> 1. A friend told me I can run 811's in it with no mods. I've noticed the
> 811 max plate voltage rating is only 1500v, while the SB200 plate supply
> is about 2400v. Is this a problem for the 811's on SSB?
You'll have to watch what 572's you use, let alone 811A's. Some
572B's on the market really don't have enough breakdown voltage,
let alone 811A's.
With 2400v on the anode of an 811A, even assuming it doesn't arc
over, you'll be able to melt the tube down in a matter of seconds
even with modest drive power.
> 2. Someone loaned me a pair of 572B's to check the amp out with. We
> noticed that my transistorized exciter had trouble tuning into the SB200,
> especially on 75 meters. I've sifted through the AMPs archives, and have
> read Rich's articles on the SB220 regarding input tuning and increasing
> the Q of the input networks. In his article he states that the Q of the
> input (PI) network is Zin/Xc = 50ohms/Xc. While intuitively this seams
> plausible (since infinite Xc would cause zero Q ie: 50ohm load), I wonder
> where this definition comes from. It's not obvious to me that Q is
> calculated this way for the PI network. Can someone point me towards a
> reference on that?
Q has many meanings and definitions, many of which don't apply
to each other.
The Q Rich is using appears to be derived from the approximations
commonly used to describe tank circuits. That Q is lower than the
actual Q, which includes the entire tank.
If you take the highest impedance end of the tank, the ratio of Xc to
load (or source) has to be about 1 plus the square root of the
impedance ratio for the network to behave like a Pi.
That's because to be a pi the network has to step the impedance
down to a value at least slightly lower than the lowest impedance
at either end of the network. In other words, the network acts like
back-to-back L networks, with a center impedance lower than the
lowest impedance at the lowest Z end.
Also, for modeling purposes of the PI network and Q,
> I'd like to use Q=Fc/BW (center freq/bandwidth). In this case, what load
> impedance do I assign to the tube? (I know it changes throughout the drive
> cycle). Finally, since the SB200 has different tubes than the SB220, I
> assume that the PI networks would be different for the two. Has someone
> determined optimum values for the SB200 using 572B's?
572B's are around 140 ohms driving impedance, if I recall correctly.
That varies with the actual circuit.
> 3. I want to buy a new pair of 572B's. From what I can gather, the
> Svetlana's may have a bit higher gain than the Taylors or Cetrons. That
> may make them more prone to VHF parasitics. Is this true? Are the
> Svetlanas recommended or should I stick with the Taylors?
Svetlana's need to be watched for voltage breakdown, they also
have more idling current than other brands. It is the increased cut-
off bias that gets you into trouble. The Svetlana 572's require more
negative grid bias to cut the tube off, without it they will draw
current when some amps are on standby. Since the input and
output circuits are not loaded when the amp is on standby, and
since 572B's have poor input to output shielding inside the tube,
some amps will break into oscillation when tuned to higher HF
bands (at the frequency the tank is tuned to, NOT at VHF).
This will zap the bandswitch.
Keep these problems in mind.
73, Tom W8JI
w8ji@contesting.com
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