[AMRadio] TL and TH tubes

John Lyles jtml at losalamos.com
Sun Feb 24 19:39:44 EST 2008

k4kyv provided good general rules for when the hi and lo mu triodes were suited for transmitters. Low mu triodes, as class C devices, would be driven hard into grid current. Another big use of these devices was and still is in dielectric and induction heating systems. These applications often run the amplifier (or usually just a power oscillator) with widely varying loads, creating large mismatches. Instead of having 50 ohm circuits, these things are direct coupled to the load, such as having ferrous metals heated in a tank inductor, or having lossy plastics in an air tank capacitor. As such, when the load shifts from zero to full load, grid current changes greatly. A high mu triode oscillator could change grid current as much as to stop oscillation, or to seriously overdrive the grid on the tube, depending on the load. By using a low mu tube, this fluctuation is much less severe, and the grid current can remain in the 'safe' region for the tube as well as keeping the oscillator
going. Thats why tubes like 8000 were seen in diathermy machine. In very large systems (50 kW), would use 3CW30,000H1 or H3 whereas such a low mu tube would be les desirable for transmitters, as they would need a bigger driver stage plus larger grid bias voltages (for cutoff conditions). 

> Message: 2
> Date: Sat, 23 Feb 2008 20:24:37 -0600
> From: "D. Chester" <k4kyv at charter.net>
> Subject: Re: [AMRadio] TH vs TL
> To: <amradio at mailman.qth.net>

> 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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