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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.
Ordered a Keying buffer/interface from associate radio on ebay. No way around this unless I make one. Keying ain't what it used to be.
Been tweaking here and there.
I had to relocate the Soviet vacuum relay. Linkage kept jamming. It is the B1B-1B model which was the cheapest I could find at $46 w/ shipping. The one I got -I'm not condemning an entire product line- needs to be mounted upright. I tried it horizontal and upside down, same issue. I ended up placing it top side where hopefully it will not jam again. At least I can see and access it without taking everything part. Eventually I'll add a flap of metal to increase the shielding from Tank section.
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Had to delete the words about the Keying Interface. There will be more about this. I can say that the only thing inside of it is a small 12v Omron relay Q6K-2F-Y and nothing else.

Low plate current has limited amp output but yesterday I got lucky on eBay. Found another 54-30 plate transformer (1800v CT @ 400mA from a Heathkit DX-100, $60 with shipping).

My intent is to connect in parallel. Still thinking through it like must the primaries be directly connected? Now that I wrote it out, it's obvious, they should be fed together to ensure balance/parity/whatever. It's still only 4 wires total!
I have enough metal to place HV connectors on side without weaking chassis next to HVPS FWB board. Amp will still run on only 1 plate transformer but the outboard transformer could be plugged-in to jacks that go to FWB AC inputs. Jacks must seal when not in use for safety. I found Millens way to expensive for what they are. BNC's are rated to 3kv and a lot cheaper. 1800vac 2 lines, 110vac on the other pair, proper wire.

NY City


  • Amp Interface.JPG
    Amp Interface.JPG
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Settled on SHV connectors. $16 for two pair, panel mount females, new from China. USA-made prices 3 times as high which I can't justify on a homebrew experiment, at best. Rated to 5kv more than double the VAC used. Damn keying circuit driving me nuts, I'm considering a CB type COR to trigger amp. A good board designed by a real Tech would end my PTT agony. :censored:
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We also have a relay that installs in a CB or "10-meter" radio to key an external amplifier.

My advice is that the higher the drive level, the more incentive for the relay in the radio. The higher the drive level, the bigger the arc you draw across the receive-side contacts when they open. A drive level under 30 Watts PEP won't cause trouble any time soon, but once you get much past 100 Watts this becomes an issue. The old-time CB ops that insisted on a foot pedal would be back every year or so to get the amplifier's relay replaced. Complaint of "relay sticking" was caused by burned contacts that failed to restore the receive-side connection. Naturally those guys were using a drive level well over 100 Watts.

The carrier-sense method should be no problem at the drive level this tube can tolerate.

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