Hi!
Currently listing down my order for caps and I was wondering what the effect of using lower than spec filter caps for the PI and preamp sections of a Twin Reverb. Thanks!
Lower filter cap values eg (10uF/500v) on preamp filter caps
Moderators: pompeiisneaks, Colossal
-
titser_marco
- Posts: 131
- Joined: Tue Mar 13, 2007 3:08 pm
Re: Lower filter cap values eg (10uF/500v) on preamp filter caps
The pre-amp filter section in the supply chain in this amp is a low-pass R/C filter - like all the other filters (except for the choke end, which is a CLC filter). Reducing C raises the roll off point of the frequency shelf, meaning that progressively higher frequencies will get shelved out of the supply rail at this point.
The formula for working out the shelf for this filter is:
f = 1/(2Pii.C.R)
However , this is only part of the calculation. To work out the total effect of all the low-pass shelving filters in the power supply rail, you have to sum the bode plots of all of the R/C filters.
To cut a long story short, the sonic effect is that the pre-amp will get slightly spongier. But you won't notice anything much until you start to push the amp a bit, and by that stage any sonic impact of reducing C will get progressively less significant as the distortion from the rest of the signal path increases. Personally, if it were my twin, I'd prefer to keep the values the way it came out of the factory.
The formula for working out the shelf for this filter is:
f = 1/(2Pii.C.R)
However , this is only part of the calculation. To work out the total effect of all the low-pass shelving filters in the power supply rail, you have to sum the bode plots of all of the R/C filters.
To cut a long story short, the sonic effect is that the pre-amp will get slightly spongier. But you won't notice anything much until you start to push the amp a bit, and by that stage any sonic impact of reducing C will get progressively less significant as the distortion from the rest of the signal path increases. Personally, if it were my twin, I'd prefer to keep the values the way it came out of the factory.
He who dies with the most tubes... wins
-
Malcolm Irving
- Posts: 139
- Joined: Wed Jan 14, 2015 12:06 am
- Location: Derbyshire, UK
Re: Lower filter cap values eg (10uF/500v) on preamp filter caps
The filter caps are also there to decouple the various gain stages. As you reduce the uF value, at some point the amp might start to 'motorboat'. Hard to guess where that point would be though.
-
Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
.
Basically any fender of the BF or silver face era with reverb does not have enough filtering if you play thru both channels at the same time!
The basic rule of thumb is no more than two gain stages per filter node if you do not want then to interact!
The basic rule of thumb is no more than two gain stages per filter node if you do not want then to interact!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Lower filter cap values eg (10uF/500v) on preamp filter caps
My Fender 75 (their Boogie inspired design) has 5 CC gain stages off the same HT node decoupled by 22uF, 4 of those stages are in cascade.
And it's not as if the high pass corner frequencies of the RC coupling are high - there's a lot of bass available.
http://bmamps.com/Schematics/fender/75_Schematics.pdf
So my guess is that there would be no perceivable difference between 22uF and 10uF on a TR AB763 HT pre-amp HT node.
Bear in mind that the value tolerance on e caps back then was often -50 to +100%, so a 22uF nominal cap may be 11 or 44uF and still be in spec.
So Fender probably spec'ed the lowest permissible nominal value to ensure that the amp would be stable even if that actual value was half the nominal.
These days, ecaps are more usually +/-20% of their nominal capacitance.
One way of looking at it is that as long as the max gain-bandwidth of the signal path is lower than the attenuation-bandwidth of the HT decoupling, then motorboating (ie positive feedback via the HT) should be kept at bay.
And it's not as if the high pass corner frequencies of the RC coupling are high - there's a lot of bass available.
http://bmamps.com/Schematics/fender/75_Schematics.pdf
So my guess is that there would be no perceivable difference between 22uF and 10uF on a TR AB763 HT pre-amp HT node.
Bear in mind that the value tolerance on e caps back then was often -50 to +100%, so a 22uF nominal cap may be 11 or 44uF and still be in spec.
So Fender probably spec'ed the lowest permissible nominal value to ensure that the amp would be stable even if that actual value was half the nominal.
These days, ecaps are more usually +/-20% of their nominal capacitance.
One way of looking at it is that as long as the max gain-bandwidth of the signal path is lower than the attenuation-bandwidth of the HT decoupling, then motorboating (ie positive feedback via the HT) should be kept at bay.
https://www.justgiving.com/page/5-in-5-for-charlie This is my step son and his family. He is running 5 marathons in 5 days to support the research into STXBP1, the genetic condition my grandson Charlie has. Please consider supporting him!