On Aug 11, 12:52 am, Patrick Turner wrote:
flipper wrote:
In class A2, there isn't any non linear increase in Ik
to charge Ck as in the case of one tube in an AB PP amp.
Grid current occurs over a threshold, that's all, and you just need a
direct coupled
voltage follower to provide it.
Patrick Turner.
First, the threshold isn't necessarily at zero grid volts. It could,
depending on tube type and the parameters the individual tube received
as its manufacturing inheritance, be several or maybe a handful of
volts either sides of no grid bias.
Second, much more amusing:
Oh, what fun it is to design someone else's amp! Since the power
tranny is the least of your worries, you could feed the transmitter
tube plate 120-180V extra and power a direct-coupled driver by a drop
over the power tube's cathode, a circuit first suggested to me by that
excellent Australian tubie Anthony Mills. My fave 417A/5842 is good up
to 25mA and the 7044 has lower plate voltage and is as solid as a
rock. Both of these would drive 211 easy-peasy, and to get the signal
elevation when you sub 845, use a step-up IT. For parts economy,
arrange the switching so that the primary of the IT is the choke
constant current load on the 417A or 7044 plate when the 211 is in the
socket, the switch simply unconnecting the secondaries and cutting in
a direct route for the signal from the 417A or 7044 plate to the 845
grid. This direct coupling via the cathode also avoids the tricky/
expensive problem of finding high-voltage switches which is otherwise
inherent in your desire to switch 845 and 211 on the same chassis.
This unconsidered detail nearly tripped me on my "Millennium's End"
which, far less ambitiously, merely switched kilovolt tubes between
PSE and shunting the plate voltage in SE. I nearly had a heart attack
when I cruised the milspec catalogues and saw the prices for high-
voltage switches; it turned out I had a handful an old ham, chief
engineer at the airport since the days of tubes, gave me with a
truckload of other stuff.
By golly, I coulda been a production "engineer"! Why, that's how many $
$$$ parts just saved?
It is one of the curiosities of tube amps that the simple ZNFB SE DHT
amp always ends up costing more than a complicated PP amp (and causes
more headscratching too), and that actually has very little to do with
the price of the DHT, even at WE/KR prices.
Andre Jute
Visit Jute on Amps at
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