[AMPS] SB-220 Amplifiers and Garage Sales!!

[Tom Rauch]

Are you guys playing Rich's word games again? 

Wes, the person who made the measurements, concluded there 
was no advantage to using nichrome. Rich, despite making an 
agreement to not "cook" Wes' data, did exactly that.

Rich re-worded things. 

> Rich says:
> 
> > Rp was 101-ohms for a resistance-wire suppresor vs. 166-ohms for a 
> >copper-wire suppressor.
> 
> Something puzzles me here. If the suppressor provided the whole plate
> load impedance at the parasitic frequency, then the gain would be
> reduced by only 4.3dB by lowering the parallel resistance from 160 to
> 101 ohms. This may be enough to just stop the parasitic, but doesn't
> appear to my mind to give enough margin to ensure stability.

A lot of things should puzzle you Peter:

1.) The frequency Rich selected is LOWER than the frequency 
where the amplifier actually would oscillate. This makes the 
difference larger than it really is, since Rich's suppressors lower 
the Q most at low frequencies.

2.) The suppressor Wes tested was NOT the hairpin Rich 
recommends, it was a larger inductor made exactly the same as 
the stock suppressor except with nichrome. If you actually use 
Rich's hairpin, VHF Q actually INCREASES over the stock 
suppressor when the whole system is considered.

> But in reality, the suppressor is only part of the plate load
> impedance. As a result, the change (percentage reduction) in the load
> that the plate sees is even smaller, so the gain margin between
> oscillation and stable operation becomes even less.

Bingo. You won't find that on Rich's web page.

The entire thing is nonsense. I measured the VHF anode Q of a 
amplifier with a pair of 3CX800's with a stock suppressor, and again 
with Rich's nichrome. The VHF Q increased when the Measure's 
suppressor was used.

That's because Rich's suppressor has less Rp, and when that Rp 
is placed in series with anode lead the reactance of the anode lead 
dominates the system.

Rich focuses on Rp because he can make a pathological argument 
that "less Rp is better". 

Of course anyone who understands the system also knows the 
exact opposite is true. When a lower Rp is inserted in series with a 
fixed reactance, system Q increases.

Measures' suppressors actually increase VHF Q, and decrease HF 
Q of the tank. Exactly what we DON'T want to do.

The saving grace is most amplifiers are very stable at VHF, and 
most suppressors have a large margin of safety. So using Rich's 
high-VHF-Q  hairpins won't hurt anything but ten meter 
performance.

Even though they don't improve VHF stability, he gets by with 
selling them because most amps have enough stability margin.

I'm amazed this stuff drags on year after year. But then I suppose 
PT Barnum was right.
73, Tom W8JI
W8JI@contesting.com 

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