[Amps] B- questions

[Ian White GM3SEK]

Carl wrote:
>> Without the diode, a short from B+ to ground will drive the B- lead to the
>> full
>> HV negative with respect to ground. Since none of the components in the
>> cathode
>> circuit are designed to ever see this much voltage, much damage would
>> likely
>> follow. For less than a dollar, cheap protection.
>>
>> 73, Bill W6WRT
>
>I'll repeat one more time. The dinky diode doesnt prevent tube and 
>circuit damage, it shorts after the damage is done. All it provides is 
>a false sense of security and a cheap way to do metering. As seen in 
>the examples Ive had in for repair over the decades.
>

Carl's experience isn't surprising at all, because older commercial 
amplifiers contain enough horrors to keep a repairman in business for a 
very long time.

Many older amps do nothing to limit the maximum B+ surge current... and 
then we wonder why "dinky" 1A rated diodes can't handle it?

But when the amplifier has:

1. A surge limiting resistor in the B+ line (so-called "glitch 
resistor") to set a firm value on the peak current

2. BIG protection diodes that can handle that peak current without 
failing

3. Careful circuit design so the diodes really do protect the meters

4. A relay, circuit breaker or fuse to bring the whole episode safely to 
an end.

  - then tube arcs and crowbar shorts can be completely survivable 
without damage.

The amplifier needs ALL FOUR  of those features. If any one of them is 
absent, poorly  implemented or has failed, that's when the damage is 
likely to occur.

There is no magic here, and no secrets - this is all old stuff.



-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek