Maco 300 pumpkin face 1 driving 4 One tube socket keeps red plating new to tube amplifiers 8950 tubes

[442FlatFoot]

 

[nomadradio]

One tube socket keeps red plating​

Which one? If I had to guess, it would be the single driver tube at the rear. It appears there is no grid-bias voltage on this tube. It should have a resistor going to ground from pin 9, but if it's there, it's obscured by the fat gray electrolytic cap.

I probably have a pic of two of where the diodes go to put some bias voltage on the tube and control the excess plate current that makes it cherry up. With any luck I'll remember to look for it at work tomorrow.

73

 

[442FlatFoot]

One tube socket keeps red plating​

Which one? If I had to guess, it would be the single driver tube at the rear. It appears there is no grid-bias voltage on this tube. It should have a resistor going to ground from pin 9, but if it's there, it's obscured by the fat gray electrolytic cap.

I probably have a pic of two of where the diodes go to put some bias voltage on the tube and control the excess plate current that makes it cherry up. With any luck I'll remember to look for it at work tomorrow.

73

Actually no it’s the final section The fins on the final section are totally open there is nothing there and that’s when it starts to act like it’s getting power

 

[442FlatFoot]

One tube socket keeps red plating​

Which one? If I had to guess, it would be the single driver tube at the rear. It appears there is no grid-bias voltage on this tube. It should have a resistor going to ground from pin 9, but if it's there, it's obscured by the fat gray electrolytic cap.

I probably have a pic of two of where the diodes go to put some bias voltage on the tube and control the excess plate current that makes it cherry up. With any luck I'll remember to look for it at work tomorrow.

73

And when I stretched the coils it stopped getting red on one tube and started on the opposite side

 

[nomadradio]

stretched the coils

So the Tune control peaks twice in one turn?

73

 

[442FlatFoot]

So the Tune control peaks twice in one turn?

73

Yes I really really need some pictures of the bottom one of these boxes some good close-up pictures cause they’re supposed to be resistors on all the number nines and there’s only a resistor on the driver and one on one other one that’s it and in the final section

 

[nomadradio]

From the picture it looks for all the world as if there is no fixed bias on the final tubes. This will exaggerate the difference between the strongest and the weakest tube. In a perfect world, all four final tubes would share the load equally, each with a proper one-fourth share of the total wattage. In the real world, the four final tubes will never match perfectly. A fixed negative bias voltage on pin 5 of each tube serves to even things out a bit.

I'll look for some pics showing where the parts go to fix this issue. It will take four generic silicon rectifier diodes and two 1000uf 25-Volt capacitors. Not sure how hard those are to scare up these days. No more RatShack.

But there's always fleabay.

73

 

[nomadradio]

Once you get both the driver and final tuned circuits to peak properly, we find that the Maco 300 will get "twitchy" when tuning the driver knob on the side. This is caused by unwanted feedback from the driver tubes to the final tubes. They're placed really close to each other.

Here's our fix.

One driver tube or two, this will stabilize a twitchy amplifier.

73