[1000mp] HEATHKIT SB-200 vs YAESU MUSEN FL-2100B

[Tom Rauch]

> You folks at Heath most certainly built what has become an Amateur

I worked as an outside contractor doing designs. We were actually 
in the process of developing several new products including a pair 
of 3-500Z's with big components (to be called the Warrior II) when 
the plug was pulled due to slumping kit sales.  

> Radio Classic, in the SB-200...rugged, reliable, stable, and most of
> all, the best performance per dollar available....I built mine, more
> or less around 1968, and have logged thousands of on the air hours.

Actually the 200 is one of the least stable amps, because 572B's 
have long thin grid leads that make it difficult to effectively ground 
the grids. Under some conditions you could make it take off, but 
with normal loads on the amp it was fine.

The problem is for some odd reason all problems are blamed on 
stability.

> have only changed my Cetron 572B's once, in all these years, and that

Cetron tubes were well built and tested properly, unlike recent 
tubes. Good tubes are getting harder and harder to find as the 
engineers all die off or retire, or manufacturing moves where QC or 
experience is poor.

> was preventive maintenance...naturally, I always minimize the grid
> current, by advancing the loading as much as possible, while tuning
> for maximum power output, and never exceeding 500 mils keyed down
> current on the grids...

Ouch. That is very high, and beyond the dissipation limit of the grid 
by a large margin. But thoriated tungsten filament tubes are very 
forgiving of grid overloads because of the materials used.  

> Regarding your comments on the bigger brother, which offers about 3 db
> improvement in output over the SB-200,  you may recall from previous
> communications, that my SB-220 has Chinese Pride tubes, which had
> operated with great instability and lack of efficiency...and yes, your
> postulation that poor vacuum may indeed be the culprit here, is
> certainly a reasonable explanation...once the tube is sealed however,
> how does one determine whether it's vacuum was pulled down
> sufficiently?....

Only by high-voltage breakdown tests. The slightest amount of gas 
greatly decreases voltage breakdown. The elements inside a tube 
tube can outgass, in particular graphite anodes. Tubes must be 
tested both before and after sustained operation. 

Tungsten filament power tubes normally have the gettering agent 
on the anode, and the elevated temperature of operation activates 
the getter. An arc can also getter the tube by breaking down 
gasses. The trick is gettering the tube without going broke 
replacing all the little parts that go boom when the anode current 
reaches hundreds of amperes as the filter caps discharge to the 
chassis through the grid of the tube.

Somehow people equate arcing to oscillations or instability, I sure 
don't know why!

> what occurred...I only can say for now, that after the changes were
> completed, all my issues disappeared suddenly...and the amplifier
> operates with both efficiency and stability...This may be due to the
> "gettering" of all remaining atmosphere within the envelope,

Yep, if you arc em often enough or if you ever manage to get the 
anodes near red-hot for a long time you'll degass the tubes.

When manufactured, tubes are operated with the anodes glowing 
cherry red for hours at extra filament voltage and allowed to arc, as 
well as cycled through temperatures because gettering agents 
liberate gasses at some temperatures and absorb the same 
gasses at other temperatures. They also have to "cook" all the 
trapped air out of the materials inside the tube.

The main problem with the Chinese and glass Russian tubes is 
they never cooked em properly or tested them right, and the 
materials are sometimes poor. When you get a good one it might 
be OK, and some factories might be OK. The factories I've dealt 
with were not, however.

Metal-ceramic tubes are a different story. They getter by cathode 
or filament heat, and are sealed much better. You can sometimes 
degas them just by running the filaments for several hours. 3-
500Z's actually have to be operated, and the anodes heated to a 
slight color to degas.

If you don't heat the anodes up, 3-500's (and 572's) will eventually 
go bad. And then when you least expect it, bang.
73, Tom W8JI
W8JI@contesting.com