Legal limit amps....

[74IN]

Will an SB220 do it? I see widely varying claims.

I'm looking to get a legal limit amp for under 1K$.

Kind of a tall order, but anything that gives over 1KW PEP is great.

 

[Captain Kilowatt]

A stock SB-220 is almost a legal limit amp, delivering a clean 1200-1300 watts output. You can drive the tubes harder to hit 1500 watts but why? Nobody will ever see/hear the differance and the amp will last a lot longer if run where it was designed for and that is at the 12-1300 watt level. I am talking pep ratings here and NOT AM carrier ratings. The stock power supply will never stand that kind of current demand nor will the tubes but the P/S will likely fail before the tubes in high carrier level AM service.

 

[BJ radionut]

SSB 220/1

74IN: The old SB/220 or 221 are decent amps but you will find them in various conditions. The full original condition ones are reaching 30+ years old...nothing wrong with that but they did have a few weak spots. The switches/caps/diodes just to point out a few. The original power transformers were just enough to get them through if you push them with a 100 watts drive on SSB...nice output but after 30 years might be a push.

There are many mods that some have done...upgraded transformers like a Peter Dahl and PS upgrade kits by W8CQ (Harbach Electronics)(also now owns PD) are good versions to look for.
The tubes in them can also vary....Eimac's/Amperex are the strongest built and most looked for...about $250 each retail...there are Z and ZG versions. The"G" is the graphite plates they take a pounding but require-slightly more drive than the "Z's" to reach full output PEP. The China versions show good output slightly above mentioned but do not IMO hold up as well over time if you push them to limit.

Personally right now I think most SB-220/1 are overpriced by most...10-15 years ago you could buy them for $400-500 now I've seen them for $750-$1000(no way)...I passed on one with all the upgrades at the Indy Hamfest for $575(Offered $500)...by the way he took it home...no takers at his price he was firm! So all this means is make sure what your getting...inspect her well!
Search around and see what mods have been done over time...meter/PS/xformers so you know what to look for.

One more thought Ameritron 811HD for the money NEW is a good amp...great buy for the money.
I have a Al-811 (3 tubes/Svetlana 572B's) I run her at 240vac puts about 1800v on the plates=800-900 PEP on 75mtrs about 700w on 10m with 65 watts drive from my FT-950...
I PAID $325 used...so shop around.

I am working on rebuilding a home brew 4/572B amp now (new PS and a few T/R switching changes)
She will be full limit maybe a smidgen's less when finished (PS/3200v B+/no load) and she won't sag much either...Paid $250...should not have more than $500 invested ....when done!...see now why I say an SB220 at $700 is overpriced MHO.
All the Best
BJ

 

[Captain Kilowatt]

I am working on rebuilding a home brew 4/572B amp now (new PS and a few T/R switching changes)
She will be full limit maybe a smidgen's less when finished (PS/3200v B+/no load) and she won't sag much either...

You might want to watch out for that. The 572B is speced for a max plate voltage of 2750 volts and a typical operating voltage of 2400 volts. If it were me I would push the power button ON with a nice long stick. Also make sure the P/S has a good glitch resistor installed. op:

 

[BJ radionut]

QRN/572B's

QRN: Yes your right! They do not make them(572B's) like they used to. The transformer I am going to rig up is a old mil-spec (Collins/I think) 2250vac/with full wave bridge should be right at 3100-3200vdc at 1500 milliamps+...
I am also going to have a 0/240vac at 20 amp variac on the primary side...so this will be my "Soft Start"...
This should do the trick. I most likely never put it in "War Mode"...2800vdc under load should give me close to the KW+ PEP I'll want.
I was going to build up a 4-1000a deck but I am still hunting parts for the RF deck...C1/C2 I would like to have vacuum vari's at 7.5kv...just have not found them yet at the price I am willing to pay. The transformer I have for that is 240vac pri/4160vac sec....also need a couple big caps there...OH well maybe after a few more Hamfests I find the goodies.
This 572B deck had 811a's when bought and had a 110v T/R relay set-up. I have new ceramic sockets and the C1/C2 caps are good Johnson 4kv spacing with B&W/band-switch set-up...should work out.
I am looking for a Bud box now for the RF deck/ I have one for the PS...Variac and transformer already set in place and got to finish the bridge and get that mounted.
Hve a Gud Day
All the Best
BJ

