Dynamic Linear Bias control - for better efficiency

[K1JJ]

This is an old idea that has been around for a while now.  (This is meant mainly for ssb, though there may be applications for AM linear)

When running a linear amplifier for its cleanest IMD, we need to idle the tube rather high, producing heat. The following circuit simply idles the tube lower until we speak and then keeps it on -  until we stop speaking and then idles the tube lower.

For example, my 8877 linear idles at 50 ma and then jumps to the normal 175 ma idle when I speak. After (an adjustable) one second delay, if I am not speaking, it drops back to 50ma idle again, etc.

The problem with some of the bias units on the market is they bring the tube into cut-off which produces a crud spike during attack.  This is solved by idling the tube at a clean running, but lower idle level - but not at cut off.

This circuit is a simple three  transistor 2N2222A VOX that drives a relay. The relay shorts out some of the cathode biasing diodes when energized, causing a higher idle.  Again, because the tube is already idling cleanly, the small delay of the relay and audio trigger is not important. The amplifier signal is clean and linear either way.  One could use an RF trigger and a MOSFET relay if speed is desired.

The bottom line is my spectrum analyzer shows no change in IMD nor do I hear artifacts up the band when in use. The take home pay?  During a normal speech transmission, my tube temperature gauges run much lower showing less wasted heat. If you pause a lot, then you will save more.... :-)

We could use a real VOX and do the same thing, but I hate the clatter of multiple control and antenna relays, so just use PTT here, thus this idea.

This simple circuit comes from a 1995 Popular Electronics magazine that I found on the web. I modified it a little to suit my needs.  When the power switch is off, then the switch shorts out the relay contacts and we are back to normal fixed bias operation.

I have added one to each of my three linear amplifiers and love it!  It's like they're not even there unless you look at the plate meter and see it cycling up and down between pauses.

73,

Tom, K1JJ

[K1JJ]

Do you think I have too many gadgets in the shack?  I'm such a sucker for this stuff.


VOX BIAS Schematic in #3 pic. 

** Whoops, there should be a coupling cap between the input pot wiper and the 10K/ diode junction. 


I have the circuit input connected to the back of the 528E compressor audio output.  A sigh triggers the Bias VOX circuit and keys the amplifier idle higher.


Pic #2:  That's Mr. Clean in operation with its new VOX Bias box.  Notice the temp gauge at a cool 68.9 F, caw mawn.

T

[WQ9E]

Tom,

Interesting stuff!

I built a dynamic bias system into my triple 4CX800 amp to cut down on the heat generated and later changed it to a standing idle above cutoff to get rid of the transient increase in IMD.  The next time I go into the amp I am going to experiment with a delay line like those used in the vertical system of analog scopes which provides for sweep triggering/start prior to signal application to the vertical deflection plates allowing you to see the triggering event.  The bias switching system will see the RF instantly but the delay line will slightly delay the RF arrival at the tube which should allow the tube to be in normal bias mode before excitation occurs.  At least it will be an interesting experiment.

[kb3ouk]

I don't really see a need to use something like that when AM can already do that naturally, just set bias past cutoff. I've done some experiments here using my FT-901 and it does increase the efficiency slightly, when biased to the instructions in the manual and the carrier turned up to give 100 ma cathode current, it puts out 15 watts. Same current but biased past cutoff gives 20 watts. May not seem like much but that 5 watts makes a huge difference when driving a linear.

[KK4YY]

This seems to be a circuit where the inherent latency of SDR radios can be used to some advantage. If latency can be "managed" it would then allow for idling the final tube at cutoff and the circuit would bias it up before it gets hit with RF as Rodger mentioned. No RF, no crud spike. Easy enough to try.

Don

[K1JJ]

Rodger,

Looks like you already have a good handle on it.  Yes, the standing idle is the key. In fact, I ran IMD tests to find the point of lowest idle before the tube fell apart and generated crud. So, even with a slow attack, the tube is still linear.  Your ideas would actuallly allow the tube to idle lower into the crud area and save more power - and pull up before actually being used. Good idea.



Shelby,

Yes, on AM we could already bias the tube way into cut-off and let the carrier bias the tube as discussed in another thread.  Another problem with AM is the bias change may cause the carrier to move up and down 3dB or whatever, almost like controlled carrier. IE, it is easier to drive a linear that is idling higher.  I haven't tried it on AM yet and will see what happens.   I love it on ssb for DXing.


Don,

The latency of the SDR rigs is also a good point and will help things here. I have an HPSDR and will see what it does.  But again, no matter if there is an attack  delay or not, if the tube is idled at a lower point that is still clean, there is no crud on attack no matter what the key up delay.

What spurred this on is I got tired of these linears just wasting power when I take a pause. After setting them up for best IMD, the idles (and heat) are higher than before, thus the need.

T

[KM1H]

The first time I saw bias control was in a late 40's CQ to control a single ended 304TL modulator; it was called a Sliding Bias.

Alpha had it in the 70's with lots of popping until someone came up with a fix.

Ameritron has it in some of their amps currently and supposedly works very well .

Since I run VOX only on SSB it has never been a concern but when running PTT and AM linear I started running the LK-500ZC amp with less idle current after being presently surprised experimenting with a customers Clipperton L.

Current amps in for service is another Clipperton L, NCL-2000, 30L1 and an Alpha 77SX 2 holer; I'll continue the experiments.