Increase Overall Linear Amplifier Efficiency With This $16 Board

[K1JJ]

This idea has been around for a long time, but I haven't seen it mentioned much.  This technique is mainly used for increasing efficiency on ssb, but could be used on AM in a more limited way..

Here's  a simple VOX board from China for $16.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=221121494057&ssPageName=ADME:L:OC:US:3160

Here's how it works... Trigger the VOX board with some audio tapped off your existing audio preamplifier output.   Set the amplifier bias so that the final tube idles JUST above class B, so that it has a small amount of idle current, enough to pass fan noise.  When our voice triggers the VOX board on, a relay sets the amplifier bias so that it idles at a normal current.  Set the delay for about one second so that during voice pauses, the amplifer goes to sleep until another spoken phrase.  So, word spoken, vox on, regular idle bias  ---   voice pause, one second later increased bias to put amp to sleep.


The problem with using these in the past is that most configs put the amplifier completely into cut-off (class C) during a pause and the first word would cause a splatter peak for a moment. (slow attack time) But if we let the amplifer idle just above cutoff, this will not occur.

I have been running my class A/ AB1 linear amp system this way for about a year now and find it works very FB. The difference in heat is about 1/2, considering the amplidier is asleep about 50% of the time in normal speech.  The SDR anlayzer shows no sign of first word splatter since it is already idling at a "reasonable" level above class B before getting kicked up to AB1 or wherever..

I am using a homebrew board, but plan to replace it with this slick Chinese board.

As for AM use....  it COULD be used the same way, but I would idle the amp a little higher during sleep time so that the carrier does not drop off and sound like controlled carrier. Maybe 1-3dB would not be noticed but could save a lot of heat.  Most amplifers need more drive when using more bias, thus the carrier drop off problem.

BIAS: The easy way to change the bias with the board relay is to use a string of standard diodes in the cathode of the final tube.  Put in enough diodes to let the amplifier sleep just above cutoff and short some diodes out with the VOX relay until the amp idles at its normal current for regular operation.

Combine the VOX bias idea with the AM linear "class C carrier technique," (as discussed in another thread) and we are talking some reasonable savings in power.


T

[W3GMS]

Tom,

That's a great idea and I wonder why more people are not using it.  Help keep the shack cool in the summer!

A way to avoid the peak of audio coming through while the amplifier is still biased for non linear operation is to use and audio delay network.  In the old days it was the famous bucket brigade chip but today I am sure we have much better audio delay lines.  So you trigger the vox with before delay audio and then a short time later the delayed audio goes to the transmitter.  It should not require much audio delay to do the trick.

In broadcasting stations use an profanity delay box and obviously that gives great audio out.  That is a lot more delay than you would want in Ham applications.   

My Inovonics 222 uses slight audio delay to make sure the limiter catches the peaks.  I will have to look at the schematic and see the scheme they use for the audio delay.   

Joe, GMS   

[Pete, WA2CWA]

I think from memory(??)  the Ameritron AL-80B does something similar to what you describe Tom.

[K1JJ]

A way to avoid the peak of audio coming through while the amplifier is still biased for non linear operation is to use and audio delay network.  

Hi Joe,

Yes, an audio  delay would certainly work. It would allow us to bias the amp into cutoff and see zero power dissipation during sleepy time.  It would complicate the system a little more, but WTF.

Though, the difference between idling just above cutoff and below cutoff is not much additional power wasted.  I find when slightly above cutoff, the amp is pretty clean. Most guys run low idling current anyway. However, for my class A / AB1 linear amp system, there is a lot of idling power to get those clean -50dB 3rd order figures.  So I have all three amplifer stages set up to use the VOX bias relay system.

I suppose that regular VOX on ssb would do the same thang, but I HATE regular VOX and do PTT only on ssb.  Of course, on AM, regular VOX is NG and reducing the carrier via bias 1-3 dB might be transparent and save a lot of power. After all, a 3dB drop in carrier during sleep is 1/2 power reduction. I have never tried this bias system on AM, so don't know. I run class C only on AM.


My previous estimate of 50% sleep time is probably more like 30% sleep.  When we talk, we tend to talk more than pausing in silence.  But with my amplifiers, I can definitely see the difference on the temperature gauges that sample air from each rig when the idling current is conserved.


Pete - Really?  I didn't know that.   I remember reading about a commercial  amplifier out there that has a bad reputation because of the momentary attack splatter (deep cut-off bias) problem I mentioned. Maybe a Google of the amp would show it up. Or maybe they fixed it due to the publicity by now.

T

[W3GMS]

I am just not sure where the technology is with inexpensive audio delay boxes with good audio quality.  A simple little line in / line out with maybe 200 ms would probably do it.

I have an AL-82 and it does not change the bias dynamically.  They do have a switch to change it from CW / SSB but the bias does not shift that much.  It would be no big deal to modify it to create a larger bias step. 

Joe, GMS     

[WQ9E]

W8JI has a good discussion of EBS on his website.  I implement it into my homebrew amp (three 4CX800 tubes) mostly to reduce heat in the contest room when it is running many hours at a time.  I based mine on Tom's circuit and it works fine.  Note his strong caution not to overdo it by biasing the tube hard into the cutoff region.

[W3GMS]

W8JI has a good discussion of EBS on his website.  I implement it into my homebrew amp (three 4CX800 tubes) mostly to reduce heat in the contest room when it is running many hours at a time.  I based mine on Tom's circuit and it works fine.  Note his strong caution not to overdo it by biasing the tube hard into the cutoff region.

I am sure that scheme works great Rodger as proven by all the previous people who have used it.   

With the audio delay scheme it appears you could have the tubes cut off and allow the bias to switch into the linear region and stabilize before the audio appears.  How much you gain, who knows but its a interesting thought.  When bias is quickly changed, I wonder if any response time numbers come into play.  I have never looked at an applied bias slope while looking at settling time for linear operation. 

Joe-GMS   

[kb3ouk]

Ameritron has an add-on bias board for the AL82, AL-1200, and AL-1500 amps. http://www.ameritron.com/Product.php?productid=EBS-1