SB-200 Modifications

The Heathkit SB-200 amplifier has been around for years and many are still in use.

I rebuilt my Heathkit SB-200 that I obtain from e-bay.  This one was in bad shape but the price was right.  It was dropped, bad tubes, and definitely been used.  I slowly rebuilt the amp and got it fully functional. Initially I rebuilt the power supply using  Harbach Electronics PM-200 Replacement Power Supply Module. Refer to their website.

I came across a web site,, which PA0FRI explains in detail on how to modify the sb200. I went ahead and modified my SB-200 according to his website and followed most of his recommendations. The following items are what I wanted to accomplish with these modifications:

  • Tube Protection. Fuse the high voltage to reduce catastrophic failures.
  • Soft Start to limit the primary in-rush of current and slow the rise of anode voltage. This protects the filaments and rectifier diodes and permits the use of a lower current primary fuse.
  • Stability
  • Change amplifier key to support modern radios.
  • Reduced filament voltage. On my SB-200,  I had almost 7 Volts on the Filaments.  I ended up using a loop of 18 gauge Teflon wire to drop the voltage until the filament voltage was in an acceptable range.
  • ALC Adjustments is now easily adjusted.  This is nice when I use it with my Drake transmitter which I can easily overdrive.  The amplifier is very stable and is well protected.
  • Faster QSK Operation.  I  wanted a faster transmit/receive transition, by  updating  the input and output relays did the trick.
SB-200Modified Amplifier

SB-200 Modified, Schematic Diagram

These modifications are based upon PA0FRI’s modifications.  I give him all the credit. I am impressed with the stability of the amp and have not had any issues.  Please refer to his  website:,

The modified SB-200 schematic diagram is shown below.  This uses a “Key-All” circuit board for keying the amplifier. This board may still be available from Jackson Press.  I also incorporated a “soft-start” from Harbach Electronics.

Here is the meter switch wafer detail.  This needs to be modified per the schematic above.

Input/antenna relay uses a reed relay for the input and a VAC relay for the output side. I had to had a 24V power supply for these relays since the original used 117V.

SB-200 Modified Amplifier Gallery: