SB-220 on AM??

[K8DI]

I have an opportunity to pick up a relatively unmolested SB-220 for less than it's worth...

I read through the manual someplace online and found no mention of using it on AM. Anyone have experience doing so? Is it a worthwhile endeavor or not so much??  Spitballing based on specs, seems like with 2000 watts PEP possible, it may be able to do a 500 watt (input) carrier...which would be beyond what is needed. Comments, please, from anyone who's been there, done this??

Ed

[KB2WIG]

E,


I don't own nor have I operated a SB 220.  But, here is a link to the Eimac Data sheet.


http://www.tubecollectors.org/eimac/archives/3500z(73).pdf


klc

[WA2SQQ]

Before I took it out of service I ran it at 200w of carrier. I don’t think I would risk much more. But never had any problem.

[W1ITT]

Keeping the 3-500Zs cool might be a problem.  Instead of using air system chimneys and a pressurized chassis, the SB220 uses transverse cooling.  This is a polite way to say that they use a phonograph motor and fan and blow air across the tubes.  The 3-500Z was designed to be used with air from below, and ducted up through a chimney.   The 500 watt plate dissipation rating only applies if it is properly cooled.   One of the problems that arises is that the filament (cathode) pins get hot and melt their solder, with predictable consequences.  I have "restored" a few of these tubes over the years simply by soldering the pins.  
With typical SSB and CW duty cycles, people have been getting good service from their amps for years.  But a densely modulated AM old-buzzard transmission at full strap may be another story.  I don't know enough about the sufficiency of the plate transformer, but one should consider that as well.  There will be a safe and reliable level at which the SB220 will perform well on AM.  Spares for those old green amplifiers are hard to come by now, so approach the problem with caution.
de W1ITT

[KD6VXI]

175 to 200 watts of carrier.  800 or so watts pep.

They have a ccs 600 Watt plate xformer.

Ditch the bias zener and throw together a string of 6A10 or 10A10 diodes.  You want about 50 mils per tube, key down, no drive for much better efficiency.  Depending on your mains voltage, this could be up to 15 diodes (I had 252vac at my old house). 

Ground the grids directly.  Bulldoze the factory NFB chokes on the grids.  Wide strap here is best.  This will increase the gain while lowering the chance of vhf parasitic.

I ran mine on am for 17 years this way, and sold it when I got a Barker & Williamson.

--Shane
KD6VXI

[KD1SH]

   I've run mine at 200 watts out for a couple of years now with no problems, but I wouldn't push it any further.  It's a wonderful old amp, but there are several issues involved in running it on AM.  As has been mentioned, it doesn't use forced air and glass chimneys with air flowing up through the sockets, as is required to enjoy the full 500 watts (per tube) of dissipation.  And, not only is the duty cycle with a steady carrier vastly higher than in SSB use, you're obligated to tune the amp for peak, not carrier, which means that at carrier output you'll be lucky to get 25% efficiency.
   When I run mine at 200 watts carrier, I tune it for 800 watts, which would be my peak assuming 100% modulation.  So, all the while I'm outputting that 200 watt carrier, I'm probably dissipating 600 watts or so, assuming 25% efficiency with 800 watts input to get 200 out.  With the actual effective dissipation - given the simple cooling method - being significantly less than 1000 watts, I'm about as close to the practical limit as I'd want to get.  I wouldn't even think about going full legal with it.
   But still, with a good antenna 200 watts carrier can be a formidable signal, and the tubes handle it well.  I've made a slight modification to mine, in the form of an extra - higher volume - fan mounted in "series" with the stock fan, controlled by a relay which closes when I key the rig.  Looking through the top of the cabinet, the anodes glow a comfortable dull tomato red; less glow, actually, than during a long "buzzard" transmission at 1200 PEP.

[KD1SH]

Keeping the 3-500Zs cool might be a problem.  Instead of using air system chimneys and a pressurized chassis, the SB220 uses transverse cooling.  This is a polite way to say that they use a phonograph motor and fan and blow air across the tubes.  The 3-500Z was designed to be used with air from below, and ducted up through a chimney.   The 500 watt plate dissipation rating only applies if it is properly cooled.   One of the problems that arises is that the filament (cathode) pins get hot and melt their solder, with predictable consequences.  I have "restored" a few of these tubes over the years simply by soldering the pins.
de W1ITT

  In defense of Heathkit, the boys at Benton Harbor weren't entirely oblivious to the need for cooling the underside of the sockets.  Look under the hood of an SB220/221 and you'll note that the chassis is cut away and the fan mounted such that the blades dip below the deck, blowing air across the bottoms of the sockets.  Not as good as real Eimac air-system sockets and forced air, but for what the amp is, it's quite ingenious.

[Opcom]

The other "problem" is the power transformer. It's rated low, like 1KVA. The advice to run low power is good.

[K8DI]

Thanks for the info and experiences, folks. I think I’ll let this ‘opportunity’ pass by.  I’ve been accumulating parts to build a 250 watt carrier rig, and I’ll just continue down that path.  Not much sense in having that and a 200 watt carrier amplifier.

Thanks for helping me not waste my time and money!

Ed

[KD6VXI]

The other "problem" is the power transformer. It's rated low, like 1KVA. The advice to run low power is good.

More like .6 kva.

Drake used a .8 kva in the L4b.

Some smallish xformers making serious power!

--Shane
KD6VXI

[W2JBL]

   I have two SB220's. The first one I bought near new in 1978 and I ran it at 300 watts carrier until the original Eimac tubes finally went soft a couple of years ago. A few years ago I put the Harbach power supply board and fan in it. The fan is the single best thing you can do for an SB220. Runs warm to the touch right over the tubes after a 10 minute old buzzard transmission. That amp has hundreds, maybe thousands of hours on it trouble free. Last I put new tubes in it and it runs like new. The second SB220 came to me about ten years ago and still with original tubes. Run off 220 volts both easily make about 1300 watts PEP on 75. Run at 250 watts carrier and 1 KW+ PEP the go all night. I have never beleived any of the BS I see here, and on the internet about these amps. The only time I tend to run lower power with them is on 10 meters, where the efficency is not very good.

[w4bfs]

all have expressed so i might as well .... I just had to scrap out ANoTHER piece of equipment with multiple power transformers where the lower power transformer did NOT have its own overcurrent protection (hint SB220) .... the fil xfmr was cooked and spewed out nasty looking/smelling gunk .... those xfmrs are hard tofind

please take the time to add the cheap fuse 

[KK4YY]

all have expressed so i might as well .... I just had to scrap out ANoTHER piece of equipment with multiple power transformers where the lower power transformer did NOT have its own overcurrent protection (hint SB220) .... the fil xfmr was cooked and spewed out nasty looking/smelling gunk .... those xfmrs are hard tofind

please take the time to add the cheap fuse 

I added a fuse holder with a 2 Amp fuse to my DX-100 LV transformer primary. I lost the LV xfmr 40 years ago in a very nice DX-100B (with the attendant cloud of wretched smoke).

Also, I'm now fusing the entire transmitter at 6.3Amps, which is right on the edge (considering the modifications I've made). If I mistune, or anything draws excessive current, pop goes the weasel.

I keep spare 6.3Amp weasels on-hand.

[W4DNR]

The SB-220 fan can be upgraded with a more efficient Comair/Rotron fan with a little sheet metal work on the rear.
Nothing that a AMPhone guy would be afraid of.
And if you should lose a tranny, if you don't mind boxing and shipping, A parted out 220 is probably more valuable
than a whole amp.... especially if you have a good set of knobs, meters, front panel, and case.

Don W4DNR