>>> >Provided that the grid (and screen, if applicable) ...... ....... ...
(Question directed to Mr. Rauch)
>>> ..... Have you ever seen a triode Class AB1 HF amplifier?
>>You ask if anyone has ever seen AB1 triode PA's. Yes, I
>>have seen dozens of them. There is no law of circuit behavior that
>>prevents operating a triode in AB1.
>Please name a few of the tube types that were used?
He has seen "dozens" of them and yet, Mr. Rauch has not named even one of
the tube types.
- Class AB1 is a indeed a strange one since a maximum results from a
nothing---i.e., maximum peak anode current coincides with zero grid
potential. This is an unfamilar concept to many because in G-G
amplifiers, maximum grid potential corresponds with maximum anode
amps/minimum anode volts. Thus, the fundamental requirement for any tube
used in a Class AB1 amplifier is that it produces high anode current with
0v on the grid, at low anode potentials--when the cathode is nearing its
peak emissive limit. .
- For example, let's say that we want to design an amplifier with an
approx. 2A-rated tube. Some choices are: 3CX3000A7 (Mu=200), 3CX3000A1
(Mu =5), 4CX3000A, and 5CX3000A.
With Zero grid volts, at low anode volts (peak emission area):
The 3CX3000A7 develops about 0.01A
The 3CX3000A1 develops about 1.5A
The 4CX3000A, and 5CX3000A develop about 6A.
- Thus, the 3CX3000A7 is not suitable, the 3CX3000A1 is semi-suitable,
and the 4CX3000A, and 5CX3000A are suitable. ..... According to Eimac's
technical specations for the 3CX3000A1, it may be used in Class AB1
service, however, the peak grid drive needed is over 1000 volts..
Although such grid drive potentials are realistic at audio frequencies,,
IMO, this not the case at HF.
- OTOH, the typical peak grid drive needed for a 4CX3000A, and 5CX3000A
is a bit over 200 volts. Such potentials are easily obtained with a
typical transceiver, driving a bifilar step-up broadband transformer,
with a grid termination R of 200 ohms.
- [note-- From my experiences, neutralization should probably not be used
by those who want such an amplifier to be capable of oscillating in the
Has anyone else seen one or more Class AB1 triode, HF amplifiers?
R. L. Measures, 805-386-3734, AG6K
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