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Linear amplifiers cold adjustments with network analyser (VNA)

by F1FRV

* Rev 0 Juin 2009 *

* Updated august 2012: added input circuits *

Part 1  Output circuit

Borrow or buy a network analyzer like the DG8QAQ VNWA, MINIVNA, or Agilent / HP.

1. Connect the VNA to the output of the amp. Amp is off and unplugged !!

2. Leave the tube or tubes connected, and install a resistor from the plate connection to the frame ground.This simulates the output resistance of the tube or tubes. 1/4 or 1/2 watt resistor is OK (preferably carbon, so it is non-inductive, but not mandatory up-to VHF).

To calculate the resistor value, you can use the EXCEL sheets:
For HF amps, see main3d_pi.html , download and use “CALCUL_PI_ F1FRV_Fr_&_Us_”.
For VHF / UHF amps, see main1L_Tetrode_Amp_Design.html, and use “Calcul Ligne SORTIE Cavite Ampli”.

The complete explanation is in the downloads, at bottom of this page.

A typical BAD HV RF choke

Typical GOOD HV RF chokes with ferrite core

Part 2   Input circuit

1. Connect VNA to the input of the amp. Amp is off and unplugged !!

2. Leave the tube or tubes connected, and install a resistor from the cathode (for GG amps) connection to the frame ground. This simulates the input resistance of the tube or tubes. 1/4 or 1/2 watt resistor is OK (preferably carbon, so it is non-inductive, but not mandatory up-to VHF).

To calculate the resistor value, you can use the EXCEL sheets:
For Grounded Grid amps, download and use “Calcul_ENTREE_Ampli_Grille_a_la_masse.zip”.
For Tetrode amps, see main1l_Tetrode_Amp_Design.html, download and use “Calcul Entree Tetrode”.

Triode VHF input example

Tetrode VHF input example

 

Download all explanations in the complete file: Linear Amplifier Cold Adjustments.zip

That's All Fox !!!!!

Enjoy !!!!

Dominique - f1frv@sfr.fr OR

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