New Tubes

I have installed a "Matched Pair" of RF Parts Inc. Taylor 572B tubes in my Heathkit SB-200. As it turned out, one of the original Centron tubes was not performing well - it also had somewhat lower idle current. Those new tubes are probably made in China and just branded under the "Taylor" brand. After installing the new tubes, I left the amp turned ON with no drive for about 4 and a half hours. This serves to condition the tubes and to minimize the chance for flash-over and in general it is a good practice with new tubes and tubes with unknown shelf life. The heat from the filament activates the "getter" which is deposited on the plate. The "getter" is a chemical composition which reacts with any free (gas, metal) molecules in the tube and improves the vacuum.
The tubes are working fine and I am getting over 850W on 80 meters when tuning with pulser. On 40,20 and 15 meters the output is a bit lower - 750+ Watts which is still pretty good for this amp! When I heated the plates with continuous drive, I noticed that one of tubes got a little bit hotter than the other (judging by the plate color). I am planing to buy another pair for spare and will try to re-match them. It is also a good practice to "rotate" the tubes in the amp with the spare ones every year or so and not allow any of the tubes to spend too much time on the shelf. Vacuum tubes don't age well when they are not in use.

The new RF Parts Inc. "Taylor" tubes to replace old Centron tubes. The tubes are from the same factory batch and have writings on the glass envelope with the test results from the "matching" procedure. I cleaned them with alcohol to prevent "hot spots" from forming due to fingerprints. As a side effect the alcohol wiped clean all of the markings along with the red Taylor logo.

Heathkit SB-200 Conclusion

The amplifier is finally done! It took me over a month, working a few hours on the weekends. The rebuild is dedicated to my patient and understanding wife (I wouldn't say "supportive" :-) as she wasn't trilled by my activities). I have a lot of spare parts now to address future needs - almost enough actually to rebuild a second one. The only thing left to do is to align the input networks for minimum SWR and to stretch a little bit the coils of the parasitic suppressors. It turned out that the suppressors were heating up at around 28.3 MHz which is somewhat low! By spreading the turns I'll lower the inductance in hope to raise the cut-off frequency towards the upper side of the 10 meter band. Lower power output is expected on 10m but this is normal! On 10m if the band is open you don't need a lot of power anyway - when it is closed, 50-100W more is unlikely to help a lot.

I never cared for the "Heathkit Green" paint. It looks very dated to me. I went for a more sophisticated look for the front panel - I think it came out really sharp. I was going to paint the panel in light grey with black lettering (like the QRO Tech. amps - similar to the "Lab grade equipment" look) but at the end black & white looked pretty good too! The other front panel modifications are the two status LEDs and the "Stand-by/Operate" switch. The left LED is a Bi-color LED displaying the status of the switch - Green for "Stand-By" mode and Red for "Operate" mode. The right LED is a blue color one and indicates the KEY down signal - it goes "on" when the amp is keyed by the exciter (Tx).

The meter is showing the Anode (B+) voltage in HV position of the meter switch. 2400V is "right on the money!" with 110V AC supply. I am planing to build a x100 HV probe for my multimeter. This way I'll be able to calibrate the meter more precisely. The voltage drops to about 1900V when the amp is fully loaded. The plate idle current is 90 mA - exactly where it should be! (with the new tubes)

The amplifier is ready for integration in my shack. I made all of the high-power coaxial jumper cables using RG-393 MIL-spec silver/teflon cable. Diamond 2-position RF Switch will switch the output between the antenna and the dummy load. Bird 43 meter /w 1000H slug will show the amp's output power for now. Later on I am planing to replace it with a Telepost LP-100A vector watt meter. I am already using one (LP-100) to monitor the input RF power. The amplifier will run on 110V until I find time to install the 220V line - most probably in a couple of months.

By popular demand, here is an old hand-drawn schematics of how the LED is wired.

Heathkit SB-200 (Part 2)

As I mentioned before, all of the wiring and most of the components of the amplifier were replaced. These old amps are using carbon-composition resistors. Such resistors change their value from stress and aging. I found that some of the resistors were off by more than 20%. I used precision flame-proof metal-film resistors and ceramic-composite resistors where low inductance is required (grid bias).

