Marshall mod in PI
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Marshall mod in PI
Hi,
Has anybody ever seen or used this mod in the PI of a Marshall style amp?
Will it change the balance in the PI or just remove some higher frequencies?
Thanks
musicheals
Has anybody ever seen or used this mod in the PI of a Marshall style amp?
Will it change the balance in the PI or just remove some higher frequencies?
Thanks
musicheals
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- zaphod_phil
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Re: Marshall mod in PI
Injecting NFB into the tail of the PI simply increases the balance of the PI + PA, thereby decreasing the nice, warm even-order harmonics. So you probably won't want it Many F*nder amps have that form of NFB, as well as bigger Marshall amps, derived from the F*nder Bassman design.
Injecting the NFB signal into the pin 2 of the PI will reduce harmonic distortion - up until when the amp starts to clip and then all bets are off! This is again another F*nder design feature from the time that Leo was mistakenly trying to make his amps more Hi-Fi-like. But the NFB also has the effect of reducing high-end detail. The additional cap to ground via the pot siphons higher frequencies out of the NFB loop to help preserve the high-end sparkle. The pot (Presence control) allows the amount being siphoned out to be varied.
Hope that explanation made sense
So without all that F*nder-designed NFB stuff, the Marshall 18W is a more dynamic and expressive amp
Injecting the NFB signal into the pin 2 of the PI will reduce harmonic distortion - up until when the amp starts to clip and then all bets are off! This is again another F*nder design feature from the time that Leo was mistakenly trying to make his amps more Hi-Fi-like. But the NFB also has the effect of reducing high-end detail. The additional cap to ground via the pot siphons higher frequencies out of the NFB loop to help preserve the high-end sparkle. The pot (Presence control) allows the amount being siphoned out to be varied.
Hope that explanation made sense
So without all that F*nder-designed NFB stuff, the Marshall 18W is a more dynamic and expressive amp
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- TriodeLuvr
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Re: Marshall mod in PI
The 250 pF cap will create an additional rolloff amounting to approximately -3.5 dB at 10 kHz. It doesn't have much affect on balance, because the contour it creates is not restricted to only one output phase.
Jack
Jack
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Re: Marshall mod in PI
Thanks a lotTriodeLuvr wrote: ↑Mon 06/14/21 5:51 pmThe 250 pF cap will create an additional rolloff amounting to approximately -3.5 dB at 10 kHz. It doesn't have much affect on balance, because the contour it creates is not restricted to only one output phase.
Jack
musicheals
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Re: Marshall mod in PI
Yes, I know about the NFB, thanks. I found this mod on the Marshall forum and it was found in a JTM45, which is said it belonged to Paul Kossoff and Andy Fraser...zaphod_phil wrote: ↑Mon 06/14/21 4:57 pmInjecting NFB into the tail of the PI simply increases the balance of the PI + PA, thereby decreasing the nice, warm even-order harmonics. So you probably won't want it Many F*nder amps have that form of NFB, as well as bigger Marshall amps, derived from the F*nder Bassman design.
Injecting the NFB signal into the pin 2 of the PI will reduce harmonic distortion - up until when the amp starts to clip and then all bets are off! This is again another F*nder design feature from the time that Leo was mistakenly trying to make his amps more Hi-Fi-like. But the NFB also has the effect of reducing high-end detail. The additional cap to ground via the pot siphons higher frequencies out of the NFB loop to help preserve the high-end sparkle. The pot (Presence control) allows the amount being siphoned out to be varied.
Hope that explanation made sense
So without all that F*nder-designed NFB stuff, the Marshall 18W is a more dynamic and expressive amp
Thanks
musicheals
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Re: Marshall mod in PI
What exactly is the mod you're talking aboutmusicheals wrote: ↑Tue 06/15/21 12:53 am
Yes, I know about the NFB, thanks. I found this mod on the Marshall forum and it was found in a JTM45, which is said, belonged to Paul Kossoff and Andy Fraser...
Thanks
musicheals
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Re: Marshall mod in PI
...the 250pF across the 1M grid resistor, which you can find in the beginning of this post
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Re: Marshall mod in PI
Yes, I agree. I suspect it might be a band-aid fix to help prevent parasitic oscillations. Likewise, you sometime see a 250pF cap strapped between the PI anodes.TriodeLuvr wrote: ↑Mon 06/14/21 5:51 pmThe 250 pF cap will create an additional rolloff amounting to approximately -3.5 dB at 10 kHz. It doesn't have much affect on balance, because the contour it creates is not restricted to only one output phase.
Jack
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Re: Marshall mod in PI
Yes, in the meantime I tried this mod and I'm sure, it was used to prevent oscillationzaphod_phil wrote: ↑Thu 06/17/21 11:28 amYes, I agree. I suspect it might be a band-aid fix to help prevent parasitic oscillations. Likewise, you sometime see a 250pF cap strapped between the PI anodes.TriodeLuvr wrote: ↑Mon 06/14/21 5:51 pmThe 250 pF cap will create an additional rolloff amounting to approximately -3.5 dB at 10 kHz. It doesn't have much affect on balance, because the contour it creates is not restricted to only one output phase.
Jack
musicheals
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Re: Marshall mod in PI
It might also cause some loss of top-end detail and sparklemusicheals wrote: ↑Fri 06/18/21 2:13 amYes, in the meantime I tried this mod and I'm sure, it was used to prevent oscillationzaphod_phil wrote: ↑Thu 06/17/21 11:28 amYes, I agree. I suspect it might be a band-aid fix to help prevent parasitic oscillations. Likewise, you sometime see a 250pF cap strapped between the PI anodes.TriodeLuvr wrote: ↑Mon 06/14/21 5:51 pmThe 250 pF cap will create an additional rolloff amounting to approximately -3.5 dB at 10 kHz. It doesn't have much affect on balance, because the contour it creates is not restricted to only one output phase.
Jack
musicheals
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