[AMRadio] TH vs TL

[D. Chester]

The Eimac designation TH and TL mean Triode High mu and Triode Low mu.

Generally, the high mu versions of transmitting triodes work better as 
class-B modulators (or rf linears), while the TL's work better in class-C. 
But either version have specifications listed for either service.  But I 
would  recommend the adjusting the circuit for correct grid bias and grid 
drive current for each type of  tube, not just swapping out the tubes with 
no changes in parameters.  The low mu's run lower grid current and higher 
bias, while the corresponding high mu's  run higher grid current with lower 
bias voltage.  See the manufacturers' data sheets, or if not available, see 
the tube charts in the ARRL or West Coast  Handbooks.

There are other similar sibling sets of RCA tubes; the 810 is the higher mu 
version; the 8000 the low mu.  203A higher mu, 211 low mu.  811(A) high mu, 
812(A) lower mu.

I have used both 810's and 8000's in the rf final of my transmitter.  The 
lower mu 8000's require about 75% as much driving power for class C plate 
modulated service as the 810's.  The HF-300's that I am now have class-C 
operating parameters very similar to 8000's.  All I had to change was the 
mechanical plate connections; the grid bias, grid current and grid leak 
resistor work equally well with each tube, and re-neutralise.  With 810's I 
had to use a lower value of grid leak resistor plus re-neutralise.

Don k4kyv