Print

[2E0ILY]

Why do you have to tweak the tune and load controls on tube amps as the power input or plate voltage is increased? Thanks.

[W3PX]

The amplifier's Tune and Load capacitors, along with the tank circuit's inductor, form a matching network that matches the plate circuit's effective impedance to the antenna's impedance.

When the plate voltage and/or current are changed, the effective plate circuit impedance also changes, requiring re-tuning for best match.

73 Frank W3PX

[N3DT]

The Acom1000 uses a 'True Resistance Indicator' tuning or TRI. It sets the tube at the optimum value for IMD. I find once I set it at say 30W in it holds for all power levels. It's a different way of tuning though. I can tune it also for min Ip and max PO, then thing change with input levels. I also get higher gain and more PO for less Ip, but the IMD values have most likely gotten worse.

[KM1H]

That was a good explanation Frank.

Also consider that the amps tank circuit is a pi network which is also an excellent impedance matching circuit not limited to use in an amp.

I dont use antenna tuners and modify the amp pi networks to tune into whatever the antenna coax presents to them.  Thankfully I dont have any SS amps to require a tuner to complicate things and become another failure point.

Carl

[VR2AX]

Another way of looking at it is to regard the pi network at matching or transforming the antenna impedance upwards in value, to correspond to the tube's optimum plate load impedance.

Typically tubes have a load impedance of a few thousand ohms. Say the antenna is 50 ohms. The pi network transforms this up so that the tube 'sees' a load more like a few thousand ohms instead of 50 ohms.

You can estimate the tube load impedance by (plate voltage divided by plate current). So for a 2500 plate voltage amp drawing 1 amp plate current, load impedance would be something of the order of 2500 ohms (actually a percentage of about 50-70% of that but the principle is the same).

To answer your question, if you turn up the power, for a fixed plate voltage, plate current rises and therefore tube load impedance falls in value. If you turn up the voltage, load impedance rises if the plate current is maintained constant. So the pi network is faced with the job of matching different tube load impedances, which it usually does by changing the tune and load controls.

When operating CW the plate load impedance is a constant value, as the plate voltage and current are fixed. In SSB the plate load impedance changes according to voice patterns so tuning for maximum output on a single tone may produce less efficiency at some other patterns.

[2E0ILY]

Firstly, many many thanks to all, some great replies here and I now understand this matter. To Carl I would ask this please:

You say the Pi network(and Pi L too, I guess?) acts as an impedance matching device. If I have a tube amp that is designed to work into a maximum of 1.8 to 1 SWR, but attach an antenna that trips the SWR safety, with plenty of either way capacitance on both the tune and load variable caps, can I assume I would have to alter or re-tap the inductor in the Pi network, and by so doing could effect a match? Would doing this screw anything else up in the amp? (Purely an academic question, I am merely wondering a bit deeper...)


Thanks again everyone.

[KM1H]

I dont know about the safety circuit design but the tank circuit may just need a coil tap moved and/or some different fixed capacitor changes.

If the "safety" circuit is just sampling a voltage that should be OK as is.

There are other ways to beat that circuit with a different length of coax at the amp.

Ive successfully used them all.

More details can narrow it down.

[VR2AX]

Here's fancy way to trip out the amp if the SWR is higher than a value that the user choses to specify

http://www.kojinsha.jp/kp-1_en/

[2E0ILY]

Here's fancy way to trip out the amp if the SWR is higher than a value that the user choses to specify

http://www.kojinsha.jp/kp-1_en/

Thanks, sounds very useful for those using the more fragile and basic SS amps!