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Henry Radio 5K Classic Amplifier
Blower, QSK and Step-Start Modifications
Completed in the summer of 2003


Download the 5K Classic Owner's Manual



The Henry Radio 5K Classic amplifier has continuous coverage from  80 thru 10 meters using a pair of Eimac 3CX1200A7s in grounded grid.  The power supply is capable of 4,000 vdc @ 1.5 amps in intermittent service.  Continuous duty is 4,000 vdc @ 800 mA.  The power supply components and blower are located in the lower part of the pedestal.  The RF deck is in its own self-enclosed and shielded enclosure that slides into the upper part of the pedestal.  The high voltage, filament and control connections are made via umbilical cords to the underside of the RF deck.

I purchased this amplifier new from Henry Radio back in 1988 and have always been disappointed with the excessive blower noise caused by the poorly built Dayton 2C915 (140cfm) blower.  First, mechanical noise was caused by induced low frequency vibrations in the cabinet from an out of balance squirrel cage (Dayton blowers are notorious for being poorly balanced).  Second, was the severe airflow restriction caused by the holes around the tube bases and the exhaust holes in the top of the cabinet being too small.  The measured airflow at the exhaust holes was only 25cfm (per tube) with the blower capable of more than twice that level.  So, after 15 years, I finally decided to do something about it.  (See Cooling System Upgrade)

The second thing I never really liked was the TR relay system.  Henry did use a Jennings RJ2B vacuum relay for 'RF Output' switching, but they also used a slow open frame mechanical relay for the 'RF Input' and 'Bias' switching.  The result was a slow and somewhat noisy TR system which wasn't up to the task for the new digital modes and high-speed CW.

After reading the Amplifier reflector extensively, and a bit of searching on the web, I was all set to start acquiring the components to make this modification.  The biggest stumbling block would be replacing the existing Jennings RJ2B 'RF Output' relay with a smaller Kilovac HC-1.  The RJ2B being slower but higher current rated versus the HC-1 being faster but lower current rated.  After talking with Paul Hewitt, WD7S, of WD7S Productions, he convinced me that using two Kilovac HC-1s in parallel for the 'RF Output' relay wouldn't be a problem.  I consider Paul to be a real expert when it comes to amplifiers.  Check out the photos of his homebrew amps in the 'Gallery' section of his web page.  I subsequently hired Paul to install the new relays, including the fabrication of new relay mounting brackets and the installation of a low current MOSFET keying system to make the amplifiers keying system compatible with modern radios.  (See QSK Relay Upgrade)

Finally, I was always annoyed by the huge thud caused by the HV power supply every time the amp was turned on.  The inrush was so great, at times it would trip the 40-amp circuit breaker on the front of the cabinet that's used as the on/off switch .  I knew the fix for this problem would be to install a step-start inrush limiter circuit for the the HV supply.  To be fair to Henry, the later production units (in the 90's) did have inrush limiting using a mercury contactor and a high wattage ceramic resistor.  (See Step-Start System)


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