DEDICATED RF, LLC  NEWSLETTER
   Summer  2013   



 Dedicated RF, LLC - Company Newsletter
 Where you get the inside news....
 
 dedicatedrf.com 

Dan Simmonds  KK3AN
Owner


Glen Bankston  K4KV
RF / Firmware Engineer

 
In this issue....

Progress made on the RF deck and sheet metal

Progress & changes made to 
the HF tank circuit

What's next....














































































































The past few months can be summarized by one word....

Progress!


rfdeck_prelim 

Decent headway is being made with the RF deck assembly, and the general chassis of the amp is coming together. Obviously the dual-8877 deck is shown above, for export & commercial use.

The RF deck will separate / unbolt completely from the power supply for simplified service.

Actual outside dimensions of the amplifier are looking like:

20"(d) x 14"(w) x 17"(h)

The 12VDC dual blower is mounted directly under the tube chamber, and will be controlled via a PWM for speed control & airflow. Thermistor readings in the exhaust air will be handed off to the microprocessor, which will in turn control the blower RPM. While we've not chosen the most quite blower available, this dual blower design moves a LOT of air, and provides some redundancy.

A good deal of time has been spent fabricating various mounting brackets above and below the RF deck, adjusting the arrangement of the plate choke and suppressors, etc. Also added is the large DC blocking capacitor (shown in blue above), as well as doorknob capacitors - one (1) for the tune side (650pf) and two (2) for the load side (650pf + 1200pf).


switch1 
switch2

The resonate tank circuit for this amplifier has undergone a major overhaul. Actually more like a complete design change. Let me explain....

I had been personally perfecting the custom variable inductor (mentioned in depth in the last newsletter) for a number of months....and it was mechanically operating well, but the bottom line is that it became clear this inductor simply wasn't right for the way we want this new amplifier to perform.

Four (4) main issues kept looming over the roller....those being (a) unwanted in-band resonances in the unused turns of the coil, (b) questionable performance in the upper end of the HF spectrum, (c) reduced Q, and (d) perhaps most importantly, the significant time it would take to re-tune the amp from band to band would be a real downfall for contesters needing to QSY quickly. For instance, taking this inductor from 160m to 10m would require approximately a full minute! Not acceptable.

So, the past couple months I've been building a custom "break-before-make" (or non-shorting) inductor switch pictured above. Measuring 4-1/2" (114 mm) wide at the outer frame, this switch is designed to switch through a bank of custom fixed-resonance inductors. At the end of the day this offers MUCH faster band changes, and comes without the other issues noted above.

No rotary switch commercially available seemed appropriate without some sort of compromise (believe me, I scoured the globe). An attempt was even made to use a bank of vacuum relays for the fixed-inductor switching, but again we kept being faced with compromises....so it was back again to the drawing board.

This new switch offers significantly increased spacing between contacts (a full .250") over typical rotary switches. This means no chance of arcing within the switch assembly. Each switch position is made with spring-temper dual contact tabs (front and rear) contacting both stators for superior contact by way of wear-resistant phosphor bronze electrical button contacts welded to the contact tabs.

Stepper control (for the inductor switch as well as the Tune & Load capacitors) will be via direct shaft drive with our sub-degree stepper motors....using no gears, belts or pulleys in the drive system.
 
next

1) Mount filament chokes under the tube chamber / air box.
 
2) Mount current limiting resistors for filaments.
 
3) Hook up HV capacitors for cathode circuit and small coax to the side of the air box using RCA connectors.
 
4) Mount stepper-driven inductor switch.
 
5) Begin power supply testing & finalize software on Atmel controlling power supply.
 
6) Wind and mount fixed inductors.
 
7) Adjust inductors for full spectrum coverage.
 
8) Power supply & general chassis sheet metal forming.
 
 
Archives of prior newsletters are downloadable from the site:

www.dedicatedrf.com/newsletter.html


That's it for now! We'll distribute these newsletters periodically, or as significant updates are made.

73

Dan  KK3AN