[AMPS] More on the SB220

[km1h@juno.com]

On Sat, 20 Mar 1999 12:03:51 +0000 "Ian White, G3SEK"
<G3SEK@ifwtech.demon.co.uk> writes:
>
>Carl wrote:
>>
>>
>>
>>>>
>>>>Sounds like a HV breakdown to me that is external to the tube.
>>>
>>On Fri, 19 Mar 1999 00:35:31 +0000 "Ian White, G3SEK"
>><G3SEK@ifwtech.demon.co.uk> writes:
>
>>>There could have been a silent arc inside the tube, with the bang 
>>>coming
>>>from the resistor. IMO a 2W resistor standing in the way of a 
>>>near-short
>>>across the HV supply is very likely to exit with a bang!
>>
>>
>>I take it that you are not familiar with the SB-220 circuit.  The
>>resistor in question is not in the HV path at all.
>>
>It's been years since I worked on an SB-220, so I didn't have the
>circuit handy, but have now downloaded it from Rich's web site
>(thanks!).
>
>R3 is not involved in the normal anode-cathode HV current path. But 
>if
>there's an arc from B+ to a grid or the chassis, the whole arc 
>current
>flows up through that 0.82 ohm 2W resistor (R3) to reach B-. 

Correct, but a typical arc in the 220 does not blow apart R-3. In fact
the original Heath and the metal film Harbach often show no external sign
of failure. So the BANG is not in that part of the amp.


(The 
>same
>current also goes through the 1 ohm plate meter shunt R1, but that's 
>a
>5W component.)
>
>However, you may be right after all about it being an external 
>flashover
>to chassis, Carl, because an internal anode-grid arc would probably 
>have
>taken out the grid RFC.

Exactly. When there is a true tube arc the choke either opens or the pi's
slam together.


>
>It seems like a lot of damage to resistors, meters and RFCs could be
>avoided by bridging R3 and R1 with big diodes (one across R3, and two 
>in
>series across R1, all cathodes pointing "up" towards B-) and also 
>adding
>the B+ surge limiting resistor that is used in all modern amps... 
>what
>do the SB-220 experts say? 


I always suggest replacing RFC-2 with a resistor since that VHF choke
just happens to be series resonant at the parasitic frequency.

ZD-1 is only a 10W device and replacing with a string of 1N5408's adds a
huge surge safety factor.
Back to back diodes protect the meters.

R-1 and R-2 almost never fail, R-3 and ZD-1 are the problem along with
the multimeter if it is in the Grid position at the time of the zap and
did not have the added protection diodes.

Would not diodes across the resistors change meter calibration?

That metering circuit is begging for a complete redesign. The meters BTW
are 200ua at 1400 Ohms.

73  Carl  KM1H



>
>73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
>                          'In Practice' columnist for RadCom (RSGB)
>                           http://www.ifwtech.demon.co.uk/g3sek
>
>--
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>

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