Not sure if one type requires much more power to drive, or the drive circuit
is wrong.
The TL versions, like the 812(a) needs lots more VOLTAGE to drive
the grid.
It might just be a matter of the grid input circuit developing the
voltage....
In the arrl handbook, they have the 100 TH taking 18 watts of drive, the TL
20.
Not a big difference....
Also, anyone ever notice the arrl handbooks had a typo on the TH-TL listings
for class B service, the 100TH does 3000 volts, 215ma for 650 watts out,
the 100TL has the same 3000 volts and 215 ma for 450 watts out.
They never corrected it as far as I know....
Brett
N2DTS
----- Original Message -----
From: "D. Chester" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Saturday, February 23, 2008 9:24 PM
Subject: Re: [AMRadio] TH vs TL
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
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