>Hello,
>I just received a tubeless, cherry looking SB200 as a first amplifier and
>have a few questions:
>
>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?
>
The extra V is likely to burn holes in the anodes. 811As only have 65w
diss.
>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.
the Q definition is Eimac's. It is not perfectly accurate but it is
close enough. The Q is actually a bit higher.
>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?
A Q of c. 2 is what gives good results.
> 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?
>
Not that I know of. C-in would be the same. Xc = c. 25 - ohms. L and
C-out are determined experimentally during a full throttle test on each
band. . Fixed values are substituted.
>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?
yes, however, Svetlanas are no longer available.
>Are the Svetlanas
>recommended or should I stick with the Taylors? I've noticed that this amp
>has been modified and does have parasitic suppression circuitry in the plate
>connections. Please, I do not want to reopen the great parasitc debate etc.
>
I don't know.
>
>4. Finally, Ive notice that one other possible mod has been made. An
>uninsulated silver looking wire has been installed in series with the
>filament choke. It is wrapped around two insulative standoffs near the
>filament choke and is about 2" long. What is this? Should it be there?
>
it is allegedly a neutralization scheme. There is no way to neutralize a
g-g amp.
cheers. Dave
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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