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@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
-----Original-Nachricht-----
Betreff: Re: [Amps] Acceptable SWR for Tube Amps
Datum: 2025-02-15T12:00:49+0100
Von: "Michael Tope" <W4EF@dellroy.com>
An: "amps@contesting.com" <amps@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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