----- Original Message -----
From: "Manfred Mornhinweg" <mmornhin@gmx.net>
To: <amps@contesting.com>
Sent: Saturday, March 14, 2009 7:51 PM
Subject: Re: [Amps] (no subject)
> Frank,
>
> The NCL-2000 indeed works quite hot. The reasons: It is designed for a
> very high idling current, which results in excellent IMD but lots of
> heat; and it doesn't have chimneys on the tubes, so that the hot air
> only slowly leaves the cabinet, and heats up almost the entire top
> cover.
> It's normal, and it will live a long happy hot life.
Ummm, Frank wasnt talking about idling current heat. Just standby.
>
> Carl,
>
>> You can also operate SSB in the CW position.
>
> Yes, you can, of course, but in the CW position this amplifier (like
> most) is biased in class C, even if only slightly so, and transmitting
> SSB in the CW position results in some audible distortion and an
> enormous amount of splatter. At least mine does.
Not even close, it runs AB2 in both positions and IMD is very close on
the SA in either position. Splatter is due to excessive drive and no
ALC.
>
> I had a particular problem with the high bias of this amp: I live in a
> country where the line frequency is 50Hz rather than 60, and so the
> blower runs slower. The manual tells to de-rate the amp in this
> situation. The fact is, the high bias almost eats up the FULL
> dissipation capability of the tubes, under the reduced air flow at
> 50Hz!
At 200ma idle dissipation is 500W for 800W of available tube
dissipation. There have never been any reports of breached tube seals
that I am aware of from excessive heat. I dont know the air reduction
formula for 50 Hz however the original xfmr is also not really
comfortable at a full 2kw input on 50 Hz either.
Why not adapt a 50 Hz fan motor to the blower? Ripley and others used to
have them available.
>
> Rather than building a 60Hz supply for the blower, I reduced the idle
> current. At 50mA still nobody reports audible distortion, and the
> splatter is no worse than that of other amps. At least not high enough
> for anyone to report it. So that's what I use. At reduced idling
> current, I can transmit SSB at full power without cooking the tubes.
>
> The data sheet for the 8122 tube recommends that very high idling
> current, for best linerity. National Radio engineers simply designed
> the
> amp according to that data sheet, I guess.
We didnt have much of a choice as any tube warranty was predicated on
going by RCA's rules. PS filter C, unregulated screen voltages and
series R were all on the spec sheet much to the consternation of
Engineering. In reality, National was taken to the cleaners by RCA.
The original concept was for 4CX250R's at 1KW and when RCA got into the
act pushing 2KW Eimac was ready to build a triode for us for GG. Think
an early 8874 if it had happened. The 4CX250 prototype still exists and
I have 3 of the 8122 versioins, one has been on 6M since 1964.
>
>> Member of NCL-2000 Design Team
>
> Oh, then you could give us more insider info about this! It would be
> interesting to know if really the amp was based on the data sheet
> recommendation , and nothing else, or what other facts influenced the
> decision to use this high idle current, which is quite unusual in ham
> amps!
See above.
High idle current is normal in most tetrodes, its a fact of life with
the AB classes. Look at the Ruskie 4CX800's or go all the way back to
the 1950's 4CX1000A which Alpha is going with now.
Its also a big reason the QRO boys have dropped the cheap 4CX5000A pulls
so fast once the YC-156 came along or moved up to the 4CX1000D. At 7500V
it idled at 3800W for 10KW PEP! Plus the ~560W filament. The 4CX10000D
idles at the same power but delivers 16KW on SSB simply by using a
bigger anode cooler and 4CX5000 internals.
>
> I'm quite happy with this amp, except for the LOUD noise of the
> blower!
> I modified the blower case to reduce turbulence, installed sound
> absorbing mats, and got some improvement, but it's STILL loud! After
> an
> hour or two using it, I start perceiving the blower noise more or less
> like a jet aircraft!
I always wear headphones so no amp bothers me, even the 2500W
dissipation GE military tetrode on 432 where I swear that tube is ready
to launch out of the modified Collins cavity.
Carl
KM1H
>
> Manfred.
>
> ========================
> Visit my hobby homepage!
> http://ludens.cl
> ========================
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