The Henry 2K Project

by KC5RT and WS0B

or how we brought this old beast back to life.

09/15/2013

Updates on 01/12/2014

The Henry 2K linear amplifier is a single stage amplifier designed for SSB, CW , FSK and AM operation. It required between 80 and 100 watts to drive a pair of 3-400Z triodes to 2000 watts PEP input. The 2K was designed in the days when the ham power limit was 1 kW input. The power supply was capable of much more. The Henry 2K is a floor mounted cabinet design, with the power supply located in the bottom of the cabinet and an RF deck attached to the top. The design is very simple as with all of these vintage amplifiers. The 2K was built from 1961 to 1965 later improved versions where available. At 154 pounds this amplifier is "built like a tank". However, the RF compartment of the 2K has a very low profile. The interior height is just enough to accommodate the height of the 3-400Z with plate cap. The taller 3-500Z will not fit into this amp without major cabinet modification. So if you have one of these, search for 3-400Zs. They haven't been in production since 1965. This particular amplifier belongs to WS0B, Glen and is Serial Number 614.

This amplifier can operate on either 110 or 220 volt house current. It has a 12 volt T/R relay system which will work with almost any modern or vintage exciter. The exciter only needs to provide a ground to the center of an RCA jack to key the Henry 2K. You don't need a soft switch modification.

The 2K Amplifier is a simple grounded grid design, the use of the zero bias 3-400Z makes the power supply even simpler than others that use 3-500Zs or any other tube requiring grid bias when operating. Most Henrys feature a Pi-L tank circuit rather than the simpler, straight Pi found in most modern amplifiers. There are important advantages to the Pi-L. The Pi-L tank circuit provides an additional 10-to 15-dB. attenuation of the second harmonic and even more attenuation on the higher-order harmonics.

Glen's amplifier was not functioning for an unknown reason when we started working on it. Since it quit without warning and didn't smoke it was assumed to have a minor problem. The manual explains operation and provides a schematic but no real maintenance information is provided. So we had to guess as to how best to go about troubleshooting this beast. We decided to first do a visual on all components and make measurements with the power off. This meant taking the unit apart so that internal components could be examined.

Reach under the RF deck and unscrew the High Voltage DC lead. Next pull the octal power plug off of the deck. Gently lift the RF deck up until it just clears the floor cabinet then turn the deck on its side allowing it to rest on the floor cabinet. Note the two AC power wires that go to the screw terminals. Remove these while observing the color code on the wires. Mark the chassis near the terminals so you will know where these wires go later. In our unit White/Red wire goes to the back terminal while a White/Yellow is towards the front of the RF deck. Note that the blower also connects here. Those two black wires make no difference as to which terminal they connect to. Once all wires are free you can completely remove the RF deck assembly from the power supply and set it aside.

Here you can see the very dusty power supply with its two 3B28 rectifier tubes and the transformers and capacitors. (Top view of the Power Supply Assembly)

See Figure 1 for a schematic of the power supply.

In the photo to the left you can see that the old bleeder resistor fell apart once the mounting flanges were removed. It's a good idea to clean off the ceramic insulators to prevent high voltage leak paths. Use warm water soapy water to wet a rag and don't leave an oily residue. Paper towels will snag on the sharp edges.

When re-soldering these bleeders make sure there are no sharp points or stray wire hairs. All joints must be rounded to prevent high voltage leakage.

Once these bleeders have been replaced or checked to be good, it's time to look under the chassis for other decaying or burned up parts.

In this photo you can see the under chassis power supply components. In our amplifier R3 was a pile of carbon. CR1 had shorted and unsoldered itself, and C4 was leaking capacitor juice.

These components along with the Low Voltage transformer make up the Low Voltage (12 volt) supply. This supply energizes the TR and Antenna relays. How it managed to work with an open capacitor and only one good diode is a mystery. (Maybe it didn't)

I didn't have a 150 ohm 2 watt resistor so for a temporary fix I put a 47 Ohm and a 100 Ohm resistor in series to make the needed 150 Ohm value for R3. I replaced C4, originally a 500 mfd 25 volt capacitor with a 470 mfd at 50 volts. I also replaced CR1 and CR2 with 1N4007 diodes. The originals were 600 VPIV @ 600 ma, so 1N4007s should work fine.

