SB200 on AM

[KF6CE]

CW is what I would use.

For low level AM modulation don't you need to have the amp linear?

[WB2WIK]

For low level AM modulation don't you need to have the amp linear?

Yeah, you do.

I'd use "SSB" on the '200 mode switch. It will provide the 100W output with less plate current and probably run a bit more efficiently.

But on either tap, I wouldn't expect much efficiency from AM linear operation.

[K1ZJH]

For low level AM modulation don't you need to have the amp linear?

The CW position has nothing to do with the class of operation or biasing for linear operation!!! The
SB 200 and SB-220 are always run in Class B. The CW position is to meet the old FCC requirements
for 1 kW max input power! The SSB position increases the plate voltage to allow a higher PEP
rating. For AM, running the amp in SSB gives greater headroom for peak AM power, which can
run over 4 times carrier power with assymetric modulation.

Pete

[WA7PRC]

The SB 200 and SB-220 are always run in Class B.

Um, no -- they run in Class AB₂. Unless the output tank has very high Q (which those amplifiers don't), single-ended Class B would produce absolutely horrid IMD₃.

[WA7PRC]

For low level AM modulation don't you need to have the amp linear?

Whether an amplifier is linear or not has to do with loading of the plate circuit, and biasing. The SSB/CW switch doesn't affect either one. The amplifier is always biased for linear operation. Then, only the plate circuit loading is of concern -- which is why I ALWAYS use an oscilloscope to determine best loading.

[WA2CWA]

Come on guys, wake up. We're only talking about a SB-200. There is no SSB/CW switch.

Pete, wa2cwa

[VE1PEW]

Guess I should have looked at the amp before I asked if I should use CW or SSB

[K1ZJH]

Quoted WA7PRC
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Um, no -- they run in Class AB2. Unless the output tank has very high Q (which those amplifiers don't), single-ended Class B would produce absolutely horrid IMD3. vy 73,
Bryan WA7PRC


Nice try, but no cigar. Semantics. If you read page five of the SB-200 manual
you'd see where Heathkit states that the amplifier is running two 572 tubes in class B.

Granted, if they show grid current under drive, they would be then in class AB2. But if
Class B is good enough for Heath, it is good enough for me.

vy 73

Pete

[W2VW]

Peak efficiency is exactly the same on ssb or AM.

Yeah, you do.

I'd use "SSB" on the '200 mode switch. It will provide the 100W output with less plate current and probably run a bit more efficiently.

But on either tap, I wouldn't expect much efficiency from AM linear operation.

[WB2WIK]

Peak efficiency is exactly the same on ssb or AM.

If you were modulating 100% continuously, that would be helpful.

But normally, you're not. The efficiency for the carrier alone is terrible, and that's what is there all the time.

I don't have an SB-200. But with my AL-80B, for example...

Tuned up at 500W CW key down output: Input power is 928Wdc (2900V at 320mA) = 53.9% efficiency

Without re-tuning, set AM carrier level to 125W output power: Input power is 428Wdc (2950V at 145mA) = 29.2% efficiency

That's 303W being dissipated continuously, all the time the PTT is pushed. On voice peaks, it's better. But the overall heating and waste is really quite something.

A Valiant running 125W of high level modulated AM runs at >50% efficiency continuously and provides the same output power as a "1kW PEP" rated linear amplifier.