Back to back diodes across a meter will not protect, for example, a meter
with a 1 ma movement. The forward voltage drop of about half a volt is
enough to destroy it.
I speak from personal experience.
There are various ways to protect meters, but this is one that doesn't work.
Victor 4X6GP
On Wed, Oct 9, 2024, 09:32 David G4FTC <g4ftc@hotmail.com> wrote:
> Hi Kouichi
> I consider the SB220 amplifier to be one of the best workhorses available.
> But it does require some respect to ensure satisfactory performance.
> Metering is basic but more than adequate. Above all, it doesn't have any
> microprocessor based monitoring systems which cause the amplifier to shut
> down and only provide the operator with an incomprehensible fault code!
> As I noted before, the amplifier doesn't require any modifications to
> operate on the WARC bands.
> However I have incorporated the following to my SB220s:
>
> *
> Replaced electrolytic capacitors – the SB220s are now about 50 years old
> and electrolytics of that age are likely to be suspect.
> *
> Added a 50 ohm wirewound resistor to the HT anode feed to limit current in
> the event of a flash-over.
> *
> Added back to back diodes across the meters to help prevent damage to the
> meters due to excessive current during a flash-over.
> *
> Added a softkeying module. The keying voltage as the phono socket is +110V
> DC which could cause damage to a transceiver's keying output interface.
> Mine is home designed/homebrewed but commercial units are available, such
> as https://harbachelectronics.com/wp-content/uploads/2014/07/SK-220PIC.pdf
>
> The only fault which I can remember happened with one of the amps and that
> was one of the HT rectifier diodes failed. I replaced the whole string with
> 1N5408 diodes.
> 73
> David
>
>
> ________________________________
> From: Kouichi Ueno <kueno@wine.ocn.ne.jp>
> Sent: 09 October 2024 00:36
> To: David G4FTC <g4ftc@hotmail.com>
> Cc: amps@contesting.com <amps@contesting.com>
> Subject: Re: [Amps] about the WARC bands of the Heathkit SB-220.
>
> Hi David ,
>
> I have summarized your valuable advice
>
> about the WARC bands of the Heathkit SB-220.
> 1) On 12m, set the band switch to 10m
> 2) On 17m, set the band switch to 15m
> 3) On 30m, set the band switch to 20m
>
> How to operate the Load and Plates
> 1)Set the load control to maximum capacitance.
> 2)Apply about 10 watts of drive.
> 3)Rotate the tune control to find a dip in anode current or a peak in
> power output on the external power meter.
> 4)Adjust the load control for maximum power output.
> 5)Gradually increase the drive while adjusting the tune and load
> controls for maximum power output at each drive level.
> 6)Repeat the procedure up to 100 watts of drive
>
> Thank you
> Kouichi Ueno Ji1FGX/DU9
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>
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