[AMPS] Re: Amp Stability

[Tom Rauch]

> Many amps will squawk if you apply bias before the output relay engages
> the load. (I know this isn't quite what you asked, but I've seen it.)
The Clipperton, FL2100, and 30L1 are examples of this.

If the tubes are biased into conduction and the load isn't applied, 
and these amplifiers are tuned on the higher end of HF, they will 
oscillate near the operating frequency.

Since there is no load, and since the tank is resonant on that 
frequency and freely passes the energy along, voltages developed 
can be quite high...enough to damage bandswitches and other 
components.

The effect is very similar to driving a PA that is greatly underloaded, 
or operated into an extreme load impedance for a moment.

While very little of the feedback is actually through the relay, it 
sometimes can be enough to push the amplifier "over the edge" of 
HF stability. But this almost always only occurs when the input 
and output ports are misterminated.

One mistake some manufacturers and home builders do is using 
two directly adjacent relay poles for RF, and a third pole off to the 
side of cathode switching. Both Dentron and Heath used to do this, 
and probably others.

A PB KA-11 relay connected that way, and terminated in 50 ohms 
has a cross talk of -28 dB at 30 MHz. If the same relay is used 
with the input and output placed on opposite end poles, with the 
cathode switching in the center, cross talk isolation from input to 
output increases almost 20 dB to -47 dB!

The relay by itself, without additional feedback, can not make a 
stable amplifier unstable but it can push a marginally stable design 
over the edge at upper HF.

Don't confuse arcs and transients caused by hot-switching with 
stability. Be sure the output contact always closes just before the 
input contact, and with unstable systems like unneutralized 572B's 
be sure the bias holds the tube off until the load is applied.


73, Tom W8JI
w8ji@contesting.com

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