[Amps] Re: 8877 dissp

[Ian White, G3SEK]

mark wrote:
>1). Does this 50% periodic filament voltage reduction stresses the 
>tube? 2). What is the recovery time from 50% to 100% caloric output of 
>the filament (for a step up voltage)? 3). What happens, if by accident, 
>HV and DRIVE is applied while the filament is still ramping up?

To answer #2, it depends on the tube and the voltage reduction you have 
applied. Fortunately you can measure the recovery time - or the warm-up 
time from cold - by measuring the heater current. The heater acts as its 
own resistance thermometer, and with a high-resolution DMM you can see 
the current change slow down until it settles at the final value.

There must be a small time lag between the heater temperature and the 
electron-emission temperature of the cathode surface, but the 
measurement should get you close.

There was a discussion of heater voltage reduction for the 4CX1500 in an 
article on a "Luxury Linear" by K5AM in QEX a few years ago. He found no 
harmful short-term effects. But since this is not a topic that the tube 
manufacturers seem to have explored, data on long-term effects may 
simply not exist.

-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek