Electravox BR60 Speaker Cone "PUMPING"

[TIMBO]

Hi guys, This is more of a fix than it needing to be fixed.
I had a two amps that had this cone PUMPING caused by the oscillator, I was not sure it was a problem.
A google search says it could damage the speaker.
I modded a BR60 amp and added another channel from another called the 408r That had tremolo and reverb.
All went well except this cone pumping that was quite severe. I ask the local guys and on fix and no good result.





I was tweaking the Marshall Popular and stumbled on a fix...

I added the voltage divider and .01 cap to help tame some of the noise created by the tone/volume.
This worked pretty good, but I was still thinking the 560p silver mica was letting some of the high frequency noise through, so I changed it to another .01 and this started the cone pumping, changed back to 560p and no pumping.
So adding a 300p silver mica between the 4700p and pin2 of the phase inverter has stop the pumping with no ill effect on the trem itself.
If someone can supply a reason why this helped that would be great.
 

[shooter]

So adding a 300p silver mica between the 4700p and pin2 of the phase inverter

like this pic?

a small cap plate to cathode, plate to B+ is a "normal" cheat to kill parasitic oscillations, most of the time I've used them is when gain is too high, beyond that you gotta get to design and I start to galze over with lots of big words n heavy math 

[sluckey]

So adding a 300p silver mica between the 4700p and pin2 of the phase inverter

Huh! What 4700p?

[shooter]

that's why I circled the 47p
I figure down under just assumes 0's 

[sluckey]

that's why I circled the 47p
I figure down under just assumes 0's 

Ah, OK. Got it. So he's adding a 3p to the 47p?   

[TIMBO]

No like this

[pdf64]

Likely to lose some bass if that input coupling cap to the phase splitter is that small. Certainly reduce the 4700pF some, eg typical BF value of 1nF.
Also reduce the 0.1uF coupling caps to power tube grids, eg 0.022uF.