[Amps] Amplifier lifetime

[Manfred Mornhinweg]

Hi Jim, and Jim,

> Interestingly, my NCL-2000 blower is rather quiet and has never posed an 
> irritation. I'd be interested in hearing comments on that issue from 
> other owners.

I would specially like to hear from other users in 50Hz countries. The 
problem with this blower is that when running at 50Hz it's very noisy. 
Not a really loud noise, but a very irritating one, like a whistling 
whine. Several years ago I tried running the blower from 60Hz obtained 
from a signal generator into an audio amp and then a transformer, and I 
found that apart from producing a lot more airflow, it was much quieter. 
But routinely running it from such a system, or from a small pure sine 
wave inverter, is a bit of an overkill, I think.

My suspicion is that at 50Hz some resonance in the impeller blades or in 
the air around them gets excited, and that this does not happen at 60Hz.

The noise isn't the only problem at 50Hz. Lack of air flow is another. 
To keep the tubes running at a safe temperature, I run this amp at much 
less idling current than the design value. In theory this should degrade 
the IMD, but in practice it remains good enough. With the low idling 
current, I can run it at its full rated PEP in SSB without the tubes 
overheating, despite the lower air flow and the fact that I'm at some 
elevation, in thinner air.

> One design issue I've never liked with the amp is the T/R relay, which 
> uses a 6VAC coil. This rules it out for use with modern transceivers, 
> unless one uses an isolation circuit. In my station, I use my StationPro 
> controllers for my amplifiers, which can accommodate any AC or DC 
> antenna, so that isn't a problem

I don't know if the Kenwood TS-450 would qualify as a modern 
transceiver, but I use this radio with the NCL-2000. Its amp relay 
output is a relay contact, I think a reed relay. I just added an RC 
snubber in parallel, built into the connector, to avoid the big spark 
that otherwise happens on switching, and which might destroy the radio's 
relay contact after some time.

> Im sure the oem blower could be replaced with one of the myriad of blowers
> out there from ebm papst, or Dayton. 

I tried to do that. I searched suitable blowers in eBay. The intention 
was to use a brushless DC blower, and run it at full speed during TX, 
switching down to a low "silent" speed 20 seconds after returning to RX. 
  But absolutely every modern blower I could find turns in the opposite 
direction, relative to the original one! Given the shape of blower 
cases, that's a problem.

I bought one anyway, a 12V-powered model that has an impeller almost 
exactly the same size as the original, runs at 3700 rpm, and has the 
motor in the center of the impeller, so it's far more compact than the 
original. It has slightly better pressure and flow specs than the 
original running at 60Hz. Running it at 11.5V produces the same 
performance as the original, and without the whistling/whining noise. 
But I found no way to install it! It would require seriously cutting up 
the amplifier's chassis, re-locate a lot of parts, and so on. Space for 
the 12V power supply was scarce too. Or else I could have cut a hole 
into the case to fit the blower on the outside. Not nice, and not well 
suited to the kind of slide-in cabinet of this amplifier. After months 
with the amplifier resting on the shack floor partially disassembled, 
and not coming up with any good idea, I put the original blower back in, 
and that was it...

I probably won't do anything more with this amp. Instead I'm working on 
my EER MOSFET system. The MRF1K50N has already arrived, some other parts 
too, while some more parts are still underway. Preliminary experiments 
have produced outstandingly good results, beyond my wildest 
expectations: IMD3 levels of -50dB at 80% efficiency, along with the 
worst spurs (located 200kHz away on each side) at -60dB. These results 
came from a lower powered test amplifier using the EER approach I intend 
to use in the big one. Now I hope that the big one produces results not 
much worse than those!

Any tube amp looks lame compared to that, so I'm not really tempted to 
invest any more work in the NCL-2000. It's a good amp, but 
representative of the technology of its day, and it's half a century old.

Manfred

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