I think. The pics most definitely help out! Good work!

Reply With Quote
  #26  
Old 04-25-2012, 07:10 AM
Rabbit Ears's Avatar
Junior Member
iTrader: (0)
 
Join Date: Jan 2009
Posts: 5
Rabbit Ears is on a distinguished road
Default

Quote:
Originally Posted by eagle1911 View Post
This is a "quick and dirty" class AB mod for the KL300p that I ran across some time ago. While this description is for the KL300p, this method of providing a small bias source can be applied to just about any class C amp. Basically any amp that grounds the bases of the transistors via an inductor is class C. In this case, the inductor is part of the input transformer.

Procedure: First I used a dremel tool to carve a notch all the way around the outside end (closest side to the switch assembly) of the input transformer, which is the smaller of the two transformers on the amp PCB. This disconnects the transformer from ground. Make very sure that you cut completely through the ground trace all the way around that end of the transformer. If you check for continuity with a DMM you'll get a reading of ~5 ohms. If you get a reading of 0 ohms, recheck to make sure that you've cut completely through the ground copper all the way around the end of the transformer.

I invented that mod back in 2004 and posted it on Plowboys mods, as I believe in 2008 or 2009!

GOOD WORK!!

I used a large 10W 100ohm resistor in line as well, one end of which I connected to the "Pre + Lin" side of the power switch which is on the topmost pin. When the switch is on, this pin is a source of 13.8V. This way, the bias is only supplied if the amp is on. This resistor limits the current that is supplied to the transistors, and it is connected in series with a large 3A silicon diode (1N5402), the side of which is physically touching the top of one of the SD1446s (there is thermal paste between the two) and the cathode end (stripe) is connected to ground right at the transistor on one of the emitter tabs. As you may already know a diode is simply a PN junction, and so this diode limits the bias voltage to the forward voltage drop of a silicon PN junction, generally .6-.7V depending on the junction temperature. At the connection point between the 10W resistor and the diode, I soldered one end of an RF choke (available from Radio Shack) which ensures RF cannot pass as we only want DC. The other end is connected to the brass tubes in the input transformer, on the same side as the cut that was made with the dremel. This replaces the ground connection that was there with the bias voltage. This completes the rudimentary bias supply. This small voltage applied to the bases (.6-.7V) is just right to keep the transistors turned "on" at all times, which means your amp is now class AB. Since the components used are fairly heavy-duty, the thick leads make for a nice stable structure so you don't have to worry about things moving around much.

This is a quick and dirty mod, partly due to limited room, and partly due to my feelings that simplicity is best for such an inexpensive amp.. however I see this same basic supply frequently in many 10-11m amps and even some Mirage VHF amps that are classified AB so by performing it you're getting very similar results to many major name manufacturer's amps and most other simple class AB amps.. In fact, since there is temperature tracking due to the physical arrangement of the diode and transistor you're actually a step ahead of many manufacturers.

There are much more complex bias designs out there that are meant to make sure the transistors are biased within precise parameters under all types of condition .. so if you really wanted to you could go much further with it and get even better results. This simple mod, combined with an effort to avoid overdriving the amp can help ensure that there is a reduction in the amount of distortion produced. If you think you're getting "crunchy" audio or fuzzy output then this mod might be just the thing you need.

Here's a pic of the end result:



Anyway, I just thought folks here might like to have a good description and pic of this mod, since I know that many people are confused by bias and amp classes in general and how they are applied in real-world hardware.. this might help remove some of the confusion. Plus, if there are people out there with KL300p's that want cleaner SSB output and less distortion this will likely help them out too..
GOOD JOB!! Employing the RF filtering!

Reply With Quote
  #27  
Old 04-25-2012, 08:59 PM
ExitThirteen's Avatar
Grumpy, Cranky Repairman
iTrader: (11)
 
Join Date: Apr 2008
Location: Cornpatch of Iowa
Posts: 1,056
ExitThirteen is on a distinguished road
Default

Great stuff here! I decided to do this mod on a little Silver Eagle 200 I got for working on a guys radio last week. It's a 2 X MRF455 class C box with no SSB delay. Since Eagle1911 has the rights to the "Quick and Dirty class AB Mod" I'll call mine the "White 'n' Nerdy class AB Mod"! Attached is a pic.



Once i got it all done, it works awesome, looks great on my O'scope, nice clean output. Does about 100W on low output, about 150W on high output, PEP. I did use a different resistor to the TIP42G transistor, as 220 ohms was keeping it "turned on", letting voltage go thru it at all times. I found that 1k ohms works perfect for switching the voltage.

