[Heath] Concerning the SB-200 and tubes...

[Kenneth G. Gordon]

FYI, recently, I had heard "through the grapevine" that RF Parts Co was no 
longer selling Chinese-made, Taylor-branded so-called "572B"s and had 
begun to aggressively search for a maker who could and would output exact 
copies of the much, much more robust ORIGINAL old 572Bs, such as 
Cetrons, etc.

After hearing this story from two different hams, I then contacted RF Parts 
Co, and was assured that, yes, that is exactly what they are doing. I was 
almost ecstatic at this good news! I am now on the waiting list for a pair. No 
delivery date has been announced.

Secondly, I was then advised that the Chinese sellers of what has been 
laughingly called "572B" s on eBay are now no longer selling those tubes, the 
Shuguang ones, as being suitable for RF use, but instead those are being 
advertised as audio amp and guitar amp tubes. I checked eBay and found 
this to be true. Furthermore, the ad now says that they are NOT suitable to 
replace the old North American 572B as they are not the same tube. (Duh!!!)  

I found this to be very interesting, and in fact, a great relief since I have had 
constant troubles with those Chinese-made "572Bs" for the past several 
years.

In the meantime, and previous to this delightful news from RF Parts Co, I 
had, in desperation, decided to modify at least one of my SB-200s to a pair of 
4CX250Bs, since those tubes are practically ubiquitous on eBay and can be 
bought, NIB, for very little money. I now have almost a dozen of those, all of 
which have, so far, tested good.

I began this project about a year ago. I am using a set of chimneys that come 
in a pair which was originally mounted in some sort of military transmitter, I 
think the T-195, which was originally paired with the Collins R-392 receiver. I 
cannot remember the military nomenclature for that setup, but it was 
extremely common. The T-195 originally used a pair of 4X150As as I 
remember it. The chimneys are a perfect fit for the 4CX250B.  

I bought the chimneys from Fair Radio sales for peanuts, I have forgotten
how much, but it was very little.

I believe Fair Radio Sales still has them.

The chimneys are a very heavy phenolic, and include sockets. They are
mounted on a small bit of chassis.

I dismounted the chimneys and sockets from the chassis, flattened it out, and 
used it as the pattern to cut out the socket holes in a small chassis I had 
made for me out of heating duct galvanized steel sheet by a local heating and 
airconditioning shop. I had them make me two chassis. They cost about $5 
each.  

I have CAD drawings and photos of my chassis if anyone would like to see
that.

I then bought a top quality "muffin" fan, I have forgotten the make but can dig 
that out, which was specifically designed to be able to deal appropriately with 
back-pressure, at the recommendation of Carl KM1H. The fan cost about 
$40, as I remember it. It is very quiet and air flow is much more than 
adequate through the chassis, tubes and chimneys.   

I then moved some components around under the chassis of the SB-200, and
the fan JUST fit, perfectly. I can send photos of what I have so far
accomplished in this modification to anyone who would like to see them.

In order to mount the chassis containing the two tubes, I will have to
dismount, temporarily, the plate tank coil. Things are VERY tight in
there, but everthing DOES fit.

With a combined plate dissipation rating of 500 watts, I don't expect to
have to replace the 4CX250Bs for many years.

I have pretty much settled on the so-called "swamped grid" grid-driven circuit, 
using a 50 ohm dummy-load non-inductive resistor available from MFJ for a 
reasonable amount in the grid. Of course the 250Bs will require a regulated 
screen supply, but that is easily enough accomplished from a circuit that Rich 
Measures publishes on his website, www.somis.org.  

All this effort was due to the fact that good 572Bs have not been available for 
some time now, and won't be until sometime in the near furture.

Even so, it is an excellent "learning experience" for me. Oh. One more
thing: in the process of doing all this, I am also adding 160 meter
coverage by adding a second original plate coil in series with the
original one, and a couple of new switch segments from Harbach, with the
necessary added fixed capacitors.

Well, that's enough for now on this subject. Even so, I suppose what I have 
mentioned here could be applied to any dual-572B amplifier. The Yaesu 
FL-2100 series comes to mind, and there were others.  

I am very anxious to see how RF Parts Co manages to find a maker who will 
output high-quality REAL 572Bs. There must be literally hundreds of these 
amps, and not only Heathkits, out there, sitting unused for want of good 
tubes.   

I hope this information is useful to some of you.

vy 73,

Ken W7EKB