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Marshall mod in PI
Posted: Mon 06/14/21 3:48 am
by musicheals
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
Re: Marshall mod in PI
Posted: Mon 06/14/21 4:57 pm
by zaphod_phil
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
Re: Marshall mod in PI
Posted: Mon 06/14/21 5:51 pm
by TriodeLuvr
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
Re: Marshall mod in PI
Posted: Mon 06/14/21 11:32 pm
by musicheals
TriodeLuvr wrote: ↑Mon 06/14/21 5:51 pm
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
Thanks a lot
musicheals
Re: Marshall mod in PI
Posted: Tue 06/15/21 12:53 am
by musicheals
zaphod_phil wrote: ↑Mon 06/14/21 4:57 pm
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
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...
Thanks
musicheals
Re: Marshall mod in PI
Posted: Wed 06/16/21 2:36 pm
by zaphod_phil
musicheals 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
What exactly is the mod you're talking about
Re: Marshall mod in PI
Posted: Wed 06/16/21 5:01 pm
by musicheals
...the 250pF across the 1M grid resistor, which you can find in the beginning of this post
Re: Marshall mod in PI
Posted: Thu 06/17/21 11:28 am
by zaphod_phil
TriodeLuvr wrote: ↑Mon 06/14/21 5:51 pm
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
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.
Re: Marshall mod in PI
Posted: Fri 06/18/21 2:13 am
by musicheals
zaphod_phil wrote: ↑Thu 06/17/21 11:28 am
TriodeLuvr wrote: ↑Mon 06/14/21 5:51 pm
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
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.
Yes, in the meantime I tried this mod and I'm sure, it was used to prevent oscillation
musicheals
Re: Marshall mod in PI
Posted: Mon 06/21/21 6:58 pm
by zaphod_phil
musicheals wrote: ↑Fri 06/18/21 2:13 am
zaphod_phil wrote: ↑Thu 06/17/21 11:28 am
TriodeLuvr wrote: ↑Mon 06/14/21 5:51 pm
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
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.
Yes, in the meantime I tried this mod and I'm sure, it was used to prevent oscillation
musicheals
It might also cause some loss of top-end detail and sparkle