[Amps] Building an Amp 101

Jim Garland 4cx250b at miamioh.edu
Sat Jan 18 12:52:31 EST 2014

Hi Colin,
I agree that one shouldn't leave the unused segments of of a tank coil
completely floating, and I don't do that. As you suggest, I connect the
wiper on the bandswitch to the cold (antenna) end of the inductor so that
the switch shorts the unused segments. For esample, if operating 10m, the
15m tap is shorted by the wiper to the 160m tap at the far end of the coil.

Unfortunately, that isn't as good a fix to the flashover problem as one
might think. A flashover in a bandswitch usually occurs when one contact
arcs over to an adjacent contact, or when a contact arcs to a grounded metal
screw anchoring the wafer to the bandswitch shaft assembly. Even with
completely floating contacts, one would unlikely ever see a flashover
between non-adjacent contacts, such as the 160m-15m contacts in the example 

Sticking with this example, assume a bandswitch which shorts the 160m end of
the coil to the 15m end. Assume also the 10m inductor is about 1 uH and the
rest of the inductor is about 20uH. Thus, in effect we have a transformer
with a step-up ratio of 20:1 with a shorted secondary, At 28MHz, the
inductive reactance of the shorted 20uH inductor coil is about 3500 ohms. At
this high reactance,  there isn't a lot of current flowing in the shorted
inductance. Thus shorting the 15m-160m contacts together doesn't have a huge
impact on the voltage developed between other contacts in the switch. 

There is another problem, which can come from stray capacitance. At 28MHz,
stray capacitance of only 1-2pF can turn the shorted coil into a parallel
resonant LC circuit, and that can lead to further arcing problems. Thus,
shorting together the unused ends of the coil may be better than leaving
them completely floating, it's not a cure-all for bandswitch arcing.
Jim W8ZR

> -----Original Message-----
> From: k7fm [mailto:k7fm at teleport.com]
> Sent: Saturday, January 18, 2014 9:25 AM
> To: Jim Garland
> Subject: Re: [Amps] Building an Amp 101
> On 01/18/14 7:41 AM, Jim Garland wrote:
> > and immediately had flashover problesm because
> > of the high RF voltages induced on the floating unused inductor
> Hi Jim:
> I used a non-shorting switch in a Heath SB-220 I was repairing because
> that was all I had.  It worked fine.  I am aware of the step up voltage
> that can be created when part of the inductor is left hanging, but there
> are two different ways to connect the switch, and one way would be more
> of a problem than the other.
> If the switch is used to short out the coil, then the end will not be
> left hanging to develop high voltages.  Only the section that is shorted
> out could develop a voltage between the windings.  In essence, the plate
> always goes to one end of the coil and the output always to the other,
> and part of the coil is shorted out to go up in frequency.  Is this they
> way you had your switch configured?  If that is the case, then a voltage
> peak would likely have occurred at one of the switch contacts.
> If you just tapped the coil and left one end float, then high voltage
> would certainly occur.
> 73,  Colin  K7FM

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