[Amps] Acceptable SWR for Tube Amps

[John Lyles]

The screen current will certainly vary if the tube load resistance is 
changed. Its often best to look for screen I max for best tuning, 
instead of a plate current dip. But as for it being momentary, no, it is 
an average screen current that moves slowly as indicated by a meter.

73

John K5PRO

On 2/19/25 8:05 AM, Lukasz wrote:
>
>
> On Wed, 19 Feb 2025, 06:09 John Lyles, <jtml at losalamos.com> wrote:
>
>     (...)
>
>     Anyway, this is how i think about these things. And, as stated
>     below, if
>     you go too far off normal, the components in the matching network
>     are no
>     longer able to compensate to bring the real part of the impedance
>     to the
>     optimal point of the tube for a given plate voltage and power. That,
>     along with the reactance that gets transformed back to the tube,
>     are all
>     bad things, tube or transistor. But how far is bad, requires doing
>     the
>     math of calculating the circulating currents and voltages.
>
>
> This explains why we (owners of this amp -it is very popular in Poland 
> , Russia and some other countries) can get away with tuning it for 
> let's say 3.7MHz and use it on entire 80m (which ends at 3.8MHz here 
> on the high end). All of the Pi tank elements are massively oversized. 
> To the point where people replace the factory 1500W output tube with 
> up to 5kW tubes and run it successfully . But it also explains why the 
> original tubes have a reputation for being "finicky"and "easy to 
> break" in sone circles and many replace them with two pentodes like 
> GU81m (which they then run at double the datasheet power). While the 
> Pi tank can definitely take it the tube is another story. If it's run 
> at 1000W I think it's going to be fine most of the time ..but let's 
> say we run it at 1800W (as people do). Then we go to the very end of 
> 80m when tuned sonewhere in the beginning....
>
>
> There is one more variable. This amplifier as it comes from the 
> factory contains the preamp and the coupling between the two is tuned. 
> I found this coupling is the first thing that limits the output power 
> (at same drive level) if I stray quite far from the original tuning 
> frequency .
>
> The most popular modification of this amp is to remove that preamp 
> completely and drive the final tube through a small attenuator with 
> 30W.( I drive it with 200mW)
>
> I can definitely see how all of this together is a recipe for broken 
> tubes.
>
> You mentioned higher momentary currents and voltages , depending on 
> where we are in the AC cycle. I wonder if this could also create high 
> momentary screen currents that could not show on the meter nor trigger 
> the protection because of how brief they are.
>
> Very interesting . These amps were made for the military/TV service. 
> There they would have 10 frequencies programmed and they would sit at 
> 100% output on one of them never straying a dozen KHz away (usually FM 
> was the modulation of choice but linearity was very important because 
> the signal they sent was a dozen telephone channels, maybe some data 
> too. They wanted as low IMD as possible ). It is only in our amateur 
> use they started to be used this way.
>
> 73, Łukasz
>
>
>
>     73
>
>     John
>
>     K5PRO
>
>