[Amps] NCL-2000 mods

[Steve Katz]

When you say grid current, I think you mean screen (Ig2) current.   It
won't matter that the B+ drops under load because the regulator will
accommodate that.  That's why you use the regulator.  J

 

It's a good idea to derive screen voltage from the B+ source, as that
way you can never have screen voltage without also having plate voltage
- well, maybe not "never," because if you blow out the plate choke this
could still happen - and that protects the screen from instant
annihilation should the B+ fail and the screen voltage still be present.

 

Personally, I wouldn't bother with lifting the cathodes above ground.
Keep the grid current very low (it is normally close to zero) and the
amp's clean enough.  Always LOAD the amplifier heavily; you'll see
(maybe you already did see) that the LOAD control can be adjusted to
reduce both screen and grid current.  If you "lightly load" the amp,
that's not good for signal purity or for the tubes.  Tune up at MAX
power, then if you want to run less power, reduce the drive.

 

73

 

Steve WB2WIK/6

 

From: jim feldman [mailto:mtnredhed at gmail.com] 
Sent: Wednesday, January 13, 2010 1:52 PM
To: Steve Katz
Cc: amps at contesting.com
Subject: Re: [Amps] NCL-2000 mods

 

Steve, thanks for the correction.  I'm in serious "learn" mode.  After
reading up on voltage doublers, it seems the voltage output is pretty
sensitive to current loads.  I know grid current is very low with
respect to plate current, but  do you think I should be concerned that
the regulated grid voltage mod is getting it's power from the HV supply?

I found a RCA datasheet for the 8122, and it seems they recommend 20ohm
"non" inductive resistor between cathode and ground to lower 3rd order
distortion at the expense of a some output.

thanks
jim



On Wed, Jan 13, 2010 at 11:14 AM, Steve Katz <stevek at jmr.com> wrote:

The HV power supply isn't half wave, it's a full-wave voltage doubler.

100uF caps should be fine, the originals in mine were 80uF and have
never been replaced, still fine in 2010 (with different owner).

WB2WIK/6