[Amps] PT-3000

[Roger Parsons]

A little while ago I wrote about some experiences with a PT-3000 that I have been working on. This work was completely interupted by an unpleasant and debilitating bout of Covid. Anyway, I am feeling somewhat better now and thought others may be marginally interested in my endeavours.

To recap, the amp originally had the blower wired between one hot wire and ground, there being no separate neutral connection. The tube filament was powered from a switch mode supply connected across the 240V supply. I decided to use a transformer for the filament and to connect both blower and transformer between opposite 120V hot wires and the main transformer CT.

Of course, nothing is ever as simple as it appears. When the main switch is operated, power is connected to the HT transformer (via 100W of soft-start resistors) as well as two series connected 120-12V transformers to provide the 24V relay supply. There is a time delay before PTT can be applied of 180 seconds. Unfortunately, this time delay applies to everything except the 24V supply, so the filament does not get full voltage until PTT is applied, and gets reduced voltage in every receive period. This is obviously rather undesirable, and FWIW is not a design 'feature' that I have ever seen before. The cure was pretty easy - to introduce an additional much shorter time delay for the main transformer - whilst retaining the long delay for PTT, etc. Perhaps a dollar's worth of components.

I also have a variable resistor in series with the primary of the filament transformer which allows me to set the filament to exactly 5V, and to reduce the switch on surge current.

Finally, one other pretty unique feature about this prototype rig - there is a TO220 7824 voltage regulator on the relay supply. No fewer than 8 connections were made to the output lead, all hanging in mid air....

73 Roger
VE3ZI