On Fri, 15 May 98 10:36:20 -0800 Rich Measures <measures@vc.net> writes:
>The most often destroyed bandswitch contacts are the10m position,
>which
>is closest to the Tune C potential.
I disagree again Rich. The 10M contacts that fail over a period of years
are due strictly to undersize contacts. The SB-220 uses a switch that
barely was accepted in a 100W AM rig such as the Johnson Viking 1 or 2.
The 10M contacts failure has almost nothing to do with parasitics, gas or
wildlife across the contacts. Once overheated the contact(s) lose tension
and finally fail. You can see that by looking at them.....one side of the
wafer fails usually and if you are astute enough in the operation of your
amp you can catch it before the other, parallel side goes.
OTOH, switch failures on 40, 20 and other so called safe bands are the
predominant failures in the SB-220 series at least. Heck, look at those
AL-82 switches...they did not blow on 10M either.
One Jr engineer with virtually no tube amp experience got wound up about
Q. Since I was talking about commercial amps with 1000+ quantities in the
real world hands of us mortals, I chose to ignore again a posting that
offered no meaningful input.
For those that missed some of this I have chosen the Heath SB-220 as the
primary focus. Some drifting off subject is fine but at least stick to
something that most readers know,own and understand. I have no problem
with someone interjecting any other single or dual 3-500Z amp...but
please, no 4-1000A or tubes with handles or SS please...it just confuses
everyone.
73 Carl KM1H
>
>>Consider this: The tube has less gain at VHF/UHF, the pi-network is
>not
>matched
>>at VHF-UHF. So how does it happen?
>
>Perform the following near-resonance experiment:
>Connect 1 ohm of XC in series with1.001 ohm of XL.
>Apply a sinewave of one volt rms.
>What is the current?
>What is the potential that appears across each component?
>
>It looks to me like the current would be 1V/the net X of 0.001 ohm =
>1000A-rms,
>the potential across L would be 1000A x 1.001 ohms = 1001V-rms,
>and the potential across C would be 1 ohm x 1000A = 1000V-rms
>
>>The Pi-network does not "absorb" out of band energy as someone
>suggested.
>> If that's the case RF theory of ideal components would break down.
>
>true
>
>>Unless I am really, really missing something VHF/UHF energy will be
>>reflected at the input of the pi-net *back* into the anode circuitry.
>
>>This is how "lossless" networks "attenuate" energy.
>>
>>If VHF/UHF energy was allowed to freely roam in and out of the
>pi-net,
>>then we wouldn't get harmonic attenuation.
>>
>agreed
>
>>No one responded to my earlier post on this. Anyhow, after mulling
>this
>>over some more I have another question:
>>
>>* Just because the VHF/UHF energy is reflected from the pi-net back
>to
>>the anode, does this mean that there is no VHF/UHF voltage across any
>of
>>the components in the pi-network?
>
>My guess is that it bounces back and forth looking for a resistive
>load.
>Metal vapour arcs certainly have resistance.
>
>> I wish I had the capability to do a
>>SPICE model
>
>... apparently, so does Prince Harry.
>
>>...on this because if you build up a voltage potential, then
>>maybe a parasitic could break down the switch. But I can't prove
>this
>>one way or the other.
>>
>I doubt that Jon or Rich could prove it to anyone who has committed
>themselves to the position that parasites are incapable of doing such
>damage. However, for me, finding a damaged vhf suppressor resistor
>after
>a bandswitch arc/meltdown is something that I find to be fairly
>convincing.
>
>
>Rich...
>
>R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
>
>
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>
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