The power amplifier build part 3 - input circuit

-


Finished board, waiting to be populated with filter components


 In part 3 we have a look at the input circuit used. Sine this is a grid driven linear, we have to feed rf exitacion to the control grid. If we look at the schematic, we can see the load - or swamping resistors - which basically sets the input impedance the transmitter will see, sans stray capacitances and inductances. The resistors sits in series with the bias supply, and are groun ochded at RF by a ceramic disk capacitor.


Input circuit shown, My build is heavily influenced by EX8A. Also load resistors (2 k) shown. I ended up 1 k load resistors.


There are designs using a broadband transformer for this kind of input circuits, but I wanted to wet my feet in filter making and tuning. On my 2x813 linear, I used an LDG Z100 autotuner, which works fine, but there is always the possibility to tune to a harmonic, so for this, I settled for a design using conventional fixed and switched PI filters. Everything is switched from a common 24V band board, located in the logic chassis. 

Pcb layout in EasyEda


I decided to use 2 rows of relays to switch the filters in and out of the circuit, and provided space for a few different sizes of toroids and mica capacitors. I also included footprints for trimmer capacitors, to be able to fine tune the match.


Surplus mica capacitors of different values. Footprints on pcb was made to accomodate a lot of different types of mica's


The input circuit uses non-inductive resistors as the load resistors. I initially used 2200 ohm load resistors , but it turned out I could not tune it properly, or have a broad enough match using them. I had to replace them with 1K resistors, which gave a better response of the input filter.

Remember, all the input power will be dissipated in the load resistor.


Filter calculation.

It's been a while since I did this, so I had to dig in the back on my memory to remember how I did. 

I used online calculators to guesstimate needed values for inductances and capacitors.

Here is part of the list of values I ended up using. Basically over 40 M, it was trial and error. We can calculate them, but in the end, turns had to be removed from toroids, turns had to be added, capacitors had to be removed, smaller ones inserted etc. On the higher bands, trimmer caps (10-60 pf ) was used on the input side, and parasitic capacitance was enough on the grid side.

input (2x1K) 500Ohm load imp.

160M

C1 400 pf + 10-60 pf trimmer

L 12.5 uH

C2 750 pf  + 10-60 pf trimmer

80M

C1 200pf + 10-60 pf trimmer

L 6.4uh

C2 510pf + 10-60 pf trimmer

60M

100 pf + 10-60 pf trimmer

L 4.6uh

C2 300 pf + 10-60p pf trimmer

40m

C1 10-60 pf trimmer

L 3.5uh

C2 68 pf + 10-60 pf trimmer


I then installed the board in place and tacked the components in. Using a nanovna, I did a sweep to determine how off I was. Using the smith chart helped me decide whether I needed more or less inductace, or more or less capacitance. I had to remove a few turns on some of the inductors, and add some on a few. In the final form, I drilled holes in the front cabinet, so that I could reach the trimmer capacitors from the outside.


Input circuit board in it's final place. 160M bottom, 10M at top

Holes for adjusting the capacitors can be seen 
on the front plate, below the tubes.


All in all, I'm happy with how it turned out.

Comments

Post a Comment

Popular posts from this blog

My HP8640 died!

-
Image
 My HP8640 died! This is me catching up on past projects. I did this back in 2022, figured someone would benefit from this information, since the 8640 is still around in great numbers at hams all over the globe. I bought a nice looking HP8640 at our local hamfest back in 2022. It was working when I got it home, but a day or two after I got it home, it started acting erratic, with RF ouput dropping out. eventually, there was no output whatsoever. Did some tests, and got to the conclusion that the cavity oscillator was not oscillating. Following various clues online, I tracked down the issue to the A3 unit (cavity), and a series resistor, which had drifted in value. By measuring the series resistance between the + feed and the A3Q1 socket and the - feed and the A3Q1 socket, I could see that the + lead seemed reasonable at 370ohms, whilst the - lead had drifted to 19Kohms, where I was expecting around 900ohms. The A3Q1 (HP21 pictured below) transistor sits under the small hex cap om t...
Read more

813 grounded grid amplifier

-
Image
2 x 813 grounded grid linear amplifier  This year marks 10 years since I built this amplifier, and it´s been in use on almost all QSO´s I´ve had since then.  When I got into amateur radio back in 2013/2014, I quite soon understood that I needed a vacuum tube amplifier. SB220´s and L4B´s were high on the wanted list, but there wasn´t anything for sale at that moment. I stumbled across a pair of 813´s and sockets, and that´s what set the wheels rolling. I begun by collecting the needed parts. High voltage transformer, variable caps, meters and such, and after that, it´s mostly a job of shaping metal and drilling holes. Most parts collected An aluminum chassis was bent using the sheet metal brake at work. Reinforcement extrusions was bolted to the underside of the chassis where the HV transformer would go. With a finished box a rough layout was determined, and the hole drilling excercise begun. Getting a feel of where to place things Lot of parts to fit. Note that pictured bandsw...
Read more

The Power Amplifier build. Part 2

-
Image
Power Amplifier Saga – Part 2: The RF Deck So, back in 2020, I was working for an old friend of mine who runs a machine shop. I was operating the CNC laser cutter – a 4 kW fiber laser – and spending my days around a machine that could turn sheet metal into precision cut panels I started planning how to use this in my amplifier build. Then, as we all remember, COVID hit . I got sick, along with the rest of the family, and ended up spending two weeks at home in the middle of winter. Tired, restless, but with just enough energy to sit at a screen, I decided to finally learn Fusion 360 – to take advantage of the situation and make plans for the amplifier. From Components to CAD During those two weeks, I modeled the entire RF deck from scratch. That meant creating accurate models of: Variable capacitors  Roller inductor and fixed coils GU81 tubes and sockets Vacuum relays Stepper motors (luckily, existing models helped here) It was a lot of work – but worth it . On...
Read more