Folks,
Just after I was first licensed, I built an HF amp. with a pair of
zero-cost, QY4-400 (tetrode strapped as triode). I put a few Volt of
positive DC on the cathode to get the ZSAC to what I thought it
should be, as otherwise it took up too much space on the anode
current meter with 4kV HV. Years of abuse later, I needed those
valves for some other project, so I put in a pair of QY4-250 with no
other changes. As I recall the ZSAC was similar, also the output
Watts were about the same and, if anything, they seemed easier to
drive than the 400's.
When one of the '250 tetrodes gave up, I didn't have a spare, so I
put in a couple of TY3-250 - at last proper triodes, I thought. When
I closed the keying relay, the anode current meter went off the top
at 1 Amp and the valves lit up the shack! After adding lots more
diodes in the bias circuit, the ZSAC was tamed and the valves worked well.
I must now try to find the relevant data sheets and look up 'mu' to
satisfy my curiosity. I bet I won't find a figure for a 4-400
strapped as a triode - anyone know it?
Regards.
Mark.
-------------------------------------------------
At 15:49 19/07/2006, you wrote:
>Lower mu tubes have wider spacing between the grid wires allowing more
>electrons to slip by each second for the same bias voltage. It also reduces
>the effective control the grid voltage has over the plate current, thus
>lower mu. You can easily see this difference in grid wire spacing by
>comparing a 250TH with a 250TL. Or a 811 with a 812.
>
>73
>Bill wa4lav
>
>
>At 11:55 PM 7/18/2006 -0400, Tony King - W4ZT wrote:
> >Tom W8JI wrote:
> > > I do think I made an error in the post about mu, but I
> > > deleted my post.
> > >
> > > Higher mu in a given tube means less bias is needed to
> > > maintain the same plate current. Lower mu means more bias is
> > > needed to cut off the tube to the same current. The rest was
> > > OK.
> > >
> > > 73 Tom
> >
> >I'm still a little confused here. I thought that a higher mu tube
> >(higher gain) would require more bias to hold it to a specific current
> >(idle?) and that a lower mu tube (lower gain) would require less bias to
> >hold a specific current. Can you help me understand where I am missing
> >this?
> >
> >Thanks and 73,
> >Tony W4ZT
> >
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>
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*******************************************
Mark Hill - G4FPH
E-mail: g4fph@mjha.co.uk
Current web pages at: www.g4fph.net
Old web pages at: www.qsl.net/g4fph
----------------------------------------------------------------------
Dipoles resonant on 1942 / 3720 / 7085 kHz
Three element wire beam coming soon on 14180 kHz!
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S witch off
I solate
D ump
E arth
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