[Amps] AMPS

[Ian White GM3SEK]

R L Measures wrote:
>
>> The manual includes an extensive, well written
>> discussion of tube protection, how it works and how to configure
>> it.  However, since it does not agree with your pseudo science you
>> don't acknowledge it - as usual.
>
>The Q1 is a TIP-147 and is rated at 5A.  Q1 appears to carry grid 
>current. Is this true?


Q1  is part of an 'electronic zener' circuit that provides cathode bias. 
In normal operation it carries cathode current, and also <normal> levels 
of grid current. The current paths for normal anode and grid current are 
shown in Figure 1 on page 6.

In the event of high grid current caused by incorrect 
drive/tuning/loading, fast electronic protection will take the amplifier 
off-line. The grid current trip is normally set about 50% above the 
maximum normal grid current, depending on the tube of course, so it's 
only a few hundred milliamps - certainly not enough to trouble Q1.

The scenario for very high fault currents is DIFFERENT. The driving 
force for these faults is always B+. Figure 2 (page 7) shows the path of 
the return current for an arc from B+, and this current path is 
DIFFERENT from Fig 1.  It goes B+ to grid, grid to chassis, and chassis 
to the B-minus rail. If the grid stays solidly grounded, this current 
path does NOT involve the cathode or Q1.

Once again, fast electronic protection takes the amplifier off-line. I 
repeat: Q1 is a bias transistor, it is NOT part of the protection 
circuit, and it is NOT required to interrupt large currents.


But there is one scenario where the bias device (zener or power 
transistor) can have to handle very large currents. That is if the grid 
is allowed to float! In that case, you don't have a functioning control 
grid any more, you have a diode and the current path is directly from 
anode to cathode.


In conclusion, this isn't a simple topic that can be debated by a 
barrage of one-liners. It does require some considered thought.

As this will be my last posting to AMPS for several days. I hope I've 
made my points clearly enough.


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