Notes on K8NZ Linear

15 October 2002

Output on 80, 40, 15 satisfactory (550 watts). Draws 400 ma plate current and about 50 ma grid current. On 20 and 10 it is only about 200 watts out, with the same grid and plate current indications. Reverting to the old 3-400Z results in about 100 watts less on all bands. On occasion output jumps up to full level on 20 and 10, but only briefly. Zener diode for cutoff grid bias seems OK because idling plate current is the same on all bands. Problem is plate blocking caps, HV RFC, or the HV DC grounding safety RFC on the output.

Tried wiring around the output T/R relay with no change. Cleaned bandswitch. No change. Inspected tank and saw no obvious problems.

Click here for info on the tubes for this amp.

7 November 2002

More extensive troubleshooting. Tested three different grid circuit mods to try to get max drive to the tube. With 50 ohms across the grid, used MFJ meter to determine best match. The final arrangement provides good load on all bands, with 10 and 15 of course being the worst. Did extensive physical inspection of all tank components, cleaned all, disconnected the final RFC safety choke. Result is that output is still somewhat spastic -- sometimes 700 watts, more often the usual 200 or so. Acts like something has to get hot before it will work. Use of an antenna tuner in the exciter helps immensely to couple RF into the input of the tube. Grid current/HV meter intermittent.

With 50 ohms in place of the tube grid, see the following looking into the input:

3.5  52 + j14
7.0  58 + j2
14.0  72 + j9
21.0  88 + j12
28.0  85 + j42.

When the exciter sees these lousy loads it throttles back the output without an antenna tuner in operation. Little wonder output is worst on 10 and 15.

8 November 2002

Tested with RFC on HV removed -- no difference. 3-500Z appears to be no good, 3-400Z is fine. 3-500Z develops full output for awhile, but after about 5 minutes on 20, 15, or 10 it is down to nearly zero. This amp will only be reliable on 40 and 80 with the 3-500Z in it, and better on all bands with the 3-400Z.

15 November 2002

Placed in service at PJ2T and blew audibly about 2 minutes after HV was turned on. Failure was the 18 V CT transformer (Archer brand) which blew its primary. N1ZZ brought a replacement rated at 50/60 Hz and W0NB installed, making mechanical modifications to the case to make it fit. Linear has not been tested on the air since this replacement.