How far can a SB-200 rebuild go?
This amp was a gift from one of my early elmers WB0SSB Larry Smith. The amp in stock form is a fine performer for SSB and CW, however, SB-200 tubes, the 572B, and RTTY do not mix well.
This project evolved in an attempt to see what kind of an RTTY duty ready amp could be constructed from an SB200.
It just got out of control... Very far out of control...
I'm unable to publish the schematics for the amp's various modules. The trouble is that, as with any project of this nature, the schematics and notes tend to trail the physical build. Which means the schematics do not serve very well as a basis for reproducing the work.
I have loved RTTY since I was a young ham. Had a Western Electric RTTY machine in my bedroom as a kid. My parents house was a pier-beam type of construction and when that machine was marking time it would set up a standing wave on the floor that would shake that end of the house. Fortunately, my parents loved the kid's radio hobby - I was pretty lucky...
The story of the evolution of the amp is explained in the page below. But it's still a work in progress. I sure hope that this year I can complete it before the fall contest season heats up.
Unfortunately, it was not until I had the amp for a few weeks that the RTTY "bug" bit me hard - otherwise I don't think I would have ever selected the SB200 as a platform for an RTTY amp project.
Presentation made to the KC DX Club as of March, 2009
Click HERE to download (12 mb PDF). Lots of pictures of the work and a good explanation of the project objectives.
PART 1 - Basic Mods
Most owners of these old Heath amps has put the basic modifications into the amp. These included, in my case:
- Low voltage keyer (Harbaugh)
- Replacement power supply filter/meter board (Harbaugh)
- Replacement fan
- Glitch protection (fuse + resistor)
- LED backlight replacing the yellow-tinted incandescent bulb
- Black face plate and plack paint on the chassis to match the FT2000 rig
Single LED replacements are available but I wanted a difused look and the only way I could figure out to do that was to stand off some white and blue LEDs. The net effect was very nice.
The unit was retubed with some China made replacements. These generally worked fine. Power output and overall performance was right in line with what you would expect from a stock SB200. About 800w on the lower bands and less as the frequency climbed to the higher bands.
The glitch protection does a great job. Normally speaking glitching is not a normal part of the day's activities but when you are trying to determine the "outer limits" of operation, it becomes a fact of life. Unfortunately, there is no grid circuit protection. And when the amp's glitch fuse opens, the grid current jumps to a very unsafe level. In the stock configuration, the series 27-33 ohm grid resistor serves some capability for current limiting but I think if I were to do a simple mod again, I would look into another way of protecting the grids.
I purchased a total of 8 pcs over those first few months for my testing. In one case, I receive one with a broken anode weld. The supplier replaced the tube for me. A funny story related to this particular tube - one somewhat well known amp builder suggested that I was abusing the tubes a bit and the broken weld was not the result of a workmanship as I claimed. Morale of the story - advice levels even from well known individuals vary in quality. :)
Vernier Reduction Mod
In practical use, the tune control was quite sensitive especially on the high bands so the next step was to add a verner control. Finding the verner
was actually more difficult than retrofitting it. The trick is to trim down the drive module so that it fits flush. The drive knob requires quite a bit of surgery but the finished result looks almost identical to the stock amp.
My hit-list of things I found the amp was lacking:
572b + RTTY = Fast Tube Death
I can say from expensive experience that there is NO combination of settings on the SB200 which will preserve the 572b in RTTY service. In fact I went around and around with my amplifier Elmer Charlie W5VIN about this - before finally determining (with a box of dead solder 572b's on the floor) that he was right.
The problem, as I view it, is that the disipation of the 572b is made primarly by radiation rather than convection or conduction. Which means, there is really no amount of air that will cool down those anodes. I even blackend the tube compartment in an attempt to improve the absorption of the tube radiation - that did help - but not enough. The 572b would still slide down in output over time. The solution, as Charlie was right to press, was in finding the proper tube for the job.
While considering a tube replacement, I got to thinking that maybe fixing all the shortcomings of the SB200 would be the right path to go. It was heading down this path that brought the SB200 Sleeper project into it's current form... I should have realized things were about to get out of hand but that's why hindsite is not as helpful as understanding cost-estimating what it would take to realize one's vision.
A big lesson learned from this project is that it's generally cheaper to buy than to build, especially if you incrementally step forward rather than working from a well thought-out project plan and have no significant junkbox from which to draw from...
PART 2 - Proof of Concept
What key features would turn the SB200 into a contemporary amp with modern creature comforts?
And what tube would do the job. Charlie was strongly recommending the Gi7b - he was possibly the first to have converted the MLA2500 Dentron to use these tubes and many mods with this tube have been published since. To compare the key metrics of the tube, this table is helpful to put things into perspective:
A common question that I'm asked is - "What does it take to modifiy my SB200 to use the Gi7b." At a very minimum, this list will cover most of the work needed. I don't recommend an untuned input - the Z match into a pair of tubes is not too bad but efficiency and harmonic performance really takes a bad hit without the helpful flywheel and matching of an input network.
At this point, I wanted to make minimum modifications to see if the amp would make power as the design suggested it should. So going forward, I considered this work as a "testing the proof of concept" project. The metering adjustments are not explained here but they were about the most complicated part of the work. The 80m tank is loosened from it's standoff in the following pictures and pushed o
Fitting the New Tubes
Heat Trend in RTTY Duty 950W Output Test to Dummy Load
Temps are too high with the stock fins for unlimited key down time. The stock coolers and this muffin fan is adequate for anything other than RTTY.
Measurements were made at various points along the tube length using an IR thermo. The limiting factor is the body temps, rather than the anode temps interestingly.
Signal Quality - Looking Good!