This picture shows Wafer C of the band-switch along with the Pi and L input networks, the large filament choke, ALC circuit components and the two valve sockets. The two grid bias resistors are of ceramic-composite type. I also installed bigger (both capacity and max voltage) silver-mica caps in the grid grounding circuit. This will lower the impedance to ground on the lower bands and should result in more output. The original caps can't handle a lot of current and often fail. The length of the leads must be kept to the minimum to avoid extra inductance. A metal-film low-inductance power resistor between the input circuit's rotary switch and the tube's cathode circuit is installed to limit the possibility for over-driving the amp. Before installing the input circuits, I made sure that all of the ferrite cores in the inductors are moving so I can align them later. Extra caution is needed since these cores sometimes get stuck or are glued inside the coils with wax and it is very easy to break the ferrite core when rotated.

Top view of the amp with the RF deck on the right and the HV power supply and power transformer on the left. Both variable capacitors were inspected for signs of arcing, cleaned and lubed. Any signs of arcing and sharp points on the blades must be smoothed with very fine sand paper and cleaned well from the dust afterwards.

Picture of the RF deck. The two original Centron 572B valves seen in the picture were replaced by a matched pair of RF Parts, Inc. 572B Taylor valves. The 10m/15m silver plated tank coil was cleaned with Deoxit D5 solution. Wafers A , B and C of the band-switch were inspected and carefully cleaned with extremely fine (0000 size) steel wool and Deoxit D5 and then treated with Deoxit ProGold. Using steel wool to clean an RF switch requires *very good* washing with alcohol afterwards, as well as using strong magnet and compressed air to get rid of ALL metal particles. The switch should absolutely clean from metal particles and contaminants before installation. After the cleaning, the switch looked almost as new. The output multi-band tank coil was encased in Kapton foil and all leads were inserted in teflon tubing.

The new Low-Q parasitic suppressors designed by Rick Measures, AG6K. The new design includes two sets of suppressors in series (per valve) for an improved performance and amp stability. Resistive Nichrom wire is used to make the inductors - the kit includes silver solder and special (very corrosive!!!) flux for soldering Nichrom. Metal-film resistors used in the suppressor can handle power dissipation in the excess of the rated 3W. An extra 1 Ohm resistor in the anode circuit of each valve serves as fuse in case of a catastrophic valve failure. 572B is a relatively unstable tube when it comes to parasitic VHF oscillations so these suppressors should help a lot.

Another view of the RF deck. I had to repair the plate choke with a new winding. The old one had badly burned wire enamel. The new choke is covered with kapton/silicone adhesive tape. It is barely visible in the top-center of the image. Right next to it is the new blue high-voltage plate filtering capacitor. The door-knob capacitors in the output circuit were inspected and one of them had to be replaced (it was showing signs of arcing). I used a layer of Kapton tape around this capacitor (the red one in the middle of the two variable capacitors) as extra precaution.

Update: I did also some work on correcting the filament voltage. It is posted here.

Restoration of Heathkit SB-200 (Part 1)

Over the past month I was rebuilding my Heathkit SB-200. When I originally bought the amp on eBay, there were a few small issues here and there - aging components, burnt HV choke, etc. The amp was actually a SB-201 converted to SB-200. It had the 10 meters band added with the original band-switch, tank coil and input network but no other mods. I wanted to update the amplifier and install all of the Harbach mods - HV power supply board, soft-key, soft-start, fan and relay. When I started the upgrades, I quickly realized that I can easily do a complete rebuild by replacing most of the components with modern versions. I stripped the amplifier down to the bare chassis and practically started from scratch, using the original Heathkit manual.

This is the Harbach High-Voltage Power Supply board. It is an improved version of the original HV board with modern computer-grade capacitors and high-voltage diodes. The equalizing resistors are of higher value and lower wattage. The board is a drop-in replacement. It also includes a string of 1% resistors for the voltage divider in the HV meter circuit.

Update: I upgraded the 6 Harbach PS capacitors (180uF/450V) with even bigger 390 uF/450V. This made the power supply "stiffer" and now the amplifier performs better than the stock version. The most obvious improvement is the lower HV drop under full load and a tad higher output power. Such an upgrade (installing high capacity filter caps) should be done only if the Soft-Start mod is already installed. The high-capacity filter caps bank draws much higher initial current (when charged for the first time during a power-on). If the amplifier doesn't have a step-start circuit, this in-rush current surge could trip the internal and/or external breakers (especially if the amp is powered with 120V) and also puts under strain the transformer/rectifier circuit.

