[Heath] Results of experiment with reactivation of 572Bs in SB-200

[Kenneth G. Gordon]

My experiment, begun 2 years ago, in which I asked for used 572Bs has 
finished, and I have discovered several things of interest.

First of all, those 572Bs which have been made in China by the Shuguang 
company are very marginal even when new. Their filaments are not made at 
all correctly, and have absolutely no reserve emission capability. Once they 
are used up, there is absolutely no possibility of reactivating them, unlike 
properly-made U.S.-made tubes.

The Chinese-made tube's plate dissipation rating is not 160 watts, as the old 
Cetrons were, but is more like 125 watts per tube. Furthermore, the grids 
appear to be unusually fragile, accepting no more than 30 mA grid current 
per tube, whereas the original 572Bs were rated at 50 mA grid current per 
tube. In fact, in the interests of longevity, I would not operate the Shuguang 
tubes regularly or for very long at any grid current much above about 25 mA 
per tube.

Plate current, rated at 250 mA per tube for the original Cetrons, should never 
be allowed to be above 200 mA per tube with the Chinese-made versions, 
for very long.

Then, tuning and loading using the Chinese-made tubes is very different 
from that using an old, but reactivated set, of Cetrons I have here. This 
indicates to me that the plate impedance of the Chinese-made tubes is very 
different from the originals.

Lastly, even 5 minutes of operation of my SB-200 with new Chinese-made 
tubes installed, at 14 WPM CW and at a power output level of 400 watts, 
resulted, routinely, in glowing red plates. This continued as the tubes were 
used.

I received several sets of used and pretty well used-up, Chinese-made 
572Bs, through at least one member of this list, with a view to attempting any 
of several different methods of reactivating them.

Any attempt to reactivate the Chinese-made "572Bs", using methods that 
have always worked for me in the past with other thoriated-tungsten-
filamented tubes, resulted in complete failure of what was left of the Chinese 
tubes.

I also bought, over the past two years, 4 complete pairs of new Chinese-
made 572Bs. I bought two sets directly from China, and two sets from R.F. 
Parts. The R.F. Parts tubes were labled 'Taylor" but were very clearly the 
same tube as those I bought directly from China.

In the two sets of tubes bought from China, one failed immediately with a 
grid to filament short. Another worked OK, although it rattled when shaken.

The two sets of tubes bought from R.F. Parts worked right out of their boxes, 
and none were defective. Obviously, R.F. Parts is very careful with their 
tubes and tests each one before they accept it or sell it.

I have absolutely nothing to complain about concerning R.F. Parts service or 
warranty.

However, ALL the new Chinese-made tubes lasted no more than 9 months 
of normal to light use in my restored SB-200.

At times, they acted very oddly. In one case, suddenly going into very strong 
oscillations on 15 meters and above...for a couple of months, then reverting 
to acting normally after that, until they finally and rather suddenly began to 
lose output and exhibit both very low grid current and low plate current at 
what had been normal drive levels.

After the output from my last matched-set of "Taylors" from R.F. Parts 
weakened, I then installed a pair of very old Cetrons I had here, both of 
which were very weak when I got them, but after I had reactivated them, they 
showed normal output with very reasonable grid drive.

I am presently using these ancient, reactivated Cetrons in my SB-200. RF 
power output is routinely between 700 (80 meters) and 550 watts (20 meters) 
with grid current of 30 mA per tube. If I overdrive the tubes to 50 mA per tube 
grid current, I can easily reach 800+ watts output on 80 meters.

I normally run them at 30 mA per tube grid drive, and around 400 mA plate 
current at 2100 VDC.

They never show red plates.

According to the Heathkit SB-200 manual, the operator should keep grid 
current below 100 mA (50 mA/tube) and plate current should be no more 
than 500 mA. and power input should never be more than 1KW. Obviously, 
this information concerned new, U.S.-made tubes, and not these modern 
Chinese-made semi-copies.

At this point, I am converting at least one of my SB-200s over to a pair of 
4CX250Bs in a grid-driven, grid-swamped, Class AB1 configuration, with 
regulated screen and grid voltages. I will use a 25 ohm 10 watt un-bypassed 
cathode resistor at each tube to improve IMD.

These tubes will finally allow me to follow Heathkit's recommendations as far 
as using the SB-200 is concerned.

I MAY attempt to buy used, flat or semi-flat, worn out, U.S. made 572Bs such 
as Cetrons, Dentrons, etc., with a view to reactivating those and using them 
in my other SB-200, but I doubt if I will ever buy another Chinese-made so-
called "572B" unless someone can assure me that they have been 
redesigned and built correctly, or if I can buy new ones for no more than 
$10.00 each.

vy 73,

Ken W7EKB