Since this box did not have any SSB delay, I installed a 2200uF 35V capacitor across the relay coil. That gives me a nice delay for SSB, but isn't too long to interfere with AM should I want to use it for that mode.

Great thread, thanks to all that posted info and especially to Eagle1911 for the continued progress of the project!

~Cheers~
__________________
Bases: Cobra 2000 GTL, President Washington, President Madison, Tram D201A
Mobiles: Cobra 146GTL, Midland 79-893 (NIB), Midland 13-893 (27.555), President P300

Reply With Quote
  #28  
Old 04-25-2012, 09:13 PM
Jock Supporter
iTrader: (8)
 
Join Date: Feb 2008
Posts: 7,534
mackmobile43 is on a distinguished road
Default

Quote:
Originally Posted by ExitThirteen View Post
Great stuff here! I decided to do this mod on a little Silver Eagle 200 I got for working on a guys radio last week. It's a 2 X MRF455 class C box with no SSB delay. Since Eagle1911 has the rights to the "Quick and Dirty class AB Mod" I'll call mine the "White 'n' Nerdy class AB Mod"! Attached is a pic.



Once i got it all done, it works awesome, looks great on my O'scope, nice clean output. Does about 100W on low output, about 150W on high output, PEP. I did use a different resistor to the TIP42G transistor, as 220 ohms was keeping it "turned on", letting voltage go thru it at all times. I found that 1k ohms works perfect for switching the voltage.

Since this box did not have any SSB delay, I installed a 2200uF 35V capacitor across the relay coil. That gives me a nice delay for SSB, but isn't too long to interfere with AM should I want to use it for that mode.

Great thread, thanks to all that posted info and especially to Eagle1911 for the continued progress of the project!

~Cheers~
Nice, do you have some heat sink compound between your tracking diode and the pill surface? if not you'll get better thermal contact with some.

Maybe try filing a flat spot on one side of the diode also.
AudioShockwav and Zman like this.

Reply With Quote
  #29  
Old 04-25-2012, 10:48 PM
ExitThirteen's Avatar
Grumpy, Cranky Repairman
iTrader: (11)
 
Join Date: Apr 2008
Location: Cornpatch of Iowa
Posts: 1,056
ExitThirteen is on a distinguished road
Default

Hiya Mack!

Yes, I do have plenty of heatsink compound on the diode, I made sure there was plenty on the contacting surfaces of both the diode and the MRF455. Good to go there. The picture doesn't show it much because of how the pic was snapped, stuff in the way.

(edit) Poor picture quality too, I might add...


~Cheers~
__________________
Bases: Cobra 2000 GTL, President Washington, President Madison, Tram D201A
Mobiles: Cobra 146GTL, Midland 79-893 (NIB), Midland 13-893 (27.555), President P300

Reply With Quote
  #30  
Old 04-26-2012, 05:15 AM
Jock Supporter
iTrader: (8)
 
Join Date: Feb 2008
Posts: 7,534
mackmobile43 is on a distinguished road
Default

If the boys over at XFORCE had a clue they would be using these simple biasing circuits instead of all that stuff they glue in their amps.
Zman likes this.

Reply With Quote
  #31  
Old 04-26-2012, 09:34 AM
liquidh8's Avatar
Senior Member
iTrader: (0)
 
Join Date: Aug 2011
Posts: 271
liquidh8 is on a distinguished road
Default

do you have a p/n for the radios shack rf choke??

Reply With Quote
  #32  
Old 04-26-2012, 08:33 PM
ExitThirteen's Avatar
Grumpy, Cranky Repairman
iTrader: (11)
 
Join Date: Apr 2008
Location: Cornpatch of Iowa
Posts: 1,056
ExitThirteen is on a distinguished road
Default

That choke is catalog PN#273-102. Hope that helps ya Liquidh8.

(edit) It's a 100 uH (microhenry) choke.


~Cheers~
__________________
Bases: Cobra 2000 GTL, President Washington, President Madison, Tram D201A
Mobiles: Cobra 146GTL, Midland 79-893 (NIB), Midland 13-893 (27.555), President P300

Reply With Quote
Reply

Bookmarks

Tags
bias, class ab bias, kl-300p, kl300p, rm italy
Worldwide Radio Forum



Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT -7. The time now is 09:25 AM.


Powered by vBulletin®
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Search Engine Optimization by vBSEO 3.6.0
Worldwide Radio Forum