Updated 11/10/2024:  This is a work in process; it will be a while until this unit is back in service; it’s hard to find much time to work on projects like this!

While taking our household trash to the local dump, I found a discarded Heath SB-1000 linear amplifier in nearly museum-quality cosmetic condition.

This is a photo of the unit as found and described below.  The front panel and meters are as new and there is only a slight scuff mark on the top of the cover.  What a lucky find!

It was odd to find a restorable linear amp, because I am currently rebuilding an 813 tube linear amplifier at home.

The SB-1000 had been discarded because the cheap plastic switch that controls the multimeter had failed and was not repairable.  It is built into the meter circuit board, so replacing it would have required a new circuit board and switch.  These parts are likely unobtanium.

Here are photos of the SB-1000, found on Flickr; I hope the original poster won’t mind me showcasing his work.

Repairs required to resuscitate this unit:

I used a Dremel motor tool to cut out the old switch from its circuit board.

Having a large junk box, I was able to source a ceramic switch wafer with the needed silver contacts and a good quality ball-bearing frame/shaft for the wafer.

This will ultimately replace the little plastic switch (shown next to the new ceramic one below), requiring a modification to the meter circuit board for mounting it to the studs protruding from the rear of the new switch.  Some re-wiring of the circuit board is also required.

The new switch appears somewhat dull and dirty in these photos, but it is not – just bad lighting.

 

 

 

 

I do not know when the unit was last used or what condition the electrolytic filter bank is in, but everything in the amp is clean and in apparent good condition.  The bandswitch is as new.  I can replace the filters and the rectifier board if need be.  The construction quality was excellent so kudos to the original owner.

I intend to make the following inspections, modifications or improvements to the unit, and comments from anyone reading this are welcome:

1.  Full inspection and cleaning (minor dust).  Back off, then retighten all screws, nuts and bolts.
2. Install a 900 ma high voltage fuse immediately after the rectifier board.
3. Install a 50 ohm, 50 watt “glitch” resistor immediately after the HV fuse.
4.  Install back-to-back 1N5408 diodes across each meter’s terminals.
5.  The 3-500Z’s maximum allowable grid current is 200 ma.  Install a quick-blow 200 ma fuse in the negative lead of the grid current meter.  Do the same for the multi-meter.
6.  Lube fan if applicable.
7.  Fortunately, a soft-start system was installed by the previous owner so I don’t need that, or even a Varistor, to slow the inrush current.  But I might put a 20 amp varistor in the filament circuit, because that will reduce the AC about 1/10 volt.  Our AC voltage here is 120-121 volts and the voltage for which this unit was designed is a few volts less.
8.  Probably:  Slip vinyl tubing over the red HV wires, just to add more insulation.
9.  Maybe:  Install an anti-corona washer to the band switch 160 meter position.  I don’t use 160 so I might skip this.
10.  Maybe:  Replace the plate blocking capacitors with better quality ones, which I have.
11.  Maybe:  Replace the HV capacitor at the cold end of the plate choke, if I don’t like the look of the small one.
12.  Maybe:  Replace the plate choke with an AL80B one that will work with the new WARC bands.
13. HV Testing:  After all mods are done,  bring the AC voltage up to 120 volts very slowly (hours!)  while measuring the milliamps flowing through the filters and equalizing resistors, to ground.  Keeping this to less than 20 ma should be safe.  This will give the filter caps a chance to reform themselves, in case the unit has been out of service for some time (3-500Z has been removed).  This will also allow testing the newly installed meter switch and its circuitry, as well as the HV.  The new 900 ma HV fuse should protect against any major failures.  The junk box will be explored to see if a high wattage (>60 watts)  150,000 ohm HV load resistor can be found.  This will provide a 20 milliamp drain on the HV, in addition to the 8 ma load provided by the 8 x 50,000 ohm equalizing resistors.
14. If testing the HV and meters goes well, a Yaesu 840 in CW mode will be used as the driver, slowly increasing its output to drive the SB-1000 into a 50 ohm dummy load. with a combination SWR/Wattage meter in the transmission line and a sensitive field strength meter present to sense any leakage from the dummy load.
15. Should the HV testing not go well, indicating worn-out filters or other significant problems, both the rectifier and filter boards will be replaced with modern components and home-brew circuit boards.  These will be smaller than the original components and will fit easily into the unit.

Should the 3-500Z turn out to be non-functional, I’ll have to rethink how to restore the unit.  I could replace it with a single 813, for 500 watts input, or a pair of 813’s for 1KW input.  Or Or I could pay $250 or more for a new tube (ouch).

I notice that the Heath standards for grid and plate current are higher than what Eimac recommends, so I think I’ll run it within the Eimac limits, which will reduce output power but no one will notice that.

Heath allows 500 ma of plate current and up to 200 ma of grid current.  Tuning in CW mode to achieve 400 ma of plate current and about 140-150 ma of grid current will meet the Eimac limits and won’t stress the expensive 3-500Z.  This produces over 700 watts input, not the 1,000 that Heath ratings will produce.   At least, that’s the plan.

Regarding tuning and loading a grounded-grid amplifier, it might be instructive to read W8JI’s page on the subject, which stresses the importance of monitoring the grid current and/or output power when tuning the unit on a band.

This beast creates HV of 3100 volts no load and 2700 volts loaded so we must remember to keep the cover on at all times and to not go poking around the inside of it while the thing is plugged in.  Death is not reversible.

I have three transmitters which might be used to feed this amp when it is placed in service.  One is the Yaesu FT-840, rated at 100 watts input.  This will have to be dialed back to about 50 watts when driving the SB-1000.  Another is a 2 x 807 tube amp used for CW and phone.  The 807 amp has a variable power of 5 to 150 watts input (about 90 watts out).  The third is a Heath DX-60B that outputs about 60 watts from a 6146.  Eimac wants 55 watts drive.

All three will have to be run at no more than about 60 watts to prevent overdriving the 3-500Z, causing splatter.  I could install a 25 ohm cathode feedback resistor in series with the cathode (the cathode is actually the filament, in a 3-500Z) coupling capacitor, so that the 3-500Z could tolerate 100 watts of drive but this will probably not be needed, if I can pay attention to how I drive it.

It does have an ALC circuit which will have to be activated with the Yaesu; don’t know about the other two transmitters yet.

At this point, that’s all I know about the condition of this unit and what I plan to do with it, but it’s a very welcome addition that will look good next to my Drake 2A receiver.