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2x 4cx300a note


Well-Known Member
Mar 6, 2020
New York
I’ve had the bottom sealed up for a few days now so it’s starting to look reliable, fingers crossed. Marking the Load cap and Roller settings for 10 - 15 - 40 & 80 so there’s less tuning guesswork (picture). Little more than half a turn of Roller causes serious decrease in output from 15-80. 10 meters (28.3 - 29) however is very broad-banded. I think it’s that fat coil, IDK. I’d rather have more 80 than higher 10 or even 6 (where I’ve never been).

The Load cap & switch combo is working well. The Tune cap is a 2 section affair where only 80 needs both sections. I need to figure out some way to switch in/out the 2nd section for 80. Tune voltage is high. I tried to learn about vacuum caps but could not understand how they connect although the mounting seems straight forward.

Generally, I was surprised at the small range of capacitance needed for 10-80 compared to say an SB200. I expected I’d need larger air caps as can be seen from my early pics. Looking ridiculous in retrospect, lol.

During operation it’s plain that PO is limited by plate current but I’m happy with the output I’m seeing. The Plate transformer comes from a Heathkit Part # 54-30 rated 1800v @ 400mA feeding FWB (Nomad board) but I see steady peaks around 500mA with 6x 330uF, 450v caps. Voltage however, sags to anywhere from 1900-2000.

Have some sturdy black mesh from an old speaker that happens to be the right size for a cover and I like the idea of being able to see through it. Hopefully, it will stay operational while I fashion some angle or channel brackets for a cover.

The tubes heat fast on transmit. Never had amp with a Blower before. The speed control is a triac board so there’s little more than Low - Med - Noisy/vibrating/fast. The 110vac motor runs smoothly though. Have 3 small DC fans inside chassis to move air across boards & relays & out the other side. All results so far have been into a DL but antenna time is coming soon.
Band Marking.jpeg
Band Marking.jpeg

Just keep those silver-plated wires on the roller coil clean. That, and the faces of the roller plates. Arcs are bad juju for a roller inductor.

The plate resistance of a tetrode is higher than for a triode. As a result a bigger coil will be needed to step down the plate impedance to 50 ohms. Bigger coil means you don't need as much capacitor to resonate a particular band.

Rock on, Neil. Lookin' good!

I'm beginning to PO with antennas. 20M remains difficult to find & Tune, almost impossible and when do-able it beats the snot out of the tubes. It's my next problem to solve. Maybe the plate choke (it lost a few turns over the years. 10 & 15 will tune with less fuss and 40 is a can't miss.

I don't have an 80m antenna I can use without an ATU. 80 is easy like 40 into a DL though. Removing the ATU is like a new-found freedom for me, I simply never considered removing it. Took me decades to figure that out, lol.

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