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Ok, fresh pics of the progress so far.. Pretty much done except for the relay/transistor circuit to key the bias only when transmitting, but for folks wanting to perform this mod IMO it's optional. I plan to implement it in my amp, but the amp will work perfectly with the bias applied all the time while the amp is powered on. If you use this amp as a mobile and decide to just let the bias run all the time it's very important to remember to turn the amp off whenever you leave the vehicle, otherwise your battery could possibly be drained. I use mine as a QRP amp, so it's always connected to a power supply.

I've included descriptions in the photos with each connection and it's components. Below is also a pic of the amp before modification with the dremel cut marked and circled. Just don't cut through the wires that are in the transformer! They need to remain connected exactly as they are.





Special thanks to Shockwave for his valuable input.. and thanks to all who have replied. Some very good ideas have been presented in this thread.

Oh and total cost of this mod is less than $10 in Radio Shack parts and maybe an hour assembling.. The signal reports are great, the output looks clean on the scope. I don't have a spectrum analyzer so I can't speak for a reduction in Harmonics and IMD, but there should be a reduction in all types of distortion as long as the amp isn't being overdriven.

EDIT:

Found a problem with the circuit.. more to come shortly. I really need another electronics geek here to error-check me lol...

So, after more testing I found that a lower value resistor is needed in the bias circuit. Using the 100 ohm 10W resistor there was too much voltage sag on speech peaks, which causes the amp to run into class C territory. I decided that halving the resistance should do it, so I took out the 100 ohm, popped in a 50 ohm and took some voltage readings at the bases of the transistors: .650V under full load, .685 at idle. I also noticed a slight increase in power output, about 5% across the frequency range, and the transistors seemed to run just slightly hotter running into an MFJ dummy load. Everything still looks just fine on the scope. So far these are promising results..

After replacing the resistor, the transistor switch that was described earlier went from optional to a necessity. The 50 ohm resistor heated up more than I wanted when the bias circuit was allowed to run all the time, so below you'll find a pic of the solution for that.

The transistor used is a TIP42G PNP power transistor. You don't necessarily need a power transistor, a small-signal PNP will do the job in most cases, but since this mod relies on thick component leads for structural support to avoid needing a PCB it seemed ideal. The resistor that the wire from the relay diode connects to is 220 ohms. The 220 ohm resistor is connected to the base pin (right in pic) of the TIP42G transistor. The transistor's emitter pin (leftmost in pic) is connected to the top pin on the front panel switch as in the picture. The last (middle) pin on the TIP42G is connected to the 50 ohm resistor in the bias circuit itself.

*It's VERY important to ensure that everything is positioned so that the tab on the TIP42G cannot touch ground, so avoiding anything metal is the best way to insure this..

the current your bias circuit needs to be able to supply to the transistor bases without sagging below about .6v to keep the amp in class AB depends on the beta of the transistors been biased and the current you draw in the collectors,
m beta sd1446's are pretty high gain devices @27mhz but your 100ohm dropping resistor is too high in value to keep the amp in class AB when you apply higher drive levels

edit you beat me to it

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tom bearden taught me that quantum entanglement allows thought to travell faster than the speed of light, this explains why at first you can think somebody is smart but if you wait a while you will see they are not very bright at all.

They say great minds think alike! That's big talk on my part really.. it was some posts I've found of yours and those from a few other people that have really helped me understand this stuff.

Great Work Eagle.
I am going to stick this thread.

73
Jeff

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Radio-active

NASA/SOHO


there is no dark side in the moon, really. As a matter of fact it's all dark

Many thanks! I very much like this forum by the way.. there is a lot of really useful knowledge here.

As I was researching the idea of adding simple bias to improve the performance of cheap class C amps I found that while descriptions were plentiful, pictures were not so my intention here has been to provide a pictorial of my progress so that people with basic questions about bias might be able to get a better understanding of how to implement it. As I said in the OP there are much better bias circuits than the simple diode clamp circuit described in this thread. This modification therefore has it's limits as far as how much improvement can be seen, but some improvement is better than none IMO. The results are promising so far..

This project was born out of necessity really though.. I'm on a small hobby budget lately, so any ways that I can find to improve inexpensive equipment to bring the performance closer to that of more expensive gear are golden to me, not to mention the enjoyment I get from getting my hands-on DIY fix..

Just wanted to update this mod..

Upon consulting the Motorola EB63 bulletin (uses a very similar bias scheme as described here in this mod) I noticed that they incorporate a 500uF electrolytic capacitor to provide extra current during SSB peaks. This seemed like a great idea, so I've added it to my mod as well.

I also replaced the .01uF caps with smaller, lower voltage units that take up less space and reduced the 10W power resistor's value from 50ohms to 40ohms for more quiescent current through the transistors:

This is a pic of a similar amp without the mod. This is just to show where I connected the (-) lead of the cap.


This is the finished mod: