Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on October 18, 2012, 07:47:17 am

Title: MARSHALL P.A. S/A 50W sag resistors ?
Post by: Colas LeGrippa on October 18, 2012, 07:47:17 am

Hi,

I had this beauty on my bench, the circuit seems to be the same as the picture but for the 3 ( light blue/white ) resistors ( caps ? ) . I am wondering if these are sag resistors ?
I couldn't see no traces of removal, in the one I had.
At the same time, does anybody have the schem of this very good sounding amp? It is a MARSHALL S/A xxxxA  P.A. model  ( 1970 make ).

Thanx

Colas

http://www.amparchives.com/album/Marshall/50%20Watt%20Series%201966-1981/1985%20PA%2050W/70%201985%20JMP%20PA%2050W%20SN%20SAxxxxA/slides/mars%201970%204.html (http://www.amparchives.com/album/Marshall/50%20Watt%20Series%201966-1981/1985%20PA%2050W/70%201985%20JMP%20PA%2050W%20SN%20SAxxxxA/slides/mars%201970%204.html)

Title: Re: MARSHALL P.A. S/A 50W sag resistors ?
Post by: TubeGeek on October 18, 2012, 08:35:22 am

The blue and white parts are 0.22uf capacitors.  I have a Marshall master pa amp that pre-dates yours and has those caps too.

Title: Re: MARSHALL P.A. S/A 50W sag resistors ?
Post by: Colas LeGrippa on October 18, 2012, 10:20:21 am

mine is 1970 and doesn't have these caps, what are they for ?

thanx

Title: Re: MARSHALL P.A. S/A 50W sag resistors ?
Post by: Willabe on October 18, 2012, 10:37:19 am

For cleaning up the SS diode switching noise. The caps short the hi freq. hash to ground. Those caps used with ultra fast diodes gives a very quite SS rectifier B+ PSU.

Marshall used them in a number of there amps.

KOC is fond of this in his TUT books. Merlin writes of them in his book too.


             Brad     :icon_biggrin:

Title: Re: MARSHALL P.A. S/A 50W sag resistors ?
Post by: TubeGeek on October 18, 2012, 10:46:34 am

Brad beat me to it!

Title: Re: MARSHALL P.A. S/A 50W sag resistors ?
Post by: Colas LeGrippa on October 19, 2012, 08:32:28 am

I see 2 caps from a/c HV to gnd, no prob about that,  but how's the third one in the amp hooked up to ?

Colas

Title: Re: MARSHALL P.A. S/A 50W sag resistors ?
Post by: Willabe on October 19, 2012, 08:43:33 am

Hi Alvin,     :laugh:

3'rd cap is in parallel with the reservoir cap to ground.


                 Brad        :icon_biggrin:

Title: Re: MARSHALL P.A. S/A 50W sag resistors ?
Post by: Colas LeGrippa on October 19, 2012, 10:41:09 am

same value, .22 uF ?

Alvin

Title: Re: MARSHALL P.A. S/A 50W sag resistors ?
Post by: Willabe on October 19, 2012, 10:42:44 am

Yes, Alvin.
           

                Brad       :m2   

Title: Re: MARSHALL P.A. S/A 50W sag resistors ?
Post by: Colas LeGrippa on October 20, 2012, 04:38:09 pm

Thanx, buddy.

Title: Re: MARSHALL P.A. S/A 50W sag resistors ?
Post by: Willabe on October 20, 2012, 07:43:13 pm

 :wink: