Thanks - I can see the logic in that.
You just need to ensure that the voltage across the shunt resistor in normal
operation never exceeds, say, 0.5V, and that the diodes used can handle the
fault current.
David G4FTC
________________________________
From: Victor Rosenthal 4X6GP <k2vco.vic@gmail.com>
Sent: 09 October 2024 13:13
To: David G4FTC <g4ftc@hotmail.com>
Cc: Amps reflector <amps@contesting.com>
Subject: Re: [Amps] about the WARC bands of the Heathkit SB-220.
Well, for example, if the meter uses the common 'voltmeter' circuit --
that is, a low value shunt resistor (typically 1 ohm) in series with the
(for example) cathode circuit and a multiplier resistor in series with
the meter which allows the sensitive milliammeter to read the voltage
drop across the shunt, then diodes can be placed across the shunt -- but
not across the meter terminals.
73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco
On 09/10/2024 15:07, David G4FTC wrote:
> Hi Victor,
>
> > There are various ways to protect meters,
>
> Please enlighten me.
>
> David G4FTC
> ------------------------------------------------------------------------
> *From:* Victor Rosenthal <k2vco.vic@gmail.com>
> *Sent:* 09 October 2024 10:31
> *To:* David G4FTC <g4ftc@hotmail.com>
> *Cc:* Kouichi Ueno <kueno@wine.ocn.ne.jp>; Amps reflector
> <amps@contesting.com>
> *Subject:* Re: [Amps] about the WARC bands of the Heathkit SB-220.
> 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
> <mailto: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
> <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 <mailto:kueno@wine.ocn.ne.jp>>
> Sent: 09 October 2024 00:36
> To: David G4FTC <g4ftc@hotmail.com <mailto:g4ftc@hotmail.com>>
> Cc: amps@contesting.com
> <mailto:amps@contesting.com> <amps@contesting.com
> <mailto: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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