You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
The Feb 2025 Radioddity Giveaway Results are In! Click Here to see who won!

Reply to thread

Message

The good news is that this specimen was made with a grounded-grid driver tube. The tuned-input circuit that takes the radio's drive power from the input side of the relay feeds it through a yellow wire to pins 2 and 6 of the driver tube. Those are the cathode pins. The grids on the driver tube are grounded.

This setup is easier to make stable.

One end of the output coax IS grounded to the antenna socket ground. So that's not the root of your problem. Bad news is that they used the white foam-plastic insulated coax, so the center-wire insulation can soften and allow the center wire to 'creep' if it gets too hot.

One thing that has helped stabilize twitchy Maco amplifiers is a shield around the driver tube.

Here is the same trick applied to a Maco 300. Much easier for this model. Only takes a flat piece of so-called "hardware cloth". Pretty sure the wires are spaced 5/16 inch on this one.

Wrapping a shield of this stuff around your single driver tube looks a bit clumsier to do. The plate-cap clip and parasitic choke would also have to be placed away from it. A short from either of those to your grounded-metal shield would be a bad idea. It doesn't have to wrap completely around the driver tube, but simply block the direct path between it and the three final tubes that surround it.

Might make a big improvement.

73

<blockquote data-quote="nomadradio" data-source="post: 605704" data-attributes="member: 22">The good news is that this specimen was made with a grounded-grid driver tube. The tuned-input circuit that takes the radio&#039;s drive power from the input side of the relay feeds it through a yellow wire to pins 2 and 6 of the driver tube. Those are the cathode pins. The grids on the driver tube are grounded.This setup is easier to make stable.One end of the output coax IS grounded to the antenna socket ground. So that&#039;s not the root of your problem. Bad news is that they used the white foam-plastic insulated coax, so the center-wire insulation can soften and allow the center wire to &#039;creep&#039; if it gets too hot.One thing that has helped stabilize twitchy Maco amplifiers is a shield around the driver tube.Here is the same trick applied to a Maco 300. Much easier for this model. Only takes a flat piece of so-called &quot;hardware cloth&quot;. Pretty sure the wires are spaced 5/16 inch on this one.<img src="http://imageshack.com/a/img198/1624/maco300ohnewtopsidesm.jpg" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />Wrapping a shield of this stuff around your single driver tube looks a bit clumsier to do. The plate-cap clip and parasitic choke would also have to be placed away from it. A short from either of those to your grounded-metal shield would be a bad idea. It doesn&#039;t have to wrap completely around the driver tube, but simply block the direct path between it and the three final tubes that surround it.Might make a big improvement.73</blockquote>

[QUOTE="nomadradio, post: 605704, member: 22"] The good news is that this specimen was made with a grounded-grid driver tube. The tuned-input circuit that takes the radio's drive power from the input side of the relay feeds it through a yellow wire to pins 2 and 6 of the driver tube. Those are the cathode pins. The grids on the driver tube are grounded. This setup is easier to make stable. One end of the output coax IS grounded to the antenna socket ground. So that's not the root of your problem. Bad news is that they used the white foam-plastic insulated coax, so the center-wire insulation can soften and allow the center wire to 'creep' if it gets too hot. One thing that has helped stabilize twitchy Maco amplifiers is a shield around the driver tube. Here is the same trick applied to a Maco 300. Much easier for this model. Only takes a flat piece of so-called "hardware cloth". Pretty sure the wires are spaced 5/16 inch on this one. [IMG]http://imageshack.com/a/img198/1624/maco300ohnewtopsidesm.jpg[/IMG] Wrapping a shield of this stuff around your single driver tube looks a bit clumsier to do. The plate-cap clip and parasitic choke would also have to be placed away from it. A short from either of those to your grounded-metal shield would be a bad idea. It doesn't have to wrap completely around the driver tube, but simply block the direct path between it and the three final tubes that surround it. Might make a big improvement. 73 [/QUOTE]

Top Bottom