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High Voltage hijinks, National NCL2000 style.

nomadradio

Analog Retentive
Apr 3, 2005
5,664
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Louisville, KY
www.nomadradio.com
High-voltage hijinks it says. The pics tell the story.

Always, always, always replace the bleeder/equalizer resistors when new filter caps are installed. This amplifier is the National brand, model NCL2000. Made from the mid-1960s to the early 70s, the two 8122 ceramic tubes can be driven to full output with around 25 Watts peak drive. An alternate hookup on the input circuit cuts the drive level to match a typical 100 Watt PEP radio. Makes it popular on 11 meters with the low-drive option installed.

The eight original 100uf filter caps were enormous axial-lead parts, with four on each side of the brown terminal board. The new replacements have both leads coming out one end, the "radial" package. The four bleeder resistors facing out when the board is installed looked okay, but for one that looked overheated. Taking it loose reveals the hidden side.

WgpqJo.jpg


That wasn't the only one. This resistor was on the opposite edge.

gWBd08.jpg


They look as if something came into contact with the hot resistor, and melted. Like black electrical tape? Maybe?

Didn't shoot pics of the new bleeder resistors. Too boring. Had more urgent things to do that day.

If you ever inspect the underside of a National NCL2000 amplifier, take the four hex-head screws loose that hold this HV terminal board in place and make sure there are no surprises on the underside out of sight.

And no, we did not install these new filter caps. What you see is what the customer brought to us.

73
 

Shockwave

Sr. Member
Sep 19, 2009
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With sockets changed and bias reset, it's not hard to get this amp to run inexpensive 4X150A / 7034 tubes. About $50 each for NOS. Even though this version of the tube has 150 watts less dissipation than the 8122, the anode radiator is the same size and there is no noticeable drop in output. It's also a good replacement for 4X250B, with equal anode dissipation. If an amp has a tuned input circuit, it may need alignment when switching to this tube. The NCL-2000 uses a resistive divider input, that is untuned. Adding a tuned input circuit can lower drive requirements down to a few watts. You'll need some resistive swamping on the control grid, or neutralization to stabilize the tetrode at these gains. Adding both are good design practice anyhow.
 

nomadradio

Analog Retentive
Apr 3, 2005
5,664
8,420
573
Louisville, KY
www.nomadradio.com
Hmm. A three-to-one stepup transformer and a 500-ohm swamping resistor like the Pride DX300 would get you a thousand Watt peaks with barefoot drive. As in single-final wimpy 'barefoot'.

Just one obstacle. The grid-to-cathode capacitance has to be "cancelled" by an inductance in parallel with the grids. In the Pride, this is right around 1 uH. Would be less with two grids in parallel. But this will kill the ham bands below 10 meters.

Naturally the lower the band frequency, the less of an issue the tubes' input capacitance becomes. Won't be a problem on 80 meters without the coil. But if that 11/10 meter coil is there, the input SWR on 80 meters will be 20 or 30 to one. No easy way to add an input-tuning section to the band selector.

Definitely room for creativity there. The 4X150 is the least sought-after version of that tube family and still cheap if you shop around.

Still want to try a 4X150 in a Maco Brute. Can't see that model is good for much other than being modded.

73
 
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