[Amps] about the WARC bands of the Heathkit SB-220.

[Victor Rosenthal 4X6GP]

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 at gmail.com>
> *Sent:* 09 October 2024 10:31
> *To:* David G4FTC <g4ftc at hotmail.com>
> *Cc:* Kouichi Ueno <kueno at wine.ocn.ne.jp>; Amps reflector 
> <amps at 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 at hotmail.com 
> <mailto:g4ftc at 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 at wine.ocn.ne.jp <mailto:kueno at wine.ocn.ne.jp>>
>     Sent: 09 October 2024 00:36
>     To: David G4FTC <g4ftc at hotmail.com <mailto:g4ftc at hotmail.com>>
>     Cc: amps at contesting.com
>     <mailto:amps at contesting.com> <amps at contesting.com
>     <mailto:amps at 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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