|[AMRadio] Difference and use of Eimac TH and TL tubes|
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
>> low mu TL as modulators. BC-610 excepted of course. What are the trade
>> offs between the 2 versions as modulators besides drive requirements?
The "L" designation means "Low" as in amplification factor. The "H" means
Many tubes come in high-mu/low-mu pairs. Some examples:
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
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).
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