[Amps] Tetrodes

[Larry Carman]

Rich, that's a well know fact in the ham community. I don't understand why 
there hasn't been more articles in QST, etc about it and why the FCC hasn't
addressed this issue. I never fire up the homebrewed 3cpx5000 amp during 
bacon, eggs and beans burrito consumption. 

Fact Or Fiction?
Regards,
Larry N5BIP

-----Original Message-----
From: R; Measures [mailto:r at somis.org]
Sent: Tuesday, July 12, 2005 9:30 AM
To: Larry Carman
Cc: amps at contesting.com
Subject: Re: [Amps] Tetrodes


The operator had a bacon, eggs and beans burrito for breakfast?  
Consumption of such is known to be especially risky around RF 
amplifiers.  One theory has it that the salt used to cure bacon somehow 
makes the methane gas produced by the beans in the lower GI tract 
slightly conductive - but only in the presence of strong RF fields - 
but especially so during contests!.

cheerz, Larry

On Jul 12, 2005, at 4:57 AM, Larry Carman wrote:

> What causes the gas release that produces an internal arc?
>
> -----Original Message-----
> From: amps-bounces at contesting.com 
> [mailto:amps-bounces at contesting.com]On
> Behalf Of Ian White GM3SEK
> Sent: Tuesday, July 12, 2005 4:14 AM
> To: amps at contesting.com
> Subject: Re: [Amps] Tetrodes
>
>
>   wrote:
>>
>> On Jul 11, 2005, at 11:36 PM, Ian White GM3SEK wrote:
>>
>>>   wrote:
>>>>>>
>>>>>> ... by stopping the flow of current into the screen PS.
>>>>>>
>>>>> If the "glitch" is an arc from anode to screen, the mains fuse 
>>>>> won't
>>>>> protect the screen at all.
>>>>
>>>> A tube that arcs from the anode to the screen is already a goner, 
>>>> Ian,
>>>> so there is nothing of value to protect.
>>>
>>> That simply isn't true. Well designed power supplies can protect the
>>> tube, the anode supply and the screen supply against a wide range of
>>> faults, including arcs. Many of these faults are completely
>>> recoverable.
>>
>> What condition would make the anode arc to the screen?
>
> As many folks will remember from previous repetitions of this topic,
> reasons for anode-screen arcs include:
>
> 1. Loss of load at a phase angle that results in extremely high anode
> voltages. With certain amps, you can make this happen anytime you wish:
> remove load, apply full drive - BANG.
>
> If the RF voltage is high enough, the anode flashes over to the next
> lower-potential object within sight. For a tetrode, this is usually the
> screen, so the screen supply experiences a very large negative current
> input from the tube.
>
> For a GG triode, the RF arc will hit either the chassis if the arc is
> external, or the control grid if the arc is internal. An external arc 
> to
> chassis probably won't harm the tube, but an interna arc to the grid
> certainly can.
>
> The so-called "glitch resistor" in the B+ supply is a vital part of
> every tube amplifier, because it limits the peak current: it's simply
> Ipk=V/R. This resistor is not intended as any kind of fuse - it simply
> limits the maximum possible current for a few vital milliseconds, until
> something *else* has time to switch off. Eimac Bulletin 17 covers this
> very adequately.
>
> If the power supply provides adequate over-current protection for the
> screen and anode supplies, and adequate over-voltage protection for the
> screen bypass capacitor, then arc faults due to loss of load can be
> completely recoverable. Re-connect the load, press the RESET button and
> the amp will come back online.
>
> 2. Bugs in the tube chimney. Don't ask me how they got in there, but in
> FD situations I have sometimes found an earwig or a moth bridging the
> anode and the screen. Again there's an arc and the amp trips out, but
> this time it won't come back online until the charred corpse has been
> removed.
>
> 3. Internal arcs due to gas release. I know that Rich doesn't like to
> accept this; but I go along with the vast majority of tube 
> manufacturers
> and professional RF engineers who have seen enough evidence to know 
> that
> it happens - so we protect against it. Again, these faults are usually
> self-clearing, and the amp will come back online when the RESET button
> is pressed.
>
> 4. Any other reason the amp may choose! The most important practical
> point is to accept that amplifiers occasionally DO arc, so they need
> some protection.
>
>
>
> -- 
> 73 from Ian G/GM3SEK
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Rich Measures, 805.386.3734, AG6K, www.somis.org