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

[Jerry Haigwood]

Hi Ken,
     Your experience with Chinese 572B tubes has been totally opposite than
mine.  I find the Chinese tubes to be of excellent quality.  The dealers I
have dealt with will also accept any defective tube back without a question.
I haven't had any defective tubes but another person I know found one of his
1+ year old Shuguang made tube down to air.  It evidently had a slow leak
around one of the pins.  The dealer took it back and replaced it even though
it was out of warranty.  I am currently running a pair of RF Parts labeled
Chinese made tubes which I suspect are actually Shuguang made.  So, you are
telling me my Chinese made Shuguang, re-labled RF Parts tubes "are very
marginal even when new." Well, my tubes are performing well and have been
performing well for about 6 months now.  I run them at 600W peak output.  I
do not see any effects of them going away soon.  They're producing the same
power as they were when they were new.  The problem with short tube life is
usually due to overdriving the tubes.  The SB-200 amp has a gain of about
10-12db.  Therefore you should never put more than 50-60 watts into the amp
or you will be overdriving it.  Forget ALC, it clamps only after the tube
has already been overdriven.  It is sort of like closing the gate after the
horses have gotten out!  At 55 watts input to the amp, I get 600-650W
output.  To see peak power you need a peak detecting wattmeter.  Very few
wattmeters are actually peak detecting.  I currently use a Telepost LP-100A
Digital Vector Wattmeter.  A single dot from your keyer will show you the
peak power output from your amplifier.  If you can't see peak power output
with only a single dot from your keyer, you do not have a peak reading
wattmeter!  I worked for a tube manufacturer for 5 years as an engineer.
All tubes require aging to activate the cathode and to burn off impurities
such as oxygen.  Your re-aging schedule for these 572B tubes is highly
suspect.  Since we don't know how the manufacturer made the tubes or his
process for aging, it hard to determine any type of re-aging schedule for
them.  We should never apply a "generic" re-aging schedule designed for
304ths or such to any modern day manufactured tube.  It is like using 1930
repair techniques on a modern day microprocessor controlled radio.  It just
doesn't work!  Since you have said, "I doubt if I will ever buy another
Chinese-made so- called '572B'," I am guessing you will be buying Russian
built Svetlana 572Bs and adjusting the bias correctly or converting all of
your amps to something else (I am not even sure Svetlana makes 572Bs
anymore).  There are no US manufacturers of 572Bs and I doubt if anyone is
going to start up making them soon (perhaps they could make 572Bs and buggy
whips at the same time?).  BTW, you will find 4CX250B tubes very hard to
cool.  You need to figure out how to move 6.4CFM of air at .82 inch of water
back pressure.  It is hard to find a blower to meet those requirements.
Dayton makes good blowers but their blowers will not fit onto the SB-200
very well.  It would be best to mount the 4CX250Bs in a sub-chassis in the
SB-200 and remote mount the blower under the desk or wherever.  A better
method for cooling 4CX250Bs is to pressurize the top of the chassis and use
a Teflon chimney which fits tightly around the tube plate and fits tightly
against the top of the chassis.  Holes for the air to escape can be drilled
at the top of the chassis directly above the Teflon chimney.  The air can
then escape through the anode cooler, up the Teflon chimney, and out of the
holes.  Small holes can be drilled into the bottom of the sub-chassis to
allow some air to escape out of the bottom past the tube pins providing
cooling for the pin connections.  Good luck with your conversion.
Jerry W5JH


-----Original Message-----
From: heath-bounces at puck.nether.net [mailto:heath-bounces at puck.nether.net]
On Behalf Of Kenneth G. Gordon
Sent: Wednesday, August 08, 2012 1:53 AM
To: Heath at puck.nether.net
Subject: [Heath] Results of experiment with reactivation of 572Bs in SB-200

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
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