 

[Captain Kilowatt]

FB there BJ. I wasn't sure you knew about the voltage rating on them or not. FWIW most four holer 572B amps run about 24-2500 volts and deliver 12-1300 watts pep out.I was going to build such a beast but since my broadcast TX creamed the modulator section COMPLETLY from one end to the other I have decided to scrap it and build an amp.The TX used four 4-400's, two in the RF section and two in the modulator. My first step will be to build it up with a pair, and possibly four, 4-400's in the RF section and then once it is working convert it to 3-500ZG's. No sense risking new tubes on a homebrew project.It's a BIG job and it should all fit nicely into a standard Hammond 19 inch rack cabinet that is 42 inches high. I am presently sanding and painting it now.I want lots of plate dissapation because I plan to run legal limit SSB and AM and up here that is 2,250 watts pep on SSB and 750 watts of carrier on AM plus modulation which means 3,000 watts pep on AM. :

 

[HiDef]

Hey QRN,

Can you get away with running the broadcast plate xfmr into a cap input filter for a few more volts? That might put it in the ballpark for a GS-35B or 2 instead of screwing around with all those sockets and filament power.

 

[gc505]

I just picked up this Drake L4B last week for about $800 with two new 3-500z tubes. I use it primarily on 80/40/20m SSB, wired for 220v it peaks at 1.3kw on 80m SSB with a 100 watt drive and I'm seeing a 1.1kw PEP on 10-15m AM. 80/40m CW is showing over 2kw and 10m CW is at 1.7kw.

The unit to the right of the amp is the matching L4PS power supply.

 

[TonyV225]

I agree withh QRN nobody will ever miss that 200-300 watts they wont hear the difference. We were talking on 75 meters and most of us agreed that one of the biggest power differences that one can tell the difference on the recieving end is within that first 500 watts a person gains . Then its within that next 1000 watts or so.

I usually on a daily basis run my amplifiers at between 400 and 600 watts and do just fine unless ofcourse its really noisey on the band. Actually to be honest I was also looking at those Heathkit SB-220 for another amplifier to play with as of recently. I like my Ameritrons but they are pricey and for just another spare multiband amplifier I dont need to spend a whole lot when there are some good deals on nice older amplifiers out there.

 

[Captain Kilowatt]

Hey QRN,

Can you get away with running the broadcast plate xfmr into a cap input filter for a few more volts? That might put it in the ballpark for a GS-35B or 2 instead of screwing around with all those sockets and filament power.

Yeah but I like the glow of glass tubes. : Like they say, glass has class.

 

[HiDef]

Yeah but I like the glow of glass tubes. : Like they say, glass has class.

Well there's no substitute for that.

I'd still get away from 4-400s. Too small.

A pair of 4-1000s with all the plate V you can get, control grid and screen voltages bypassed for RF and cathode driven would be about right for the power level you mentioned.

If you had a way to make 5 or 6KV they would work just fine without control grid and screen voltages.

 

[Captain Kilowatt]

The thing is, I have all the parts on hand to use 4-400's or 3-500Z's.I figured I would build it up using the 4-400's that I already have and get it working first and then maybe buy some 3-500Z's. It should just be a matter of readjusting the taps on the tank circuit.Four 3-500Z's have the same plate dissapation as a pair of 4-1000's and I would need to buy a socket and filament transformer and possibly a plate transformer for the 4-1000 not to mention the 4-1000's themselves.I can make up to 9000 volts but at only about 0.5 amps CCS and it would have to be a capacitor input supply. I already have the sockets as well as the chimnies and stacks for the 4-400/3-500Z's and a bunch of tubes along with the proper filament xmfr and a plate xmfr good for 1.2 amps CCS that will give me a 3100 volt good stiff PSU using choke input.In the b'cast TX the power supply was 3200 volts unloaded and only dropped to 3100 at a tad over 1 amp load,not too shabby for a HV supply.A pair of 4-1000's would be nice but a little too much cash for me right now. The amp is going into a standard Hammond 19 inch rack cabinet 42 inches high and will be built in separate levels making it easy to later change the RF deck if I so decide.

