[Amps] AMPS

[R L Measures]

On Jul 22, 2006, at 1:29 AM, Ian White GM3SEK wrote:

> 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.

If the anode potential fell to a very low level during parasitic  
regeneration, electrons from the 3, 8874s' cathodes would mostly flow  
into their grids, causing a >18A burst of grid-current which would  
quite likely cause the 5A-rated Q1 to C-E short.  .
>
> In the event of high grid current caused by incorrect
> drive/tuning/loading, fast electronic protection will take the  
> amplifier
> off-line.

I agree, and the circuit will adequately take care of blunderous  
tuning/loading, however, the sticky wicket is an intermittent VHF  
parasitic oscillation. because during one there's essentially no-load  
on the anode, the anode potential drops to near zero, a large number  
electrons begin to flow into the grid, and bad things can happen.

> 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.

All grid current flows through the cathode in all cases.
>
> 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.

Even during a VHF parasitic?
>
>
> 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.

I do not agree.  Perhaps an interested  third party will  
experimentally float a grid and report back to us. However IME, when  
a grid fusing R blows, nothing notable takes place except that the  
input SWR increases and the RF out drops like a rock.
>
>
> 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 I see it, the presumption that a conductive path from anode to  
grid would suddenly appear is clearly not an example.
>
> As this will be my last posting to AMPS for several days. I hope I've
> made my points clearly enough.

cheers, Ian
>
>
> -- 
> 73 from Ian GM3SEK
> http://www.ifwtech.co.uk/g3sek
>
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R L MEASURES, AG6K. 805-386-3734
r at somis.org