When working under no-load conditions the circuit should produce 14.5 volts DC as measured across the 150 Ohm resistor. The transformer has a 110 volt primary winding and the circuit is a full wave rectifier with capacitor input filtering.

The Power Supply was reassembled and set aside while we examined the RF deck.

Figure 1.

During the RF deck inspection we noticed that one of the 3-4000Z s had a loose filament pin. This could cause the filament not to light. It was re-soldered and the filament checked for continuity. We also cleaned the tube sockets to insure that the filament voltage would make a low resistance connection. Thanks to Ron, NE0X we were able to quickly track down these filament issues.

Keep in mind that if you do have poor filament connections, the filament pin can heat up and the solder can actually melt out of the tubes pin and cause the circuit to open. I used some emery cloth to clean the contacts of the tube sockets.

This tube had previously been repaired and flux was left on the repaired pin. The pin that comes out of the tube's glass envelope is soldered to the socket mate at the factory. Due to age and filament pin heating these pins often become open and must be cleaned and re-soldered.

Not only should you re-solder the pin and clean the flux off of it, but you must clean the mating tube socket pin. This will ensure that the filament makes good low resistance contact and reduces future pin heating.

The repair was successful and both tubes are lighting as shown in the photo below.

In Figure 2 below you can see the circuitry of the RF deck. The circuit is relatively simple and it only takes a few minutes to check the components visually and electrically.

We found no problems under the chassis. Glen did say that the Plate Current meter had stopped working. An Ohm meter check of the plate current meter confirmed the meter was open. These meters cost over 100 dollars on the surplus market when they can be found. They are rare but from time to time show up on ebay.

The meter was removed and disassembled. Inside we found an open shunt wire. This wire was repaired and the meter was checked at half scale (500 ma). The meter was reading 5% low but well within a usable tolerance. The meter was reassembled and reinstalled and confirmed to work during final testing. Be careful not to use magnetized tools when working on meters. Plastic or aluminum tools are recommended.

One other item that needs a good visual inspection is the plate caps and suppressor assemblies. It our amplifier one of the solder joints had broken away from the suppressor coil. This is the point were the resistors solder to the coil. All of these solder joints were checked and two were re-soldered.

Also one of the plate caps was loose on the tube and it was secured with an Allen wrench.

The band select switch on these amplifiers take a lot of force to move. This is partly due to the large RF switches that are used and the dial cord type system that turns the 3 rotary switches. A bit of lubricant seemed to help but not that much. The knob on the band select switch was cracked and it was necessary to replace it. It's still hard to turn the band selector but we assumed that to be normal.

Burn-InTesting.

The Amplifier was reassembled into a complete unit with the RF deck married to the power supply and floor cabinet. Initial tests indicated that the unit was ready for service. There were no load noises, hissing high voltage or fan squeals and all the functions seemed to work. NE0X and I fired her up and applied a modest 10 watts of CW drive at 7.2MHz and the old Henry tuned up smoothly with a 300 watt output. We advanced the drive to 60 watts and observed 650 watts. Later we tested at a full 80 to 100 watts of drive and saw 900 to 1100 watts peak into a dummy load. Now for the ultimate test, WS0B will use the amplifier on the air.

Good News ! WS0B reports that his Henry 2K is producing 650 to 700 watts with 60 watts of drive. He is getting great reports with 12 to 14 dB of gain at the receiving end. Audio reports are excellent as well. Not to bad for a pair of original 1963 tubes. It's time to start looking for a new set of 3-400s or maybe even 3-500Z tubes.

Mike N5RXP just completed his Henry restoration and is on the air.

According to Mike, Ted Henry is supplying replacement HV diode modules that are better than the originals. "The OEM HV diodes that comprise the bridge are discrete components and when you look them up on the web, the mfgr does not recommend using them any longer. Ted Henry's replacement's package is of two HV diodes in series and he claims they are an upgrade, so you might want to look into that. (You will need two to make a full wave bridge)"

Ted Henry is still in business and supporting these amplifiers with parts. He can be reached at:

Ted S Henry
HENRY RADIO, INC.
2050 South Bundy Drive #285
Los Angeles, CA 90025
310-820-1234
310-826-7790 FAX
henryradio@earthlink.net
www.henryradio.com

Thanks Mike for the update on your Henry project and FT101 exciter.