Thank you all for interesting replies.
Indeed the amplifier is operated remotely or if I'm sitting next to it I
still don't touch the tuning and I operate it the same as if I was remote.
So the comments by Michael are spot on. To simplify. Imagine you take a
classic tube amp and you tune it to a certain frequency. Then you move your
trx to frequencies that are higher or lower without touching the amp. How
bad can the SWR get before there is trouble?
I'm not sure it can be explained in function of the Pi tank Q, because the
Q will determine how far off the tuned frequency your SWR will remain good.
This is one step before. Assume the SWR is 1.6 or 1.8 or 2 (wherever your
Pi tank Q happens to put that spot).
What is the threshold "bad stuff"starts to happen? Is it different for
different tubes or tube types? (no doubt) My amp uses a tetrode in common
cathode configuration so I'm mostly interested in this tube.
Also what are the symptoms that happen? I imagine they would be similar to
mistuning so increased screen/plate current while power drops culminating
with the grid current once we get into "the bad stuff" happens region.
73,
Łukasz - SP4IT and SQ4KW
On Mon, 17 Feb 2025, 19:38 Michael Tope, <W4EF@dellroy.com> wrote:
> 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..............
>
There are at least 3 different versions of this amp, but the only Pi tank
difference is the load capacitor (fully adjustable in my version,
switchable only in at least one other version).
See this link for a fairly accurate schematic https://ibb.co/mF0wjCYS
Other than that the plate capacitances are switchable by a large 5 position
rotary switch that can be switched manually, but most of the time memory
recall drives it to one of the 5 positions. These positions are described
on the front of the amp as frequency ranges (1-1.3 to 3Mhz, 2-3 to 5Mhz,3-5
to 10Mhz, 4 - 10 to 16Mhz and 5 - 16 to 30 Mhz). So the plate capacitor is
always fixed into its range.
Then there is the variable coil (variator?) this is initially set manually
on each of 10 memories, then it is driven there by the mechanism. There is
a fairly precise scale that allows one to make sure it really ends up where
it should. However the switch changes that too by shortening/lengthening it
so the range is different on each setting.
Likewise the load capacitance . It also is mechanically driven to a
manually specified target, but as Peter said below the switch adds extra
fixed capacity to the adjustable cap depending on the range.
This is all about the Pi tank, but there is another tuning mechanism
between the stages (between the preamp and the final tube) this is tuned in
exact the same way and the big switch also switches capacitors for it while
the knob adjusts a variable coil.
>
>
>
>
> 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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> >
> >
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