Cathode resistor value on 120 Mk. 4

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Cathode resistor value on 120 Mk. 4

Postby SMF on Mon Jul 09, 2001 8:24 pm

Hi everybody!<P> Does anyone know the correct value for the cathode resistors on the SC L120 head? The schematics I bought through e-bay were blurry and I cannot read the values. It looks like it may be 68 or 27 ohms (yes the print is that bad). The amp has obviously been worked on and has 75 ohm, 5 watt cement wirewound resistors from the cathode to ground on all the power tubes. Does a higher value reduce the gain of the stage? My SMF does not have any cathode resistors but the amp is very similar in the power section. <P> On another thread someone stated SCs have active EQ stages. This true for some amps but not all. My '74 SMF has passive tone controls whereas my '75 L120 is active, very active. A turn of 1 number on the bass or mid controls yields an amazing amount of boost or cut. It seems like the presence control doesn't do anything so I'll have to fix that.<P> I enjoy reading others' comments on the board and hope to work with y'all on various problems, #1 quieting these things down.<P>Dan
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Re: Cathode resistor value on 120 Mk. 4

Postby hcmaxson on Mon Jul 09, 2001 8:43 pm

My schematics are 11x17 and relatively clear. Are you referring to the resistors going into the power tubes? It seems that on mine the values of tubes 6,7,and 8 the top three on the schem are 68 ohms. The values for tubes 9, 10 and 11 are 27 ohms. That seems weird to me, but that's what's written on the schem. I'd have to check with my tech to be sure.
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Howard
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Re: Cathode resistor value on 120 Mk. 4

Postby SMF on Mon Jul 09, 2001 8:57 pm

Okay, I'm not going blind. It seems very strange that the two halves would have different values though. Maybe we are looking at the same original that has been Xeroxed 400 times. Does anyone have an L120 open out there?<P>Signed,<BR>Curious and yellow
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Re: Cathode resistor value on 120 Mk. 4

Postby chrism on Mon Jul 09, 2001 9:24 pm

Damn, both those values seem high for such an amp. I mean without bypassing there is maybe 1/4 amp per tube over 68 ohms = 17 volts. 4 1/4 watts per tube lost in the resistor. As for the assymetry, it is also in the LB50 plus schematic in the Aspen Pittman book.<P>chris m
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