[Moon-net] Fw: Lifetime of GS35B, your experience? WA6PY comment..

[Bodo Heyl]

Hello,

here another very interesting mail...

73 Bodo/DL2FCN





----- Original Message ----- 
From: "Paul Chominski" <pchominski at maxlinear.com>
To: <bodoheyl at gmx.de>
Cc: <pchomins at san.rr.com>
Sent: Tuesday, May 08, 2007 6:53 PM
Subject: Lifetime of GS35B, your experience?


Hi Bodo,

I would summarize following reasons for shorten life time:

1. Working outside max ratings: Vf, Va, Ik, Ig Pa. Especially
overdriving tube creating high Ig and flash over inside the tube due to
excessive Va

2. Lack of protection inhibiting driving tube without Va and limiting
Ig_max

3. Insufficient cooling - overheating. For most of power coaxial tube
important is cooling grid and cathode. Many cavity designs use finger
stock, and important is to provide adequate cooling to the grid(s)
structure. Removing some fingers from finger stocks will increase
thermal conductivity. This is OK if there is air blow cooling this area.
Please remember that brass is poor thermal and electrical conductor. For
critical components am using only aluminum and copper.

4. Self-oscillations on other frequencies which dissipate inside the
tube, UHF oscillations can cause local overheating heating of internal
structure.
Neutralization is usually designed to work on frequency of operation,
bad can create more problems on other frequencies. Neutralization can
decrease Ig if the reason id RF feedback, otherwise will not have
influence on life time.

5. Frequent switching filament/heater on and off - I keep my power tubes
all the time on during entire activity weekend

I never use tube outside max ratings. I always use protection against
lost of voltages and excessive currents. I lost one TH328 only once when
I came to San Diego and did not check that I have 250V AC and heather
voltage was much too high. ALL my tubes work for years without failure.
PA with TH327 which I used first for 70 cm and now for years on 23 cm
show on the time meter over 1000 hours during last 13 years. My YL1050
in 70 cm PA works from 1985 and during some periods of time was very
heavy used. GU84B in 2 m PA works from 6 years without any degradation.

Good example of well designed cavities is those manufactured by Hans
Plish.

I hope this helps.
GL 73 Paul WA6PY

----- Original Message ----- 
From: "Bodo Heyl" <bodoheyl at gmx.de>
To: "moonbounceboard" <moon-net at list-serv.davidv.net>
Sent: Monday, May 07, 2007 9:15 AM
Subject: [Moon-net] Fw: Lifetime of GS35B, your experience?


> Here the comments of G4FUF,
> very interesting!
>
> 73 de Bodo/DL2FCN
>
>
> ----- Original Message ----- 
> From: "Keith Naylor" <keith.naylor at micronet-uk.com>
> To: "Bodo Heyl" <bodoheyl at gmx.de>
> Sent: Monday, May 07, 2007 12:03 PM
> Subject: Re: Lifetime of GS35B, your experience?
>
>
>> Bodo,
>>
>> In over 20 years of professionally conducting "post mortems" on
>> commercial amplifiers
>> the root cause is invariably operating the tube outside of the
>> manufacturers ratings.
>> Unless there is some other obvious reason in the PA tank or load then
in
>> 99% of cases it is
>> usually inadequate airflow.
>> Even for large manufacturers i.e Marconi, Thomson etc , employing an
>> airflow dynamics specialist
>> has been a problem. These guys command high salaries and usually work
>> for F1 or motor sport  teams.......
>> Of the remaining 1% of failures its totally incorrect heater voltage,
>> usually DC heaters with a fault in the PS
>> resulting in terminal emission....
>>
>> For ham amps I do not know a single amp that has adequate airflow,
>> despite claims of key down
>> conditions, the tubes will eventually fail if you do this each
>> day......thats if the power transformer does not go first......
>>
>> The worse thing you can do is cycling the heaters/filaments. Once a
amp
>> is switched on its best left
>> running all day.
>>
>> There are so many GS35b tubes around. Many have been subject to
>> mechanical damage by dropping.
>> I had 4 suspect tubes xray-ed and you can see the grid is disturbed
>> inside. Even small 3-4G drops will
>> damage this tube. In these cases an arcover is then possible
especially
>> up at 4kV+ , usually you can see
>> this condition onset by very high grid current at low drive.
>> Poor emission seems to never be a problem unless you get the heater
>> voltage badly wrong.
>>
>> GS35b -> Rule 1, if it has even the slightest dent/ding in the anode
>> cooler, look out for trouble.....
>>
>> 73'
>>
>> Keith G4FUF
>>
>
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