"4CX250B" wrote:
>
>Paul, I don't believe there is any reason to treat a B- return as if
>it were a B+ lead. With your planned diode clamps, B- will always be
>within a few volts of chassis ground.
Exactly. Whoever recommended SHV connectors for the B-minus return of a
normal amateur amplifier is showing a serious lack of understanding [1].
>Running a separate B- cable,
>with its own connector, is potentially a dangerous situation. In some
>circumstances, unplugging the B- connector could raise the entire RF
>deck up to the B+ potential.
Also correct. That is the fatal flaw in using those black, single-pole
Millen HV connectors for the B-minus return. As "4CX250B" says, they put
the user one step away from a situation that can be lethal. The worst
thing of all is, such a connector isn't even needed [1].
This is a perfect example of a so-called "safety feature" that
introduces a new danger of its own.
>In my opinion, it is far better to run
>the B- wire in the same cable with the other control wires, so that if
>the main control connector is unplugged everything safely shuts down.
>
Over the past several years I have been recommending 5-pin XLR
connectors between the amplifier controller (assumed to be part of the
RF deck) and a remote HV PSU. The aim is to keep the HV-minus, control
and ground connections all together, so that the HV PSU cannot be
enabled separately without that connector plugged in.
This goes together with using SHV connector in the B+ line - but
**ONLY** in the B+!
Details are here:
<http://ifwtech.co.uk/g3sek/in-prac/best-of.htm#0901>
Click on "January" to download a schematic in the first of three
articles from Radcom magazine (RSGB). You can also download a set of
Powerpoint slides from a talk about HV Safety - there's no sound, but
the slides highlight the important points that are explained in the
written articles.
[1] And finally... yes, there are a very few amateur amplifiers that do
have the B-minus return at a high negative voltage. These include
tetrodes with a chassis-grounded screen, and Traveling Wave Tubes. But
unless you actually own one of those amplifiers, you can forget I ever
said that.
The main message still stands: anyone recommending SHV connectors in the
B-minus return of an ordinary amateur amplifier does not understand what
he's doing.
73 from Ian GM3SEK
>-----Original Message-----
>From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of MU
>4CX250B
>Sent: 12 March 2014 23:20
>To: Paul Christensen
>Cc: amps@contesting.com
>Subject: Re: [Amps] SHV Connectors and B-
>
>Paul, I don't believe there is any reason to treat a B- return as if
>it were a B+ lead. With your planned diode clamps, B- will always be
>within a few volts of chassis ground. Running a separate B- cable,
>with its own connector, is potentially a dangerous situation. In some
>circumstances, unplugging the B- connector could raise the entire RF
>deck up to the B+ potential. In my opinion, it is far better to run
>the B- wire in the same cable with the other control wires, so that if
>the main control connector is unplugged everything safely shuts down.
>
>Sent from my iPhone
>
>> On Mar 12, 2014, at 4:55 PM, Paul Christensen <w9ac@arrl.net> wrote:
>>
>> At the risk of starting a raging philosophical war, I am looking for
opinions
>on how to best manage wiring between an outboard HV supply and an RF
>amp deck.
>>
>> Here's the project:
>>
>> An old Alpha 77D is being used as the foundation of a quasi-homebrew
>amp. The plan is to upgrade the PS and move the HV transformer and oil
cap
>out of the RF cabinet and into its own enclosure -- similar to what
K2VCO
>and W8ZR have done. I am doing this for two reasons: (1) I can no
longer
>manage the weight of a 77Dx amp by myself. It already takes a
meditation
>exercise to move my older Alpha amps from the ground and onto the desk;
>and (2) Ep will be slightly elevated with better regulation under full
load. A
>new 2.0A CCS Dahl plate transformer will be purchased from Hammond.
>The existing 25uF/4KV oil cap gets upgraded to 35 uF/5KV. A dedicated
>filament transformer will be mounted in the RF deck area once occupied
by
>the old Hypersil HV/Filament transformer.
>>
>> A new PS cabinet is being fabricated and will house the HV
transformer,
>filter cap, diode stack, and perhaps a bleeder pair. I would like to
use a SHV
>connector for HV, but after reading the AMPS archives, I'm seeing
widely
>varying opinions on how to best manage B- I've seen everything from
just
>running it like a control conductor at one extreme to using a dedicated
SHV
>connector and treating it like HV at the other extreme. A 2-3 diode
string
>will be used between B- and the chassis as it is now.
>>
>> The new amp is getting a complete facelift with a new engraved front
>panel, round 250-degree meters, and a vacuum variable cap will be used
for
>loading. Dedicated metering for Ep, Ip, Ig, and filament voltage. The
Pi-L
>section will be be upgraded but remain mostly the same. Bias is
redesigned
>using a 3-stage bias system with the entire bias and T/R function being
>software-controlled from my S-QSK board. A matching PEP analog
>wattmeter with Bird ThruLine section has been designed and can be used
>either above the amp or remoted to a small desktop enclosure.
>>
>> http://72.52.250.47/images/77D_Panel.pdf
>>
>> As I move forward with this project, opinions regarding current
trends in
>SHV/B- distribution are very welcome. Tnx!
>>
>> Paul, W9AC
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