This picture shows the multi-meter, bypass capacitors on the filament voltage line used for the backlight lamp and the new rectifying circuit for the front panel status LEDs. New, larger solder lug terminal strip (with more positions, from Radio Shack) is installed to accommodate the extra components of the low-voltage rectifying circuit - a diode, bypass capacitor and electrolytic filter capacitor. I am using half of the center-tapped filament circuit to supply the LEDs with about 3.5V DC . The half-winding is the same half used to power the Soft-Key module.
Two paralleled diodes (A-K, K-A) are installed right across the meter's terminals for protection. In the event of a high-voltage glitch the diodes will clamp the maximum voltage across the meter's coil, saving it from damage.

The underside of the amplifier. All wires were replaced with silver-plated/Teflon insulated solid wire. The coaxial lines were also replaced with silver/Teflon MIL coax cable. Almost all components were replaced with the modern type version - the new resistors are metal-film and ceramic-composite type. All capacitors and diodes were also replaced (most of the parts I ordered from Mouser. I also bought some components on eBay and from West Florida Components). The meter switch was carefully cleaned and lubed. Both UHF connectors (SO-239, input and output) were replaced with new silver/Teflon bulkhead type - the old ones were worn out and in pretty bad shape. For the KEY and ALC line connectors on the back panel I installed new female bulkhead BNCs. RCAs will work fine too but they don't have a locking mechanism.

The Harbach mods - Soft-Start (left) and Soft-Key (right). I installed the Soft-Start mod by gluing a piece of Plexiglas (with High-temperature RTV silicone) to the chassis and attached the PCB with cable ties through special L-shaped holes in the Plexiglas. Note that while this method worked just fine for me, Harbach recommends different location and way of installation. Harbach sells two versions of the Soft-start mod - for 240V and 120V AC mains. Each version is using different value power resistors. The best way to go is to order the "240V version" plus an extra set of the 20 Ohm resistors even if you are using 120V AC. For 120V AC mains, the extra set is soldered in parallel to the existing resistors. This makes changing the operational voltage more convenient. The directional coupler of the SWR/power metering circuit is seen near the top of the image. The coupler assembly was removed to allow access to the input coaxial cable and for replacement of the RF connectors. I cleaned and re-installed the coupler assembly after replacing the connectors and cable. One should be very careful with the germanium detector diodes (small glass body) - they are brittle and can be overheated during soldering, not to mention that they are also hard to find nowadays. The thin coaxial cables between the coupler's detector diodes and the front multi-meter rotary-switch were replaced with small diameter silver-teflon type coaxial cable. The power cord was changed to a heavy gauge one (AWG #12). Stress-relief is provided by a metal cable clamp. Visible at top left is a ferrite toroid (mix 43) with a few turns of the small diameter coaxial KEY line. This should take care of stray RF in the key circuits.

The new fan motor from Harbach and the new Tx/Rx relay. It turned out that the new relay had lower coil resistance compared to the original relay. During Tx mode, the relay coil is part of a voltage divider for the tube's operating negative grid bias. This lower resistance in the new relay coil had to be corrected by changing the value of the coil's paralleling resistor. I did some voltage divider calculations and shared the results with Jeff from Harbach. He adopted the mod and is selling now a new resistor together with the relay upgrade kit to fix this issue. Test measurements showed that the corrected circuit is providing the needed -2V grid bias in Tx mode.

This picture shows the high-energy "Glitch" resistor on the right. This resistor is encased in Kapton foil and mounted on teflon stand-offs. The high-voltage red wire going to the RF deck was salvaged from the power supply of a microwave oven - it is rated for 10kV. At the top of the picture are visible components of the front panel status LEDs as well as the current limiting resistors for the LEDs (located in teflon tubing). Also visible is the back side of the Operate-Stand-by switch. I used a bi-color LED to show the amplifier's operational status - Green when the amp is in stand-by mode and Red when in Operate mode. A blue LED is used to indicate the status of the KEY signal. Extra bypass caps are installed at the Stand-by switch line and LEDs. The switch is DPDT type - one pole of the switch is used to interrupt the KEY line (between the KEY jack and the Soft-Key module) and the other pole is switching the bi-color status LED.

Picture of the glitch resistor, main wire harness, some of the front panel LEDs circuit, the multi-meter switch and potentiometer. Also visible is the HV diode string (3 diodes in series) used to protect the meter during a HV "glitch" event. I added an extra RF bypass capacitor between ground and the output of the high-voltage supply. Teflon stand-offs are used to raise the glitch resistor above the chassis and away from other components/wires to prevent arcing. Kapton tape is applied as extra insulation around the high-voltage line.