Marshall PI fizz cap

[12AX7]

 Is there a particular reason why some marshalls have the 47pf fizz cap before the coupling caps to the power tubes(in other words, across the plates) and others after the caps?

[Willabe]

Because Marshall copied the Fender tweed bassmen amps (5F6-A) and they had them.    

I'm not sure if it makes a difference if their before or after the power tube coupling caps. Marshall might have been experimenting with their position?

Fender probable used them for layout/lead dress issues they were having on some but not all the amps that came off the line.

They were bass amps so cutting a slight amount of very high end was not a problem.


                Brad      

[The_Gaz]

You can go up in value before you hear any audible change, especially with clean tones. Even up to 500p won't really effect clean tones. The cap also increases stability with NFB. It does not really matter which side the cap is on, however I think the frequency response is slightly different - I remember asking a similar question once.

[loogie]

I'm under the impression that the cap is there to prevent high frequency (dog) oscillation that is occasionally present.

[12AX7]

It just hit me why they probably put them after the coupling caps....they can use low voltage caps there. So probably a cost thing. As to tone, i don't know why but on this amp it makes a big difference in tone. Removing it makes tha  amp a lot brighter. And i've used several caps there in the same range, the last being two 100pf in series for 50pf. I swear it's not at all a subtle difference. Maybe my preamp is just so bright with all the brightening tweaks i've done. But whatever it is it's quite obvious. I prefer the brightness without it, but theres something happening thats not quite right and i bet it's oscillation as was mentioned. It just seems a bit harsh in a way i can't describe. Maybe harsh isn't even the right word, but at times it seems the treble can go back and fourth from moment to moment being good one moment and seeming too bright and in your face the next. Thats not a great description but i can't find the words to explain it. In any case it'll be going back in, tho i will miss the cut it has without it. I'll try the post coupler spot and see if that works better.

[tubenit]

As to tone, i don't know why but on this amp it makes a big difference in tone. Removing it makes tha  amp a lot brighter.

I find the snubber cap to be somewhat a tone killer to my ears.  You might try an "enhance cap" which goes across the LTPI entrance plate resistor.  Not only did I find it did NOT kill tone and high end,  ........  it seems to help with some of the blooming effect on the Tweed BluezMeister and D'Mars OD Special as well as making the high end frequencies smoother, IMO & experience.  I have this on all 3 of my amps now with similar positive results.

http://www.el34world.com/Forum/index.php?topic=12723.0

With respect, Tubenit

[12AX7]

I've done that and in fact just did today. Not sure it sounds any better but i'm trying 4 different scenarios including that one back and fourth till i start to notice one that stands out.