Heathkit SB-200 Modifications
The SB-200 from Heathkit is a 1200W PEP input linear amplifier, which will develop an output of approximately 600W PEP when operated on a 120VAC line. It made its debut in 1965, and covered the 80m-10m bands available at that time. The straight forward grounded-grid design, using two 572B triodes, has stood the test of time fairly well. I highly recommed the SB-200 to anyone in need of a compact desktop amplifier that plugs into a common duplex outlet and still pumps out a respectable signal.
SB-200 mods schematic
SB-200 mods photos
There are a couple of things I didn’t like though. The high voltage metering circuit uses three 4.7M 2W 10% carbon composition resistors in series as the upper half of the divider. The lower resistor was a 15K 1/2W 10% carbon composition resistor. Such resistors are a poor choice for reasons of accuracy and stability. My SB-200’s meter was telling me that the plate voltage was only 1,800V. The manual says it should be 2400V! The Actual voltage is just under 2300V when connected to a 124VAC line. I replaced the three 4.7M resistors with nine 1.5M 1/2W 1% metal film resistors, and one 1M 1/2W 1% resistor in series. The 15K resistor was replaced with a 15K 1/4W 1% resistor.
The original electrolytic capacitors were 125uF 450V units. They measured 72uF on average so I replaced those too. I used capacitors purchased from DigiKey in Minnesota. They’re a 270uF 450V 105C part, Panasonic part number ECO-S2WB271EA. They’re printed circuit board snap-in caps, so I had to drill some holes in the PC board. This was fairly easy to do using a pin vise and a sharp drill.
The original rectifiers were also replaced, but for no particular reason. Eight P600M 6A 1KV diodes purchased from RF Parts to replaced the sixteen original diodes marked Sens 57-27. I enlarged the holes for the original diodes leads with a drill bit. A jumper is installed in locations where there were no diodes installed.
When I started off on this project I only intended to fix the metering. Then I noticed that one-half of a filament in one of the 572B tubes wasn’t lit, so I bought a matched set of Taylor 572B tubes from RF Parts. Soft starting the filaments of the tubes will increase their life. Since a pair of tubes cost $99 bucks I thought this would be a good feature to add. A Ketema NTC thermistor (surge guard), part number SG140, fills the requirement nicely. Its 25 degree C resistance is 2.5 Ohms, when hot the resistance drops to 0.09 Ohms, and it’s rated for 9A. The 572B tubes draw 4A each at 6.3V. I mounted the thermistor where it would receive some cooling from the fan. This makes it recover more quickly to help protect the filaments against the AC line blinking. The CL30 made by Keystone, part number KC003L-ND from DigiKey, should work as well, but I haven’t tried it.
I added a glitch resistor in series with the high voltage lead too. The literature recommeds values ranging from 10 Ohms to 100 Ohms. The glitch resistor should be a vitreous enamel insulated wire-wound type. The value I had on hand was 24 Ohm 30W. It’s mounted on a piece of G10 material, near the high voltage power supply printed circuit board, on nylon snap-lock stand offs.
The original ALC circuit unbalances the pair of tubes. So I moved the connection of C29 from the grid of V1 to the common connection between R21 and R22.
I also increased the value of the grid by-pass capacitors by paralleling a 470Pf silver mica across each of the existing 200pF units. This should increase the gain in the 80m band.
Finally, I added two 1N5404 diodes (in parallel, connected cathode to anode) across the meter movement terminals. I’d hate trying to find a replacement meter.
572B Typical Operation, Grounded Grid Linear Amplifier (frequencies to 30 MHz) ICAS
|DC plate voltage 2400|
|VDC grid voltage -2 V|
|Zero-signal DC plate current ** 45 mA|
|Single-tone DC plate current 250 mA|
|Driving power 50 W|
|Single-tone useful output power ** 300 W|
** Approximate value. From www.svetlana.com
The Heathkit SB-200 and other amplifiers using the 572B seem to use these values as their design parameters too. I haven’t checked the bias voltage at the grids… need to do that. For my amplifier the bias current reads 65mA and the plate voltage reads 2200V. At least that’s what the SB-200’s meter is telling me.
SB-200 Keying circuit modification
This circuit allows keying the SB-200 with transcievers which have no keying relay. The 3.15V is full wave rectified filament voltage (the 6.3V filament winding is center tapped.) An optocoupler could replace protection diodes D2, D4 if ground loops are a concern. None of the values are critical.