Hi Carl,
> Ummm, Frank wasnt talking about idling current heat. Just standby.
Ouch! My mistake, sorry...
> Not even close, it runs AB2 in both positions
Mine doesn't... And didn't. As I got it, it idled at 200mA in SSB, and
no idle current in CW. In CW mode it required a drive above a few watts
to START delivering ANY power. In my books, that's class C. Now I don't
know if this is by design, or because any components drifted, or because
of operating it on 220V, 50Hz. But it's as it was when I got it. And
then I reduced the SSB idle current to 50mA, by adjusting the bias
voltage, and this moved the CW operating point even deeper into class C.
It works very well for me in this configuration, but definitely it's
class C in CW mode.
> and IMD is very close on the SA in either position.
Again, with mine it isn't! Clearly mine must be biased a little
different than what you are accustomed to. It has an additional
transformer (apparently an autotransformer) for 220V operation, which
seems to be used for the bias voltages and the fan. This is not on the
schematic that came with it, so that must be some adaptation, which
could have changed the operating points. I never investigated this further.
> Splatter is due to excessive drive and no ALC.
Splatter will indeed show up in that situation, but it will ALSO show up
if you use a class C amp on SSB.
> At 200ma idle dissipation is 500W for 800W of available tube
> dissipation.
Yes. And 500W is about all the tubes are rated to handle, with the
airflow this blower delivers at 50Hz.
> There have never been any reports of breached tube seals
> that I am aware of from excessive heat.
That's good to know. I was a bit scared about this. I had the bad idea
to ask an industrial tube supplier for a price on pair of spare 8122
tubes, and took three days to recover from the shock! ;-)
> I dont know the air reduction formula for 50 Hz
I measured the effective air flow through the tubes, and checked the
8122 data sheet. It turns out that at the airflow I effectively get with
50Hz, the tubes can work at about 250 watts each, before RCA would no
longer stand behind their seals.
It's a lot of reduction.
> however the original xfmr is also not really
> comfortable at a full 2kw input on 50 Hz either.
Yes, in the SSB position it's really somewhat saturated. That's very
noticeable because the cabinet vibrates and hums loudly, from the stray
magnetic field! At the CW position, instead, the transformer is
comfortable, and nothing hums. I usually switch off the HV during
prolongued receiving, but at times I have had the amp on line for
several hours, with perhaps 30% transmission over that time, and nothing
has blown up so far.
> Why not adapt a 50 Hz fan motor to the blower? Ripley and others used to
> have them available.
I don't think that would help at all! 50Hz squirrel cage motors, by
nature, run slower than 60Hz ones. I guess the original motor is
supposed to run at about 3300rpm at 60Hz, and at 50Hz it will do perhaps
2700rpm. A different 50Hz motor will not be much different. I would need
an universal motor (with brushes) to get more than 3000 rpm at 50Hz.
The better option is using a larger blower rotor, possibly with the
existing motor. But that doesn't fit weel enough in the blowe rcase, and
a larger blower doesn't fit in the amp. I would have to install it
externally. Maybe I do it someday, but it's likelt that I will rather
use the NCL-2000 as it is, until finally building my very own solid
state amp.
Or perhaps start playing with the two 4CX1500B which I was given some
years ago. But I'm hesitant to keep building stuff based on components
developed half a century ago! A solid state amp instead would be
instant-on, almost zero power consumption during standby, and the fan
would need to run only during extended transmission. These three
advantages are powerful for me!
> We didnt have much of a choice as any tube warranty was predicated on
> going by RCA's rules.
Ah, I see. So it was a bureaucratic limitation!
> High idle current is normal in most tetrodes,
But why? Looking at old 100W transceivers with 6146 or TV sweep tubes,
they all (to the best of my knowledge) use low idle current. And an 813
amp I saw also does. Is that because these are pentodes? If so, why does
a tetrode need higher idle current?
I was born in 1965, so please excuse me if I don't know all the ins and
outs of tubes!
> I always wear headphones so no amp bothers me,
I find them uncomfortable. The first moment they are great, but after an
hour at most, I really want to throw them in the corner! And often I
operate digital modes, turning the volume down and listening to music
while I operate. Amplifier blower noise is very unwelcome in that situation!
Whatever, at this moment my amp is out of service. My only electrical
supply is a water turbine, and now is the driest time of the year here,
and the water is enough to generate barely 500 watts! When the rains
come, I will be back to QRO.
Manfred.
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