Marshall mod in PI

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musicheals
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Marshall mod in PI

Post 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
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zaphod_phil
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Re: Marshall mod in PI

Post 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 :D
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Re: Marshall mod in PI

Post 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
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Re: Marshall mod in PI

Post 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
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Re: Marshall mod in PI

Post 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 :D
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
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Re: Marshall mod in PI

Post 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 :?:
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musicheals
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Re: Marshall mod in PI

Post by musicheals »

...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

Post 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.
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Re: Marshall mod in PI

Post 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
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Re: Marshall mod in PI

Post 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
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