[Drake] S-meter

[Garey Barrell]

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
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