[Amps] Why mu TSPA

[John T. M. Lyles]

Exactly, a variety of tube Mu's is a good thing for designers. 
Another application of lower to medium mu triodes is for dielectric 
and induction heating. In these applications there is a wide swing in 
load impedance, as produce is inserted into the work applicator or 
removed. I suppose that plasma excited by RF is similar, in that 
before it lights, the Z is higher, and then jumps down to very low. 
Where the tube is TPTG oscillator, you want to tolerate these wide 
fluctations in load Z, with only minor fluctuations showing up on the 
grid current. When these changes in plate loadline occur, you don't 
want the tube to fall out of oscillation or to have overcurrent and 
excess grid dissipation. Medium mu tubes are better in this respect 
than high mu as the grid voltage can very (from feedback in the 
oscillator) while the tube continues to stay in class C and not be 
driven far into positive grid current. Low Mu tubes are best for 
series pass tube regulators, as they have a low drop across the tube 
with widely varying load current. Not that many of these are built 
anymore.

73
John
K5PRO



>
>
>Message: 3
>Date: Thu, 20 Jul 2006 05:53:28 -0400
>From: "Tom W8JI" <w8ji at w8ji.com>
>Subject: Re: [Amps] Hi Mu, Low Mu
>To: "Barrie Smith" <barrie at centric.net>, <amps at contesting.com>
>Message-ID: <008701c6abe2$5f043e30$640fa8c0 at radioroom>
>Content-Type: text/plain; format=flowed; charset="iso-8859-1";
>	reply-type=original
>
>>  Is there a reason for having high and low Mu versions of
>>  the same tube?
>
>There are a number of reasons for having the choice
>available Barry.
>
>In an RF amplifier with widely varying load resistance, or
>with class C operation, the designer generally want lower
>mu. The reason is load impedance affects grid current less
>with low mu tubes, and a low mu tube can be switched harder
>into and out of conduction. This makes a better plate
>modulated stage, where power output more closely follows the
>square law of the anode voltage. They also work well in
>class AB1 modulator service as grid driven audio power
>amplifier where distortion is critical. You can have a high
>anode current on "positive" peaks without driving the grid
>positive, making the audio driver stage much easier to
>design.
>
>High mu tubes find more favor in grounded grid applications.
>High mu tubes have lower driving impedance making matching
>easier and producing more gain in GG. They also require less
>bias voltage which makes the bias design much easier and
>wastes less power in cathode bias systems.
>
>73  Tom
>