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

[Bodo Heyl]

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
>