On Wed, 02 Jun 2010 22:01:13 -0700, Roy Lewallen
wrote:

The lengthy discussion about tube plate resistance has muddled the DC
operating point (equivalent to the diode DC bias) and the plate
resistance, which is the plate's AC or dynamic resistance. They're
related just the same as for the diode. And just like the diode, if you
were to send an AC signal to the plate of a biased tube through a
capacitor, you'd find an AC voltage and current which are in phase
resulting in power being delivered to the tube, and an increase in plate
dissipation in the amount of that power.

Hi Roy,

That is a good analogy of power, non-linearity and dynamic resistance.

As for the DC operating point being muddled into the discussion, I
don't think I have seen anyone raise that separately - which is
perhaps what you mean.

For Walt, and all,

So, to take that part, and give this RF final stage a better view of
the dynamic resistance's part in source resistance, let's consider how
much power is dissipated in the plate that is strictly due to the bias
and a zero input condition.

There are few typical characteristics that match Walt's data, and I
have to abstract both from the TS520S schematics, service manual, and
the published data for the 6146B for this. As I had to juggle these
considerations to give Walt some perspective of the plate resistance
being far lower than his anticipated 5 kOhm to 14 kOhm, so I must
similarly reckon for the DC baseline.

By all things considered, we have a grid one bias of -60V. This
exceeds the RCA published data for class AB, but not for C (which is
in excess of this at -100V or more). My readings of all these
variables puts the plate current for a zero input at 20-25-30mA which
yields from something under 20W to something over 20W for a plate heat
load. Subtract this from the 53+W Keith figured and we have a ball
park 30W heat load attributable to RF. This is a back of the napkin
kind of balance sheet, mind you. I say this because the drive has a
cooling factor in that it drives down that zero input plate current
during part of a cycle. Having said that, it is enough to simply note
that there is significant heat dissipated in the plate that is
attributable to the RF current that passes through it and the load.

73's
Richard Clark, KB7QHC