[AMRadio] Difference and use of Eimac TH and TL tubes

D. Chester k4kyv at charter.net
Wed Nov 14 12:19:30 EST 2007

>> What is the current feeling about the use of those old Eimac jugs? 100T,
>> 250T, 304T, etc
>> If memory serves me OK the high mu TH was mostly used as a RF tube and 
>> the
>> low mu TL as modulators. BC-610 excepted of course. What are the trade
>> offs between the 2 versions as modulators besides drive requirements?
>> Carl
>> KM1H

The "L" designation means "Low" as in amplification factor.  The "H" means 

Many tubes come in high-mu/low-mu pairs.  Some examples:

811A,  812A

203-A, 211

810, 8000

100/152/250/450TH, 100/152/250/450TL

Taylor 822, 814 (different from RCA 814)

Generally, the high mu version makes the best class-B modulator.  The  lower 
mu version is better for class-C rf service, and also better for class-A 
audio service.

So usually, a pair of 211's would be modulated by a pair of 203-A's.  A pair 
of 8000's would be modulated by a pair of 810's.  A pair of 812A's would be 
modulated by a pair of 811A's.  Of  course  the reverse could be used, since 
all those tubes have plate modulated class-C rf ratings as well as  class-B 
audio ratings.  But the most optimum performance can usually be had using 
higher mu for class B and lower for class C.

For example, a pair of 810's will work as class-C plate modulated finals. 
They are widely used as such in broadcast transmitters.  But a pair of 
8000's will operate just as well, but at about 75% of the rf driving power. 
Even zero-bias tubes like the 805, 811-A, 3-500Z, etc. may be run in class-C 
plate modulated service, but those are not the best tubes to use if 
something else is available.

One exception, the very low mu 845, is recommended by RCA only for use in 
class A or AB1 audio service.  It is not recommended for use as an rf 
amplifier.  They make excellent class-B driver tubes (if you can find one 
that the audiophools haven't already scooped up).

Don k4kyv 

More information about the AMRadio mailing list

This page last updated 16 Dec 2017.