I'm redoing this Zenith 10H20 and now that I'm nearing completion I was trying out a few big (6-7") speakers I have around here to see what this thing using PP 6v6s can sound like. I don't have any really big 8 or 10" speakers.

Now on both AM and FM I have noticed that on very strong bass notes where that speaker cone really starts to move air...
I occasionally hear a crackle that seems coincident with the lowest bass notes or max movement of the cone. There also seems to be some rumbling "cone-oscillation"? is that it?.. whatever it is the bass starts to "rumble".. I know that can't be right.
I tried 3 or four similar sized cones older and newer and they all make the static-like crackle at the most bass sounds but some rumble more so than others.
The worst rumbling cone I treated with some flexible speaker cone treatment right at the outer edge folds.. and that seemed to reduce rumble quite a bit as it is more flexible now I think.

What causes the crackle? Something in the tubes? Arcing in the output transformer?
I tried changing them with some improvement but can't be positive.

I'm confused as to what exactly would cause static-like crackle, on deep bass only.

Might be that you need a speaker that can handle higher wattage output. The speaker cone sounds like it is bottoming out causing the crackle. Your radio with the two 6V6 may be putting out more wattage than your test speakers can handle.

Hi Peter, just had the same problem a couple days ago. It was a tube causing it in my case. Replaced the tube and all's well. It was the Beam Output tube. Hope this helps, George T.

:I'm redoing this Zenith 10H20 and now that I'm nearing completion I was trying out a few big (6-7") speakers I have around here to see what this thing using PP 6v6s can sound like. I don't have any really big 8 or 10" speakers.
:
:Now on both AM and FM I have noticed that on very strong bass notes where that speaker cone really starts to move air...
:I occasionally hear a crackle that seems coincident with the lowest bass notes or max movement of the cone. There also seems to be some rumbling "cone-oscillation"? is that it?.. whatever it is the bass starts to "rumble".. I know that can't be right.
:I tried 3 or four similar sized cones older and newer and they all make the static-like crackle at the most bass sounds but some rumble more so than others.
:The worst rumbling cone I treated with some flexible speaker cone treatment right at the outer edge folds.. and that seemed to reduce rumble quite a bit as it is more flexible now I think.
:
:What causes the crackle? Something in the tubes? Arcing in the output transformer?
:I tried changing them with some improvement but can't be positive.
:
:I'm confused as to what exactly would cause static-like crackle, on deep bass only.

Peter, try increasing that .002 across the opt primary to a .02 or experiment with little higher values than the .002.
Bill

okay .. I'll try those ideas.. thank you guys..