Thank you for the information. This is a very interesting read and leads one to
believe that many things would improve with these chokes in place, especially
in noisy settings.
As far as a transistor for the keying circuit, one might first want to
determine the parameters I had suggested earlier. What is the collector to
emitter voltage across the keying transistor when unkeyed?
What current (if any) flows through the transistor when it is unkeyed. It won’t
be much buy will likely be non-zero.
R, a 150K Ohm 2 watt resistor is in fact in place across the emitter to
collector points of the 2N3055 (Q1 as shown in the TT schematic). I am certain
its intent is to suppress just exactly the charge accumulation I mentioned
before. This amp is in reality a Cathode keyed design and would be expected to
show key clicks but for the presence of R2 and, to some extent, C1 (0.1 uF).
Without measuring, I would be hard pressed to guess what the voltage on Qi
collector would be.
2 Watts is pretty big for a 150 K resistor. In order to heat this resistor to 2
watts you would need 550 volts applied! TT had a reason for using a 2 watt
resistor here so I am guessing one might expect a few hundred volts to be
possible at this point. This would make a 2N3055 quite uncomfortable, even a
perfect one.
The MJL4302AG
<http://www.futureelectronics.com/en/Technologies/Product.aspx?ProductID=MJL4302AGONSEMICONDUCTOR3062501&IM=0>
from Future Electronics would be a nice choice. Collector to Emitter rating is
350 volts. It is NPN. Current rating is 15 amps, more than enough to handle the
Cathode current for the tubes. It is a power tab, not an SMD so installing it
should be easy (relatively). In this circuit, gain is not much of a factor
since the thing is only operating as a switch anyway.
Some of the horizontal output transistors from CRT color TV’s should also work
as they have high CE voltage ratings and sufficient currant capacity. Many are
TO-3 and virtually all are NPN.
Gary
W0DVN
> On Feb 22, 2016, at 7:07 PM, Stuart Rohre <rohre@arlut.utexas.edu> wrote:
>
> Allen,
> Look for higher collector breakdown voltages to mitigate any transients.
>
> The 2n3055 is a bit puny in the voltage breakdown dept.
>
> Pick silicon with higher PIV, and similar Beta as the 2N3055.
> I will have to look up the part nos. for what I have used in industrial
> circuits.
>
> Stuart Rohre
> K5KVH
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