Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on January 26, 2014, 12:49:39 pm

Title: Amnesia - Which is the purpose of the small capacitors between plates of PI ?
Post by: kagliostro on January 26, 2014, 12:49:39 pm

In some amp (SLO - OD3 ....) you can see a small capacitor between the plates of the PI

Which is the purpose of this cap, is it there to avoid self oscillations ?

I'm not able to remember this thing with enough security

K

Title: Re: Amnesy - Which is the purpose of the small capacitors between plates of PI ?
Post by: Jack_Hester on January 26, 2014, 02:12:30 pm

I saw it first in the '59 Bassman schematic (47mmf, as they put it), and I figured that it was to cancel higher frequencies.  Someone with more knowledge will speak up and give the better explanation. 

Jack

Title: Re: Amnesy - Which is the purpose of the small capacitors between plates of PI ?
Post by: Ed_Chambley on January 26, 2014, 02:32:49 pm

I have seen anywhere between 47 and 100pf.  Jack is correct that it cancels frequencies higher than needed for guitar.  Removes oscillations.  I have changed values between these and removed it as well without hearing any difference.

I just put it there "just in-case" if I am using a LTPI.

Title: Re: Amnesy - Which is the purpose of the small capacitors between plates of PI ?
Post by: Willabe on January 26, 2014, 02:41:03 pm

Quote from: Ed_Chambley on January 26, 2014, 02:32:49 pm

Removes oscillations. 

Yep.

Title: Re: Amnesy - Which is the purpose of the small capacitors between plates of PI ?
Post by: tubenit on January 26, 2014, 03:40:23 pm

My experience is that those snubbing caps really kill the liveliness on the particular amps that I've tried them on.  Never bonded with using them. I only have used them successfully on one amp.

I do like the "enhance cap" and on the specific amps I've used it on, it seemed to improve the tone including the highs making them smoother.

I do think perhaps the specific amp can make a difference of what method of "killing oscillation or high end hash" seems to be best, IMO?

with respect, Tubenit

Title: Re: Amnesy - Which is the purpose of the small capacitors between plates of PI ?
Post by: kagliostro on January 27, 2014, 12:16:23 am

Many thanks to All

K

Title: Re: Amnesy - Which is the purpose of the small capacitors between plates of PI ?
Post by: PRR on January 27, 2014, 02:08:27 am

> Amnesy

I think you want the word "amnesia".

Amnesty (http://en.wikipedia.org/wiki/Amnesty) is when the King used to be angry at you, but then agrees to forget it. After the US war of 1861-1865, president Jackson granted amnesty to rebel soldiers; Treason was still illegal but their treason would be forgotten. Many libraries have an amnesty week where people can return late library books and they will not be charged a fine. From the Greek ἀμνηστία amnestia.

Amnesia (http://en.wikipedia.org/wiki/Amnesia) is loss of memory. Usually after brain injury. From Greek ἀμνησία, "ἀ" meaning "without", "μνησία" memory.

Yes they are practically the same word, but it came to English in twisted ways.

Title: Re: Amnesia - Which is the purpose of small capacitors between plates of PI ?
Post by: kagliostro on January 27, 2014, 02:33:39 am

Thanks PRR

as you say I was meaning amnesia

english and italian are wrote in the same way amnesia

I wrongly wrote amnesy

K

Title: Re: Amnesia - Which is the purpose of the small capacitors between plates of PI ?
Post by: kagliostro on January 28, 2014, 04:12:14 pm

Ciao Drgonzonm

You can correct the title of each your post

so I corrected my first post

each post that come after this (as a replay) keeps the last (corrected) title

if you give a look you can say that the replies before the post of PRR

has still the wrong title

Franco