[Amps] Acceptable SWR for Tube Amps

Tue Feb 18 12:31:51 EST 2025

All the critical voltages and currents should be independently limited 
somehow by the amps protective circuitry.  And while operating the tube 
with a higher Ig (generally speaking) is not optimal, unlike a SS amp, 
it's not terminal.

What is another consideration in the context you mention (tuned to QRG=X 
and then QSY to QRG=Y) is the impact on harmonic filtering.  I remember 
getting a notification that I was creating QRM on 80m when operating on 
a 160m contest once for that very reason.  The antenna was very narrow 
banded on 160m and the amp (a triode GG-type Alpha) was pretty sensitive 
on 160m band tuning as well.  I would tune it for 1825 and then work +/- 
25 Khz and not worry about touching up the amp.  Cannot remember what 
the SWR was at that point but probably ranged up to 3:1 on the extremes. 
Unknown to me at the time, as I got further from the tuned point, the 
2nd harmonic levels got to be significant and that brought the OO 
notice.  oooopppsss!

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com

On 2/18/2025 1:27 AM, Lukasz wrote:
> 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 at 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 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
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> -----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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