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Dentron Clipperton L

OK fine, so then what box should be considered if swinging up to 1500 or so should be considered? Thanks.

Since it's for AM, you need to get something bigger. Say something good for 2000pep. Then run it lower for longer life so it isn't ran full bore. Components are not getting any cheaper.

Yup. Something with at least a pair of 3-500Z's in it will do as long as the power supply is up to it as well as the airflow. That rules out something like the Kenwood TL-922 however. The plate voltage was a little weak in that as well as the airflow. That's what I would do. AM carriers really take their toll on amplifiers and you really cannot simply go by pep ratings of them when using a carrier mode like AM.
 
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If you really want something to do close to 1500 -2000 watts with AM, start thinking about 3 to 4 times that output power capability. A 572 tube, unless you have around a dozen of them, will never get close. You might also start thinking about how unrealistic most power out put claims are with 'CB' amplifiers.
All of the ham amplifiers from the period when that Dentron was made measured 'input' power, not output power. In general, cut that claimed rating in half for SSB. For AM, cut in to quarters. That's really how it works.
Paul
 
If you really want something to do close to 1500 -2000 watts with AM, start thinking about 3 to 4 times that output power capability. A 572 tube, unless you have around a dozen of them, will never get close. You might also start thinking about how unrealistic most power out put claims are with 'CB' amplifiers.
All of the ham amplifiers from the period when that Dentron was made measured 'input' power, not output power. In general, cut that claimed rating in half for SSB. For AM, cut in to quarters. That's really how it works.
Paul


If he is looking for 1500-2000 watts of unmodulated carrier then I agree but I believe he is looking for that pep so as a MINIMUM he would need something that will do 400-500 watts of carrier all day long and maintain peaks to four times that. Thanks for reminding us that the old amateur amps did indeed spec their power as DC INPUT which does tend to run about half that in actual useful RF output. It scares me when I hear people talking about running their FL-2100 and trying to get "the rated 1200 watts" OUT of it. I can just hear those 572B's screaming from here. :eek:
 
The Heath SB220 does a fine job on AM if you read the manual and select "CW", or low side.

It's built for sideband, and the tubes throw a few hundred Watts of heat with no drive in "SSB", or high side. This is called "Idle" or "Zero-Signal" current, and you need that to get a clean signal on sideband.

But each tube is dumping 200 to 300 Watts before the radio's drive power is applied. The added heat from only a 300-Watt AM carrier will overheat the tubes. This is why the instruction manual says to use only low side for AM. That will bring the heat down to a safe level. AND cut your power nearly in half. But that's the manufacturer's advice.

Never have met a AM operator who would stay on "Low" side. The solution is to change the bias on the tubes. Rather than the 100 mA of idle current on each tube, better to hold it down to about 50 Watts per tube. With a 300-Watt carrier, the tubes will be tossing only about half the heat they're rated for. Leaves you the headroom you need for 1500 Watt modulation peaks.

The original 5-Volt 10-Watt zener diode sets this idle current. Taking the two wires loose from it and substituting a series string of rectifier diodes will boost the bias voltage from 5 Volts to about 20 Volts. This will nearly, but not quite cut off the tubes' idle current.

homebrewbiaszenerpz7.jpg


You can just build your own. No secret sauce to wiring thirty 3-Amp rectifier diodes in series. The good news is the voltage rating. That number tells you how many Volts of reverse (turned-off) voltage a diode can handle before it breaks down. You can use the ultra-cheap 50-Volt version, the 1N5400. The diodes will ALWAYS be forward-biased, showing about 2/3 of a Volt per diode. Reverse-voltage rating is irrelevant for diodes that will never have that reverse voltage applied to them. These won't. The regulation of the bias voltage is not quite as tight as the zener was, but plenty close enough to a 20-Volt zener diode. But with about a 20-Watt effective rating. And zeners are famously sensitive to surge damage. Rectifier diodes are a lot more robust in this department.

Or, you could just buy a commercially-made bias board that comes with brackets to mount it behind the meters.

AMBias_v5_sm.jpg


The 10-meter input circuit in the SB220 is pretty flimsy. Okay for 100-Watt peak sideband, but a problem for AM. You can't easily reach the tuning slug to re-peak for 11 meters. And the drive level people try to use on AM just roaches the stock 10-meter coil and capacitors. The easy solution most folks go for it to just cut the coax from the input side of the relay where it goes to the input side of the band selector. The coax gets run directly from the relay to the tubes' cathode circuit.

This eliminates the wimpy-coil problem, but creates a new one. Your driver is now feeding into a 3-to-1 SWR. The amplifier's input impedance with direct drive will be about 150 ohms. This reduces the power you can get into the tubes. Just the same as a high SWR reduces the power you can pump into an antenna.

Most books about grounded-grid amplifier design will mention that a tuned-input circuit works best the closer it is to the tube sockets. Heathkit chose to locate their input circuits on the other end of a 12-inch long piece of coax so they could mount it in a convenient spot. A perfectly-decent compromise, but a compromise.

