[Amps] Acceptable SWR for Tube Amps

[Michael Tope]

The difference between the case Lukasz is describing and what you are 
describing, Dennis, is subtle but important. When specifying what level 
of VSWR a conventionally tuned amplifier can handle, you are indicating 
the range of antenna mismatch the output tank can transform to the plate 
impedance the tube wants to see when tuning the amplifier into that 
particular antenna mismatch. For this case, the tank circuit is in 
essence also serving as a limited range antenna tuner.

For example, say you tune-up the plate and load controls on a 
conventional amplifier with a 50 ohm load so that you get optimum tuning 
(i.e. the tube sees an impedance that produces an optimum combination of 
efficiency, output power, and linearity). Now, you switch from that 
ideal 50 ohm load to a higher VSWR (e.g. 2:1) . In that case, you have 
the luxury of re-adjusting the tune and load controls to attempt to get 
the impedance presented to the tube back close to what it was seeing 
when the amplifier was driving a perfect 50 ohm load. As the VSWR gets 
higher, eventually you will get to a point where you run out of tuning 
range in the tank circuit (e.g. the capacitance range of tune or load 
capacitors is insufficient, the tank inductor starts to overheat, the 
load capacitor starts to arc, etc).

An amplifier like Lukasz describes is a slightly different animal. Here 
the tune and load controls for each band are preset to fixed values (the 
ETO Alpha 78 when operated in "bandpass" mode is an example of this type 
of amplifier). These presets can be optimized either for a perfect 50 
ohm load or the impedance of a particular antenna at a particular 
frequency.  Whichever is the case, as the load impedance departs from 
that optimum preset value, you can NOT re-adjust the tune and load 
controls to bring the impedance presented to the tube back to the 
optimum. Here the antenna impedance range is limited by the range of 
plate impedances that the tube can tolerate (in addition to whatever 
limits the fixed tank components impose in terms of voltage and current 
stress when they see a non-optimum antenna impedance).

The Alpha 78 manual states "A load VSWR of 2:1 or better is required for 
safe manually-tuned operation of your Alpha 78. For safe and efficient 
operation in the bandpass (no-tune-up) mode, a load VSWR of 1.5:1 or 
better is desirable". The fixed tuned "bandpass" mode preset capacitors 
in the Alpha 78 are pretty small compared to the variable tune and load 
capacitors used for manually tuning mode. Also, the 8874 tubes used in 
the Alpha 78 are sensitive to grid overcurrent. Both of these things may 
factor in to the more limited VSWR range for "bandpass" mode. It may be 
possible to accommodate a VSWR range greater than 1.5:1 in a "bandpass" 
mode amplifier with more beefy tank components and more forgiving tubes.

73, Mike W4EF.............

On 2/14/2025 9:44 AM, Dennis W0JX via Amps wrote:
> I think that the specs on my ACOM 2000A are up to 3 to 1 from 80 through 10 and 2 to 1 Max on 160.
> My old SB-220 seems to be more tolerant with its simple PI net out. I have run it as high as 4 to 1 with 500 watts out.
> 73, Dennis W0JX