Topband: SB-1000

Tom W8JI w8ji at w8ji.com
Mon Dec 21 21:34:06 EST 2015


>
> Both are N750 types. You need a #6 lug, a short #6 screw (3/16th long), 
> and
> some #16 bus wire plus a brass washer that Ameritron will have. The chokes
> come from Ameritron no matter where you buy them.
>
> Thanks Tom,Re. the lug, screw and wire, I'm assuming the capacitor I 
> ordered from them will install with the lug, screw and wire that already 
> exists? Didn't realize they had the washer also but will give them a call, 
> possibly they can include it in the order. I did get the choke from them 
> also. I also got one of F1BXL's parasitic suppressors he sells on Ebay. 
> Hopefully all this will calm things down a little. The knowledge 
> generously shared by folks like yourself, Jim and others on their web 
> pages and this forum are greatly appreciated by all. Have a good Christmas 
> and New Years to all.
> Jimk2hn


I don't know who F1BXL is,  but parasitics have nothing to do with anything. 
I would not go sticking hairpins in there. That stuff is all heebie-jeebie 
voo doo.

The 160 tab is under the most stress when the amp operates 80 meters. There 
is almost 3kV peak across that 160 plate padding terminal to the switch 
rotor.  Every amplifier that switches a plate padding cap in has highest 
peak voltage on that tab when on 80 meters.

To reduce the electric field gradient around all the pointed areas of the 
contact, a washer is used. The washer acts just like one of those 
anti-corona rings that used to be in TV sets, or that you see on HV power 
lines. It spreads the field out, and reduces the chances of the corona 
setting off an arc when you are on 80 meters.

Voltage between the switch rotor and that contact is highly dependent on how 
you set the load control, because that sets the anode impedance. This is why 
people should **always** tune an amplifier up for maximum possible power at 
full drive and then back drive off to safe power. That reduces peak voltage.

I can, for example, make the anode of a 3-500Z reach 3 or more times the dc 
supply voltage if I underload the amp. As a matter of fact if it is severely 
underloaded, the voltage increases until something someplace absorbs the 
energy.  If it is a switch contact, then the contact goes away.

People can cast all the spells they want  with magical suppressors and, if 
the PA gets grossly underloaded for the peak drive power, something will 
arc.  That is just how these class AB amplifier systems work.

Ask Ameritron to include the parts. They do not normally come with the 
capacitor. When I released the SB1000 design to Heath, the release was real 
early in the run. I think we were at the first 100 or 200 AL80A's. That 
washer, plus a buck-boost winding to the transformer, came after Heath 
kitted the unit. What you have is a very early release of the AL80A, just 
after the AL80 was dropped. The AL80A was progressively refined until it 
couldn't be refined any more in that chassis. The next major revision was 
the AL80B, which had major changes. The AL80B remains pretty much unchanged.

73 Tom 



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