ACOM 1000 Tetrode Deck - Input Circuit

For some practical purposes I decided to replicate the input circuit for GU74B from ACOM 1000 1kW amplifier.
The tube is Grid driven and the maximum input power is around 55 W.

According to the user manual, when this power is excedeed, a 3db attenuator is activated so the G1 maximum dissipated power is not exceeded. The attenuator can stay in the circuit as long as the maximum power is greater than 50-55W.

I was very interested in the power detection circuit and  3 db attenuator. It is a very clever circuit and it was a pleasure to analyse it.

Original schematics

The basic ideea behind the circuit is a dummy load of 50 Ohm/100W, frequency compensated.

As the measurements was made without the tube in the socket, I simulate the G1 circuit with a 150kOhm in parallel with a 68 pF capacitor.

Simplified schematics

I made all the coils from 0.8 enameled copper on a 8 mm diameter core.  The lenght of the coil was around 12 mm but after fine tuning, some coils are at 10 mm and some at 15 mm lenght.

The home-made circuit inside the pressure chamber

The input SWR is less than 1.02 from 1.8 MHz to 54 MHz and is rising if the G1 is removed (the 150kOhm and 68 pF).
I do not have here (at my second location) the Marconi analyser to the circuit loses but the SWR is very sensitive to the G1 circuit so i can expect a low insertion loss.

Here are the results from measuring the voltage at the RF detector with various input power. At 100W the voltage swing is not so linear to the frequency but this can be due to the 1N4148 diodes.
At 65-75W, the voltage line is very linear so may be a good point to set the threshold at 31-32 V for activating the 3db attenuator and keep it inline as long as PTT is pressed.

 

	20W	40W	50W	60W	70W	100W
1.85 MHz	16.00	23.68	25.85	28.50	32.37	41.10
3.7 MHz	16.00	23.17	25.90	28.43	31.81	39.62
7.1 MHz	15.89	23.15	25.89	28.22	31.81	40.20
10.12 MHz	15.79	22.64	25.41	27.90	31.81	40.20
14.15 MHz	15.90	23.10	25.87	28.15	31.88	39.06
18.12 MHz	15.40	22.90	25.75	27.94	31.55	39.52
21.2 MHz	15.35	22.16	25.14	27.83	30.89	37.57
24.9 MHz	15.67	22.72	24.99	28.09	31.94	34.50
28.2 MHz	15.44	22.46	24.60	27.75	31.65	39.67
29.5 MHz	15.39	22.33	24.45	27.58	31.46	36.40
50.12 MHz	14.75	21.30	23.96	27.15	31.09	32.64

And a nice graphic with the values:

 

73 de YO3HJV