[Elecraft] Problem with K3 driving old tube amp

[Bob McGraw K4TAX]

Sounds as though ALC from the amp is causing the transceiver to step 
power.   One should NOT use ALC from the L-4B to the K3. The ALC circuit 
in the L-4B was designed for exciters which did not have precise power 
control.   The use of ALC to control power in this configuration is 
likely to cause rather poor sounding CW notes.    Yes, everything you 
describe points to an ALC issue with the amp.       WHOOP WHOOP WHOOP

73

Bob, K4TAX


On 5/31/2019 6:21 PM, William Stewart wrote:
> All,
>
> I am trying to bring a Drake L-4B to life. I have done the power 
> supply rebuild, the soft-start and other mods. I have also directly 
> grounded the grids of the 3-500Zs (Eimac brand). In driving it with my 
> K3 (SN 51XX, software up to date) I find that the RF power stair-steps 
> to the set power in a hesitating way over a few seconds. This happens 
> sending Morse code or using the "Tune" button.
>
> The supply voltage, as monitored on the K3 display, is at about 13.4 
> volts when transmitting.
>
> I have a power/SWR meter between the K3 and the amp and it shows a low 
> SWR at all times, as does the K3 SWR meter.
>
> The power meter between the K3 and the amp shows the power gradually 
> increasing. As the K3 increases power out, the amp power out increases 
> in a similar fashion.
>
> I see no indication of oscillation. The power meters on the output 
> side of the amp show no power when the amp is keyed but no RF is 
> supplied. I placed a low-pass filter (41 MHz cutoff) between the K3 
> and the amp to no effect.
>
> I notice that if I send a series of Morse code characters such that 
> the K3 transmit delay timer never times out, once the power ramps up 
> to the set value, it remains solid until the timer expires - when I 
> start transmitting again, the power step process repeats.
>
> When I put the amp in Standby, the K3 functions normally.
>
> This smells like an oscillation problem, but I see no evidence of one.
>
> Any advice would be appreciated.
>
> Thanks,
>
> Bill K5EMI
>
>
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