[Elecraft] Problem with K3 driving old tube amp

[William Stewart]

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