AL-80B Dynamic Bias Defeat


If you listen closely to someone running the stock dynamic bias circuit you may hear a gritty first syllable or a mushy first cw note. If you own an AL80B with the dynamic bias circuit in place, listen to your own signal in another receiver. When you key the mic without speaking you will hear the sound of an amplifier at cut-off with a few pops and clicks blaring through now and then. Personally, I can't fathom why anyone would want this circuit active in a linear amplifier. The amount of power consumed between syllables is incredibly small, and the amount of heat given off by having the tube drop to its normal resting current between words rather than complete cut-off is not enough to warm a teaspoon of water. After a few months of operating the AL80B I decided it was time to bypass the dynamic bias circuit.

This is very easily done with a single resistor to turn Q101 on all the time. What they have done with the dynamic bias is to couple RF from the input circuit through C102 and R101, which is then rectified by D101 and D102 to produce a small positive voltage to only turn Q101 on when RF is present at the input. The solution is to insert a 47k resistor (actually anything between 10k and 80k works fine, but I had a pile of 47k resistors so I grabbed one of those) between the VCC line at the junction of R105 - R103 and the base of Q101. This causes Q101 to switch on all the time and defeats the dynamic bias circuit. If you trace things out you will find that those connection points are actually accessible at several different convenient spots in that area of the board. The amp will now function like a normal amp and you will see the correct resting current immediately when you key the amp. The modified schematic below shows where the new 47k resistor is inserted. The photo below that shows the 47k installed in a convenient way.

The 47k resistor will bias Q101 to turn on

The 47k resistor soldered to the points shown in the schematic above


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