Grid coupler change?

[Mike Watterson]

The cap was an unlikely kind to be leaky.

Maybe try cleaning the board more, but desolder socket first.

[Danndad]

The valve base has been removed and you can clearly see the board has been affected by the heat.
   
   
   

[Mike Watterson]

Try cutting away that part of board and then solder in a socket with tags using stiff wire with sleeves to suitable component pads?

Nothing to loose and shouldn't take long. Scoring and snapping off will work if you have no "Dremell" style tool.

[Murphyv310]

A work around would be to lower the grid resistor or just add a 220k between G1 and ground, audio level may drop slightly as might bass response but changing the grid coupling cap to a 0.02uf would restore the bass.

[Murphyv310]

Hi.
Also increasing the cathode resistor value helps. You need to measure the cathode voltage and subtract the grid volts, this with tell you the grid volts WRT cathode that should be a negative value in the order of 5 to 7v, also if you measure the cathode volts and calculate the current by ohms law you'll know if the valve is over driven.

PS I've seen boards far worse than that and the G1 volts are not excessive.

Also why not lift the track to the grid away from the PCB and just use insulated wire to make the connection away from the overheated section

[Mike Watterson]

Hi.
Also increasing the cathode resistor value helps. You need to measure the cathode voltage and subtract the grid volts, this with tell you the grid volts WRT cathode that should be a negative value in the order of 5 to 7v, also if you measure the cathode volts and calculate the current by ohms law you'll know if the valve is over driven.

PS I've seen boards far worse than that and the G1 volts are not excessive.

Also why not lift the track to the grid away from the PCB and just use insulated wire to make the connection away from the overheated section

Better than my suggestion.

And drill out the PCB at the grid pin of socket so grid is only wired to cap and resistor via insulated wire.

[Danndad]

Thanks for the replies and suggestions.
I have to think about this long term as the more I use this the worse its going to get, I actually like the radio.
My initial thought was to drill a hole where the original valve base was and drop a ceramic base with tags on through the hole.
This would be then lifted off the board a bit by a couple of small standoffs then secured by 2 small nut and bolts,all traces to the new holder would be removed and the connections would be made with insulated wire.
I will have to think about what I'm going to do with it.
Danndad.

[Mike Watterson]

I think only the grid connection on PCB needs drilled out. Then use original socket. Cut away all grid PCB track to coupler cap and leak resistor and wire those direct to now isolated pin.

The other base connections are not an issue.

[Murphyv310]

If its a plastic base they too can add to the issue.
If you have a ceramic base replace it for good measure.

[Danndad]

Following some of the advice given here, all the traces and pads have been removed in the blackened part of the board that go to the grid. The grid cap and leak resistor connections have been made point to point and raised off the board and the plastic base has been replaced with a ceramic 1.
After monitoring the voltage on the grid for around 3hrs the voltage is around 08.3mv on the dmm, loads better but I'm still not seeing a negative voltage on the grid??
I've tried different UL84s but the voltage is still round about the same.