Um die Frage richtig beantworten zu können musst Du auch noch angeben WO der 39k hängt . . .
Die Antwort (für Marshalls, aber nichts anderes ist der SLO ja endstufenmässig) ist hier:
"Negative Feedback Circuit
Negative feedback is an out of phase signal fed back into to the amp to cancel out certain frequencies, designed to bring the amp out front more in the mix. The negative feedback circuit consists of the presence control potentiometer, feedback resistor and where the feedback resistor is connected. The negative feedback affects not only gain and frequency response but also dampening, a lot of tonal variation can be achieved, simply by changing out the value of this resistor or by changing which output transformer tap it is connected to. The output signal is fed back into the amp before the phase inverter and the presence knob controls the amount of negative feedback.
The negative feedback circuit was changed over the years by Marshall, from a large amount of negative feedback, to less and less, over the years. Also the changes were not constant, but actually varied wildly at times.
The entire JTM series used 27K and 16 ohms. Pre '67 the 27k and 16 ohm combination was pretty universal, but there are several rare occurances of a small valued bypass cap across the 27k resistor (this would increase the bass, but have less overall gain). I believe this was changed to the 47k and 8 ohm setup for Lead amps only, circa late '67, even though there was still lots of variation from the factory. The NFB lead was normally connected to the 8-ohm tap in SL amps, to the speaker jacks in Bass and PA. In late '68-'70 some of the SuperLeads had the NFB connected to the far side of the impedance selector or directly to the speaker jacks (which made the NFB variable). The 100k resistor seems to begin appearing around '70, again Lead amps only. The Bass, PA, etc., retained the original NFB setup (even though there are exceptions).
Typically this depends on if you have a 50 watter or 100 watter. 50 Watters have a 47K on the 8-ohm tap and 100 watters have the 100K on the 4-ohm tap. They are *supposed* to sound the same but if you A/B them you'll hear a sonic difference. Be aware that there is lots of variation in Marshalls as they came from the factory, every now and then you'll see a 47K on the 16ohm tap (which is a lot of feedback) or a 100K on the 8ohm tap but those were pretty far and few between. Marshall used to experiment a lot in the 60's.
Using 100K will increase the gain and mids. Finally, The lower resistor value gives a bit more smoother sound, while the higher value gives more of a harder edge at the expense of smoothness. Larger value means less nfb, therefore more gain.
On some older Marshalls (like my '69) the negative feedback (purple) wire was connected to the speaker jacks, so the feedback varies depending on which speaker impedance is selected. The more negative feedback means less gain in the power section; less nfb means more gain. The 16 ohm tap is gonna have more voltage coming off of it than 8, and 8 will have more than 4, so you can control the overall gain of the amp by choosing which tap you use. Use 16 for the least gain, 4 for the most. Remember, this is only power amp gain, not preamp. Also, the Presence control will make more of a difference when there is more voltage to work with. So, if nfb is on the 16 ohm tap, Presence will seem to do more than if nfb is on 8 or 4.
A little known fact (and all the George Lynch follows may like this since he preferred the '73 hand wired heads) is that the common .1uF cap on the Presence pot was a .68uF cap resulting in a very cool "cut" to the upper mids that was not as harsh as the .1uF.
With the 100k feedback resistor, the 0.68uF cap bypassing V2a may be too much. With a 47k feedback resistor, it's just right though. My '72 has the 100k feedback resistor and no .68uF cap on v2a. "
