[Hallicrafters] Re: When do you replace tubes?

[Ken Kaplan]

What I do is test the tubes annualy and determine their trends. If a tube hasn't gone legs 
up before its annual test, I'll find out if it is starting its downward spiral. That way I hope 
to catch it while it is spinning and crashing but not yet burning. This assumes that the 
radio is still working. Otherwise I test sooner. I'd be inclined to start getting worried 
about a tube that tests below 50% of book Gm. Maybe that number should be 57.3% but 
I like round numbers.

If I had enough tubes and time (especially time), it might be fun to measure or hear the 
difference between a tube at 100%, 75% and 50%. Gm is the ratio of a small change in 
plate current to the small change in gird voltage that produces it. Sounds like it is a 
measure of its sensitivity. Does this mean that a tube with 50% of its speced Gm which 
is used in a circuit with a designed gain of 50 will only produce a gain of 25? Or should I 
look more at u factor (amplification factor)?

73 Ken kb7rgg - every day forgetting more & more about tube theory

> On Tue, 4 Jan 2005 14:56:17 -0800 Waldo Magnuson <magnuson at mac.com>
> writes:
> > Hi,
> >     My question is when do you replace tubes in Hallicrafters 
> > receivers. 
> >   I use a Hickok 6000A tube tester which is a dynamic mutual 
> > conductance 
> > and reads out Gm.  As an example in a SX-100 receiver I'm working on 
> > I 
> > read some of the following measurements:  For a 6AU6 (book value) 
> > 2300 
> > (measured) 1475 or 64%, For a 6C4 (book) 2200 (meas.)1700 or 77%, 
> > And 
> > for a 6SC7 (book) 1000 (meas.) 350 & 200 or 35% & 20%, and so 
> > forth.
> >     My inclination is to replace any tube below 75% Gm reading of a 
> > new 
> > tube.  What are other opinions?  Thanks.
> > 73,  Skip Magnuson  W7WGM