1nEsOa.jpg


This simple setup does require a couple of one-terminal tie strips like the one already found on the inboard tube socket. This setup has a 68pf disc cap to ground from where the coax feeds from the relay, into a 7-turn coil wound on a half-inch form. The other end of the coil connects directly to on side of the tubes' cathodes. A 47pf disc cap goes to ground from there, also. The coil gets fine-tuned by squeezing or stretching the turns for lowest input-side SWR with carrier drive. Should get it down to 1.3 or better.

To make room for this mess, the original fat .01uf disc cap C32 gets moved. It was strung from the single tie point where the coax feeds the tubes, to the tube cathodes.

It gets mounted on the other 1-lug tie strip, between the relay and the input coax.


hISakR.jpg



So long as you use disc caps rated for 2000 Volts or (better) more, this input-matching circuit will take whatever you throw at it. Won't make the amplifier one bit bigger, but just moves the weak link in the chain elsewhere.

This post has already gotten way out of hand. There are other measures to adapt the SB220 to high-power AM, but those are the highlights.

Maybe it's time for a more-detailed "how-to" post adapting the SB220 for heavy-duty AM service.

But it can be done. Just don't expect the factory-stock SSB-oriented setup to withstand 11-meter AM service. Things will break.

73
 
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That's great Nomadradio!! It all makes perfect sense. I guess looking for an SB-220 all done up with those mods will be hard to find unless someone is doing it. Thans for the insight.
 
How does the SB220 power supply hold up in AM service? Most Heath amps had a barely suitable power supply for SSB service. A lot of die hard AMers replaced the plate transformer and mounted a beefier transformer external to the main chassis.
 
The stock transformer tends to die of old age whether you use it for AM or SSB. Hammering it too hard on AM would probably speed up the failure. I recommend no more than 300 Watts AM carrier from this box after the bias is modified. So long as you're not too long-winded it won't work a lot harder than it does on sideband.

73
 
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If you're going to modify an SB-220, the first thing is to beef up the power supply, it's barely adequate as is. You can (or used to) start with a separate chassis and transformer (P.Dahl!). Some got really ambitious and added two more tubes in that empty part of the original chassis. Not for the faint of heart (or cheap).
Have fun...
 
For Sale/Trade: HB 4-1000A 80-10 amp
A really nice BIG home-brew single tube 4-1000A 10-80m amp for sale or trade for ??. It includes all interconnect cables, 4-8KV (variable!)/2A DC slave power supply, motor-driven Variac, a free SPARE socket, and schematics. On the bench, 30-50w drive easily produced 1500W+ out! Power supply is built commercial grade with custom built power transformer and large (8kV)electrolytic caps. This will be the last amp you need. Blow away your competition - not for the QRP minded. This is a physically large amp with 300CFM blower. Photos on request (only really serious inquiries/really, no tire kickers). All for $2250 firm. You could part out all of the components and make a big $$ or two. Pick-up in Cincinnati, OH.

This may fit his needs:rolleyes::LOL:
 
BJ,
A 4-1000 type amplifier is very nice, I do agree with you. (There's always a BUT though, right?) Here's the 'but'. How easy can you find a decent 4-1000 tube anymore? I haven't looked so have no idea.
Paul
 
Well yes getting scarce...but still around...I got a New Eimac/4-1000a and new Thompson 4-1100(1000a on steroids for Broadcast use in Europe) for $300 for pair maybe 2-3 years ago...You can still find old BC pulls for about $250 or less...NIB Eimacs are like $5-600...but I have seen good ones at Hamfest for like $150-200...
With demand also sagging prices have come down...
Now 3-1000z is a whole other animal $$$$...Glad I have kept those (have 2 good pulls) One good one in an old BTI RF-2000, I restored (still doing Legal Limit++ at 80w drive) and a NIB Eimac...I know I could get a pretty PENNY for that one!:love:
AND NO FELLOWS...it's not for sale:love::D
All the Best

Gary
 
If he is looking for 1500-2000 watts of unmodulated carrier then I agree but I believe he is looking for that pep so as a MINIMUM he would need something that will do 400-500 watts of carrier all day long and maintain peaks to four times that. Thanks for reminding us that the old amateur amps did indeed spec their power as DC INPUT which does tend to run about half that in actual useful RF output. It scares me when I hear people talking about running their FL-2100 and trying to get "the rated 1200 watts" OUT of it. I can just hear those 572B's screaming from here. :eek:

I knew a guy that had an SB-200, and he told me he was getting 1200 watts out on AM. Yes, AM. I tried not to laugh in front of him. Then I thought... 1200 watts, maybe... but for how long? :)

73,
Brett
 
Sounds like 1200 watts output is really stretching it on AM. Its not unheard of to get 900 watts pep SSB out of an SB-200 as a mono bander, but 1200 AM is pushing it on AM. With enough drive it might but like you say, for how long.
 

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