VK3ZL amp mod

[ZL1TPH]

Work done by ZL1RS re my 144 MHz amp from VK3ZL. The output line was shortened and a loading flapper was installed, rather than the original output line and variable capacitor.

Plate line shortened and bend

Full view plate line

Flapper load bottom

900 Watts

[ZL1TPH]

At 900 Watts

Amp before being modified

[VK2KYP]

Nice job, I have had great success with loading flappers in my 4CX250b's amps.
Is that a GS31B?

73 Gary vk2kyp

[VK4WTN]

nice amp another masterpiece by Bob VK3ZL and great mods by the other Bob ZL1RS
regards Wayne

[ZL1RS]

Bob VK3ZL made a superb job of Steve's 2m GS-31 amplifier ... the construction methods and finish were of 'commercial quality' and a credit to his abilities. There were more than a few moments of quiet reflection here when his passing was announced ... http://www.vklogger.com/forum/viewtopic ... 75&t=12479

However, the 2m PA was sometimes unstable and the tuning/loading action was not as it should have been (unable to load properly at high power levels). I already had experience with a commercial GS-31 2m amplifier belonging to ZL1IU that used a similar output link coupling/loading system and had similar 'issues' with the tuning/loading action (but not the instability). Having had great success in the past with 2m valve amplifiers using the 'capacitive probe flapper capacitor' output loading method (a 2kW K1AGB 2m 8877 amplfier and a 800W HB 'portable' 2m 8877 amplifier), I decided to try that to overcome the loading problems on Steve's VK3ZL amplifier (as well as tackle the instability issue).

The mods worked a treat ... completely stable operation, and tuning/loading behaving just like it should.
- shielding added to the cathode compartment cooling fan hole
- the input tuning capacitor 'earth' connection shortened
- replaced the low voltage feed through caps passing the filament voltage with a couple of 500V rated items (precautionary measure)
- add extra bypass caps to the filament transformer secondary (probably not necessary, but can't hurt)
- bypassing on the filament transformer primary (same reasoning)
- bypassing on the rear panel +24V and PTT RCA connectors (same reasoning)
- move the 470pF 'door knob' HT bypass cap to the outside of the anode compartment wall
- add a 'plate sandwich' HT bypass capacitor to the anode compartment inner rear wall
- flapper capacitor style of loading capacitor added
- shortened the plate line about 10mm so it tunes with the 'Tune' flapper some way in

Photo showing the 'sandwich' style HT bypass capacitor

Following those changes the meter readings were smooth when turning the 'Tune' and 'Load' controls (i.e. no instability), and the amplifier could be 'loaded' properly at any drive level. The results were ...

50W drive
660W out
420mA Plate current
100mA Grid current
approx. apparent efficiency 53%

75W drive
920W out
500mA Plate current
200mA Grid current
approx. apparent efficiency 61%

73,