[Drake] S-meter

[Richard Peterson]

Garey, and the group,Yes, I am sure it is not the mic tube! :-) That is the problem with trying to do these things from memory. But, yes, I did juggle some tubes in my TR-4 to get the S-meter to behave ... the bridge was not balanced the way Drake had intended it with a tube that someone had stuffed into the circuit. To further protect my diminished integrity, I will not try to recall which one it was!RichardWB5NEN----------------------------------------> Date: Sat, 4 Oct 2008 14:46:11 -0400> From: k4oah@mindspring.com> To: drake@mailman.qth.net> Subject: Re: [Drake] S-meter> > Richard -> > I don't understand how the Mic Amp tube (V18) could affect the S-Meter! :-)> > The tubes involved in the S-Meter bridge are V4, V11, V15 and V17. > Essentially the bridge compares the current drawn by the first three > against that drawn by the fourth one. Problems of this type are often a > result of "grid emission" or "gas" in one or more of the tubes. The > result is that the current through the tube is not controlled entirely > by the bias voltage and tends to increase as the tube heats. This will > show up in increased cathode voltages, as a result of higher than normal > current. The 12BA6 seems to be the most susceptible to this fault, with > the 6EJ7 close behind. As you say, even "new" tubes can vary > considerably, and it is often necessary to select a "good" one. The good > news is that once one is selected, it tends to last for years. A "bad" > tube will typically look ok at first, but get progressively worse after > several hours of operation.> > 73, Garey - K4OAH> Glen Allen, VA> > Drake 2-B, 4-B, C-Line & TR-4/C Service Supplement CDs> <www.k4oah.com>> > > > Richard Peterson wrote:> > That is a very critical bridge circuit associated with the meter (all bridges are tricky and critical!) and not just any tube will work. V-18 can be problematic, especially if someone replaced it. That happened to me, and to get my bridge and S-meter to behave, I had to try several tubes. I believe that is a 12AX7. That could be your problem. No guarantees, though.> >> > Richard> > WB5NEN> >> >> >> >> >> > _________________________________________________________________> > See how Windows Mobile brings your life together—at home, work, or on the go.> > http://clk.atdmt.com/MRT/go/msnnkwxp1020093182mrt/direct/01/______________________________________________________________> > Drake mailing list> > Home: http://mailman.qth.net/mailman/listinfo/drake> > Help: http://mailman.qth.net/mmfaq.html> > Post: mailto:Drake@mailman.qth.net> > ------------------------------------------------------------------------> >> >> > No virus found in this incoming message.> > Checked by AVG - http://www.avg.com > > Version: 8.0.173 / Virus Database: 270.7.5/1708 - Release Date: 10/4/2008 11:35 AM> >> > > ______________________________________________________________> Drake mailing list> Home: http://mailman.qth.net/mailman/listinfo/drake> Help: http://mailman.qth.net/mmfaq.html> Post: mailto:Drake@mailman.qth.net
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