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RE: Push-Pull Tube Amps for Dummies: Questions 1 & 2

Posted by 6bq5 (A) on August 5, 2020 at 17:47:46

In Reply to: Push-Pull Tube Amps for Dummies: Questions 1 & 2 posted by immatthewj on July 25, 2020 at 22:45:08:

Question 1:
Bias is the current that is applied to the plate to make the pentode/tetrode operate-
each tube has a slightly different (or sometimes dramatically different) set of operating parameters.
It is generally accepted that the parameters set forth in the RCA tube Manual or the GE tube Manual are gospel - they are mostly on-line today, but a few of us have original copies...

the tube characteristics for a given tube in a given circuit is presented in both listings, and includes bias voltage and current (in milliamps).

Many designers choose to operate a given set of tubes at a certain set of parameters, but each designer chooses the operating set points for their amp(s)

You, the user, get to maintain those set-points - increase the current, you will get a bit more power, a bit less headroom (dynamic range) and a shorter lifespan of the tube(s).
Lessen the current, and you will have slightly less power, about the same (design) headroom and normal to slightly longer tube life -


2.
The amount of power that a designer/manufacturer gets from a set of tubes is - as indicated above - a direct result of the design parameters that the designer chooses to employ
There was a very well though of amp in the late '80s that used a pair of 8417 to get 60 wpc - but they were run hard and tube life was ~6-9 months, possibly a year....
Another designer made an amp that could use EL-34s, OR 6CA7, OR 6550, or KT-88 - 4 tubes per channel and would get 100 wpc w/ the EL-34/6CA7, and slightly more (~115wpc) with the 6550/KT-88.
It all depends upon the design and the execution...

Happy Listening

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