[Amps] Acceptable SWR for Tube Amps

[Michael Tope]

Thank you for this info, Peter. I was presuming that Lukasz question 
pertained to the case of remote operation where a fixed preset was used 
without adjustment at different levels of VSWR (like you might see when 
switching between different antennas or when the operating frequency was 
moved off the frequency where the preset tuning was optimized). This is 
not clear to me from the schematic. Are the three 5 position switches 
for C26, C27, C28, C29, C32, and C37 manually switched or electronically 
actuated? How about the variometer, is it manually adjusted or 
controlled by servomechanism?

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



On 2/15/2025 3:51 AM, dj7ww at t-online.de wrote:
>
> The amplifier by Lucasz is tuned as other tube amplifiers and can be 
> preset in addition.
> Plate tuning is L tuned by a variometer and uses fixed capacitors, 
> load tuning is by a variable 1200pf capacitor in addtion to fixed 
> capacitors.
>
> Attached diagram by F1FRV.
>
> 73
> Peter, DJ7WW
>
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> -----Original-Nachricht-----
> Betreff: Re: [Amps] Acceptable SWR for Tube Amps
> Datum: 2025-02-15T12:00:49+0100
> Von: "Michael Tope" <W4EF at dellroy.com>
> An: "amps at contesting.com" <amps at contesting.com>
>
> 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
>
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