SB220 on AM?

[WA1HZK]

OK
Todays exercise is to figure out what to do with a HK SB-220 amp that I'm supposed to mate with a Ranger. I did all the ranger mods yesterday so that's good to go. I have the SB-220 open and found I'm short a 3-500Z tube. There is one tube in there. It lights so if I can latch onto another one maybe this will come to life. I figure I can go this way or just put a Russian tube in the space & be done with it. Assuming I had two good 3-500Z's, can this amp make 300 watts? The power supply looks like it would be light for a modulator let alone a power amp? Is it worth screwing around with or just put the stock tube back in? If it was mine I would go Russian with an external power supply and new tank/bandswitch components but it's not so I'm looking for advice on the AM performance of the original setup. It's set up for 120 volts. Does 220 gain you much with this thing? I'm worried about any rig that does not require some effort to lift up on the bench.
Keith
WA1HZK

[flintstone mop]

Hi Keith
It might be a good idea to wire for 220 vac and it should do 300 watts carrier easily. I think the 2-3-550Z's in that amp was legal limit for SSB (1550 Watts P.E.P.). 300 watts carrier plus the modulation for AM would be 1200 watts P.E.P. The extra 75 watts for the AM legal limit of 375 would not be missed on the receiving end.
We hope to hear you soon on TOP band. This is antenna building season.
Fred

[K1JJ]

Keith,

Run an audio tone through and find out what pep it will cleanly do on the scope. Then simply set your carrier so that the actual voice audio peaks do a little less. A lot depends on the guy's assymetrical voice and headroom required. My voice is 110% max. Big Al had 140% peaks when we tested on the same rig.

Over here I have two linears with 3-500Z's. I usually give them a break and run them at 200W carrier max. I even remounted both sockets so they use real chimneys and good air flow.

The difference between 200W to 300W carrier is nothing (<1db) and at 200W you will see less tube heat and have the big audio headroom. (X6 audio)  It's real easy to get non-linear when kissing the max peak output and producing garbage that wud normally not be there. That last few watts near peak saturation is what does it.... not counting flat topping at all.  Hard to see this critical point on the scope. Just stay away from it and you're real clean.

T

[W2VW]

I'd think twice about running that box as a one holer. The filament voltage might be high enough without the second bottle to damage the toob over time. You can stick a 4-400 in there if you get a low profile plate cap or pop a small dent in the H.V. cover AND directly ground all the non filament leads on the 4-400 socket. I used a 4-400 next to a 3-500 in one of these when I needed that last extra 500 bux to buy a house. Cooling is the name of the game with these amplifiers on AM. The more air you run, the more R.F you can get without launching something. Watch for fatigued filament pins on the sockets. I used to get 180 watts carrier out with a mudified ultramodulated Ranger at 200% positive. Worked this setup for a few years.

[WA1HZK]

OK
I put some good 4-400's in this afternoon and seemed to get a good 200-300 Watts of AM. I did notice right away that there was not enough loading cap. I padded the dual section with 1000 pf. and I was able to properly adjust the amp for AM. This amp will only be on 80 so the tubes & padder will be OK for this setup.
Thanks
Keith

[W2VW]

That doesn't sound right. 1000pF is a lot. You loading it to 4X P.E.P.?

[W2JBL]

    i have an SB220 on AM since 1975, with few problems, until it tossed it's filament transformer last month. i did rebuild of the power supply with Harbach filter block and rectalfyer boards, but lacking the right fil xfrmr stuffed one for a single 4-400 in. works fine with one tube, but the tank circuit Q is not right. i now have the correct fil transformer, and soon to buy two new tubes and finish the job. that loading cap is only a two section job, Dave. they switch in big mica caps for addtional C on the lower bands. mine blew up years back and the 80 meter cap i use now is a .001 bathtub style. i have run this amp at 300 watts carrier for many years. keep LOTS of air on it though. back in the 70's when i put it on the air i got kind words of encouragement from a few AM "gurus" in New England: waste of power, too much heat, 30% efficient, not enough power out. funny but i'm still using it while some of those guys have been reduced to tinkering with microwave oven transistors to stay on the air...

[W2VW]

Hi Chris,

   I'd look for a bad switch before suspecting a cap that plays across 50 ohms. It's not too hard to put these on 160 too if you are there with the junkbox parts out.

[W2JBL]

the bandswitch is a weak point in that amp and i replaced mine with a bigger one ages ago. they usually go from abuse on the higher bands. mine went while i was on "10" meters...

[k4kyv]

Theoretically, with a pair of 500's in the final, you should be able to make 500 watts carrier output modulated 100%.  Half the total plate dissipation.  The only problem is, would the power supply hold up?  It would  pull 1500 watts DC input to the final @ 100% duty cycle.

[Steve - WB3HUZ]

500 watts would be pushing it, although you could probably do it if you put 4 kV on the plates. With that voltage, you can get about 2 kW out of a pair. That would be enough PEP for 500 watts of carrier and 100% modulation. But you would be beating the tubes pretty good!

[WA1HZK]

I got him running anyway. Listen for KA1EUK. When he finds his own 4-400's or 3-500's I'll take the amp back and check the band switch/loading cap & fix it. How about putting this rig on 160? It looks like there is plenty of room for the parts. Does anyone have the plan? I'll do that also when it comes back if it's not too much of a pain in the ass.
Keith
WA1HZK

[W2VW]

Check AG6K's site for his idea on conversion. I did it once myself about 15 years ago. Used #12 solid house wire for the coil. Used 2" PVC for a form. Aded this in series with the 80 metre coil. Dunno how many turns. Also padded the loading cap with a doorknob. I bypassed the tuned input. Of course you would lose 80 metres with this mod.
A far better way with commercial ham gizmo sales potential would be to make up an outboard happy box containing the L section of a 160 metre Pi-L network. This could be cut in with a HD relay only when transmitting on 160. Same idea could be carried over to the tuned input if one was needed. It depends on what you drive it with (a Ranger pi-net should match anything). The existing 80 metre tank in the linear would now act as the first part of the Pi-net and the external happy box would form the rest. No bands would be lost and you wouldn't even have to open up the SB-220 to fool with it. The only fly in the ointment would be extra voltage on the SB-220 output SO-239 because it is now working at the Pi-net's transition impedance. Maybe swap to the teflon type connector.