[AMRadio] TH vs TL

Brett Gazdzinski Brett.Gazdzinski at verizon.net
Sun Feb 24 17:15:14 EST 2008

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 

In the arrl handbook, they have the 100 TH taking 18 watts of drive, the TL 
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....


----- Original Message ----- 
From: "D. Chester" <k4kyv at charter.net>
To: <amradio at mailman.qth.net>
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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