Re: [Amps] Re: Drake L7 newbie - Summary

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

rlm wrote:

[I've changed the topic order slightly here]

> > ð The aforementioned ECBS obviates the need for the relay.
> > ?
> > You could install QSK without changing the biasing method, or change 
> > the biasing without installing QSK, right?
>
> ð  A conventional bias relay is too slow to keep up with the high-speed 
> RF relays.
> However, an ECBS can be used with a slow RF relay.
Providing the changeover sequencing is correct. It's best to keep the 
tube cut off until after the RF relays have made contact, so that the 
amp has an output load before it wakes up. And the reverse when changing 
back - cut off the tube, then remove its load.

A stable amp shouldn't mind being operated with no load... but better 
safe than soldering!

> > ð But still not fast enough to keep up with modern transceivers.
> >
> >  I operate 95% CW and almost never QSK. Don't really care for the 
> > mode  although I know the relays used in QSK would last longer than 
> > the open  frame type.
>
> ð  All modern radios switch so fast that the amp needs quick relays to 
> avoid hot-switching -- even if PTT is used.

That's a very good point. Full QSK switches between RX and TX much more 
often than semi-breakin does, but there is absolutely no difference in 
the speed and sequencing requirements whenever you hit the key. And the 
only difference when coming back to RX is in the hang time.



> > ð Glitch resistors are not supposed to open. Their job is limit peak 
> > discharge current during a fault. The trouble with the stock 0.82-ohm 
> > resistor is that it does not limit current to a reasonable amount.
It's just being used as a cheap and very poor fuse.

> > Suppose you have?the worst case?fault (shorted tube?). 3000 V across 
> > a  10 Ohm resistor means 300 Amps. Not much of a current limiter. 
> > Should  the Glitch resistor be a higher value?
>
> ð  With smallish tubes, yes.  With the typical amp, there's c. 5-ohms 
> of ESR in the filter caps, so the total = 15-ohms-ohms, which limits 
> peak fault current to c. 200A, a value that 3A diodes can tolerate.

But the tube might not. For tubes up to the 1500W class, Eimac 
recommends limiting the current to 40A peak - see their Bulletin 17 at:
http://www.cpii.com/eimac/eiapps.htm

The revised edition of 'Care & Feeding' is also available for download, 
and contains a shorter summary of the same information.

In practice Eimac's 40A limit seems very conservative, as many amateur 
amps have discovered by using no glitch resistor at all... but it's 
still on the statute books.

[I'll be off e-mail now for 10 days. Listen out for GM3SEK... CUL]


--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek
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