 

[HiDef]

The thing is, I have all the parts on hand to use 4-400's or 3-500Z's.I figured I would build it up using the 4-400's that I already have and get it working first and then maybe buy some 3-500Z's. It should just be a matter of readjusting the taps on the tank circuit.Four 3-500Z's have the same plate dissapation as a pair of 4-1000's and I would need to buy a socket and filament transformer and possibly a plate transformer for the 4-1000 not to mention the 4-1000's themselves.I can make up to 9000 volts but at only about 0.5 amps CCS and it would have to be a capacitor input supply. I already have the sockets as well as the chimnies and stacks for the 4-400/3-500Z's and a bunch of tubes along with the proper filament xmfr and a plate xmfr good for 1.2 amps CCS that will give me a 3100 volt good stiff PSU using choke input.In the b'cast TX the power supply was 3200 volts unloaded and only dropped to 3100 at a tad over 1 amp load,not too shabby for a HV supply.A pair of 4-1000's would be nice but a little too much cash for me right now. The amp is going into a standard Hammond 19 inch rack cabinet 42 inches high and will be built in separate levels making it easy to later change the RF deck if I so decide.

I see.

I just can't imagine 4 surplus 4-400s in parallel all working in harmony.

Still like the idea of control grid and screen supplies instead of hooking all grids together. Separate bias pots for each tOOb could help.

You will land up pulling almost 2 amps at 3kv to make the Can A Da power level quoted earlier. Time to find a brother for the plate transformer.

You may already know but FYI 4-400 and 3-500 chimney heights are different. IIRC you can use a 4-400 chimney on a 3-500 but not the reverse. A socket mounting scheme where spacers can be used could make the taller chimneys work on the triodes and have nice airflow at the plate cap.

Hammond rack to keep it all Canadian?

Fred Hammond used to get on 75 AM with a great signal. Nice guy too.

 

[Captain Kilowatt]

Yeah, I checked and double checked the heights of my chimneys and they will fit either tube. I also have the fiberglass stacks to fit on top of the chimneys to direct airflow straight up over the anode connection and out the top of the cabinet. I'm not too worried about the current rating of the transformer,it was meant for 100% continous duty forever with it's 1.2 amp CCS rating so peaks of 2 amps ICAS should not hurt it a bit.It weighs in at 85 pounds all by itself BTW. As for the tubes, they will be in GG service and should work resonably well 20m and down which is really what I am after. Anything above that is gravy.Yes,there are better ways to do some things but I am going to use what I have on hand and if/when a problem comes up I will deal with it in whatever manner it takes to solve it be it technical changes or monetary comittment. I may even just start off with a pair of tubes and see how things go.The RF deck I have from the old b'cast TX has four sockets installed below chassis level and a ready made airflow path under the chassis.It is a piece of cake to simple block two socket holes and just run a pair for starters.I am just in the process of cleaning out the basement and moving things out to the barn and with each large storage box I dig out I find yet more parts.After I get them all together I will have a better handle on just what I will do.

 

[HiDef]

Yeah, I checked and double checked the heights of my chimneys and they will fit either tube. I also have the fiberglass stacks to fit on top of the chimneys to direct airflow straight up over the anode connection and out the top of the cabinet. I'm not too worried about the current rating of the transformer,it was meant for 100% continous duty forever with it's 1.2 amp CCS rating so peaks of 2 amps ICAS should not hurt it a bit.It weighs in at 85 pounds all by itself BTW. As for the tubes, they will be in GG service and should work resonably well 20m and down which is really what I am after. Anything above that is gravy.Yes,there are better ways to do some things but I am going to use what I have on hand and if/when a problem comes up I will deal with it in whatever manner it takes to solve it be it technical changes or monetary comittment. I may even just start off with a pair of tubes and see how things go.The RF deck I have from the old b'cast TX has four sockets installed below chassis level and a ready made airflow path under the chassis.It is a piece of cake to simple block two socket holes and just run a pair for starters.I am just in the process of cleaning out the basement and moving things out to the barn and with each large storage box I dig out I find yet more parts.After I get them all together I will have a better handle on just what I will do.

Sounds like you have it covered. See you on the airwaves.