[ARC5] Back to basics: Capacitor breakdown voltages

[Richard Knoppow]

     Any text from the time AM was prevalent should give you information 
on the voltages appearing at various points in the circuits.  My 
understanding is that high voltages are confined to the parts seeing 
modulated RF and not to the power supply.  It is these high modulated 
voltages that can cause arc over in tuning caps.  My BC-375 did not have 
the original power supply but rather a conventional supply using mercury 
vapor tubes. It had conventional filter caps. I did sometimes use it for 
AM. Its been in storage for decades now and there are some details I 
don't remember but I never had arc over problems.

On 1/28/2016 11:12 AM, Dennis DuVall wrote:
> True, but the capacitors at issue here are upstream from that point. 
>  The radio here is a BC-375 and the apparent function of the
> three-section HV capacitor under discussion was to suppress dynamotor 
> hash.
>
> Three section unit here, one section bypasses the HV feed to the RF 
> circuits, a second section bypasses the HV feed to the modulation
> transformer and a third section bypasses the HV feed to the speech 
> amplifier and is connected at the low tension end end of a
> voltage dropping resistor string.   The failed section here was the 
> one bypassing the HV feed to the RF section.  Jim reports that
> the radio appears to function normally with this section cut out of 
> the circuit (no surprise) and the set is likely to work OK with
> the other two capacitor sections removed as well considering the fact 
> that he is using an AC, well filtered high voltage power supply
> with the radio.  Back on a dynamotor, results TBD……
>
> Dennis D.  W7QHO
> Glendale, CA
>
> *******************
>
>> On Jan 24, 2016, at 11:33 PM, spr at earthlink.net 
>> <mailto:spr at earthlink.net> wrote:
>>
>> From: Dennis Monticelli
>> Sent: Jan 24, 2016 11:28 PM
>> To: "WA5CAB at cs.com <mailto:WA5CAB at cs.com>"
>> Cc: ARC-5 Maillist
>> Subject: Re: [ARC5] Back to basics: Capacitor breakdown voltages
>>
>> Let's not forget that the voltage at the plate of the final can 
>> exceed twice the HV rail under certain load load conditions (i.e. 
>> SWR).  This is true whether the rail is steady (CW) or time varying 
>> (AM peaks).  This is one the big reasons why arcing sometimes occurs 
>> in tubes, var caps, and switches.
>>
>> Dennis AE6C
>
>
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL

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