Negative Feedback Question
- Thread starter Cap217
- Start date
1 meg audio push pull and what brand/type of cap? I’m ordering from amplified parts I guess.
May as well tell me what resistors and caps needed for the other nfb so I have it. And anything else I would need.
glpg80
Well-known member
I’ll tell you a little secret. The caps you use in the NFB path are extremely, extremely important to the tone end result as much as the value.
I absolutely only use MKP (metalized plastic film polypropylene) WIMA capacitors, or ceramic disc for anything in the NFB path. These are the most aggressive sounding that don’t slew the top end of notes like sozos IMO.
You can use sozo caps but I only like them for true vintage sounding reproductions. This is a personal preference of course.
Whatever you do stay away from paper in oil caps marketed today - straight snake oil.
I absolutely only use MKP (metalized plastic film polypropylene) WIMA capacitors, or ceramic disc for anything in the NFB path. These are the most aggressive sounding that don’t slew the top end of notes like sozos IMO.
You can use sozo caps but I only like them for true vintage sounding reproductions. This is a personal preference of course.
Whatever you do stay away from paper in oil caps marketed today - straight snake oil.
RedPlated
Well-known member
Another tip in the NFB circuit…
Put a cap across the NFB resistor and you’ll remove very high shrill frequencies. 100-220pf or so. I usually do not use a cap here but some amps benefit from it.
As for which pot, I use Alpha push pulls. Some like CTS push pulls but I don’t. The switching on the Alpha is also more useful. I can control two separate parts of the circuit with one Alpha pot.
For the cap, Valvestorm has the .0047 (listed as 4700pf) ceramic disc caps I mentioned for 30 cents. I would just order the pot and cap from him.
http://valvestorm.com/Products/Components/Capacitors/Ceramic
http://valvestorm.com/Products/Components/Potentiometers/Amplifier
Put a cap across the NFB resistor and you’ll remove very high shrill frequencies. 100-220pf or so. I usually do not use a cap here but some amps benefit from it.
As for which pot, I use Alpha push pulls. Some like CTS push pulls but I don’t. The switching on the Alpha is also more useful. I can control two separate parts of the circuit with one Alpha pot.
For the cap, Valvestorm has the .0047 (listed as 4700pf) ceramic disc caps I mentioned for 30 cents. I would just order the pot and cap from him.
http://valvestorm.com/Products/Components/Capacitors/Ceramic
http://valvestorm.com/Products/Components/Potentiometers/Amplifier
Last edited:
glpg80
Well-known member
Ironically I do something similar but in a different method.
The only difference is that I place mine in parallel with the presence pot to ground since it’s a better/easier location to solder. I use 47-220pF. Yes I have a shunt 5k resistor across the pot as well which is why it works.
It lets you get more aggressive with the treble pot to get that last little bit of gain without notes having such a harsh sharp poignant attack to them that hurts.
I find it so helpful that it will be made switchable on my 74. Some days I want the value I chose, other days I want it out of circuit and to turn the treble down. I find that the lower the volume the amp is played, the more dynamics that are removed in circuit, which makes sense given its in the NFB path.
Last edited:
This is my current cap on the depth pot.
What kind and brand is this?
Are you saying I should replace this while I’m in there with another .0047 and add a .0068 on the push pull? To me this makes sense. When you guys start talking about:
"Put a cap across the NFB resistor and you’ll remove very high shrill frequencies. 100-220pf or so. I usually do not use a cap here but some amps benefit from it."
Then I get lost.
What kind and brand is this?
Are you saying I should replace this while I’m in there with another .0047 and add a .0068 on the push pull? To me this makes sense. When you guys start talking about:
"Put a cap across the NFB resistor and you’ll remove very high shrill frequencies. 100-220pf or so. I usually do not use a cap here but some amps benefit from it."
Then I get lost.
Attachments
He doesnt have a .0068 there and I think thats what Id want to use with the .0047. And I dont see an alpha push pull 1meg audio, only cts. Is that the right one?
I have this bit of info from Dave Friedman about the be100 deluxe settings:
To make it as close to the JJ you do frequency switch left and response right. Voice middle. Thump on 6-7.
To make close to the SS you do frequency right and response right. Voice middle. Thump and resonance 6.
So these would be the 2 positions Id be interested in. I own the SS100 and I think it sounds great in the SS+ and dark in the SS mode. On my old Be100 deluxe I liked thump switched to the left most of the time.
Im not sure what the switchs switch to. I dont think its in order because he usually does middle "normal".
Last edited:
FourT6and2
Well-known member
I keep it simple with modded Marshalls. 2200pF depth cap and NFB is 47K/8ohm. I'll alligator clip a pot in for the NFB and play around with it at various settings until I find something I like. When I measure, it's usually around 47K. Always. So I just go with that.
scottosan
Well-known member
Since you want more aggression, I interprete that and more mids. The .0068 is going to take you in the the opposite direction as it fills out the bottom end. That said, it’s a dance right? All parts work together. Bump your NFB up to get your immediate goal of aggression, then see if you want more thump
More aggression meaning more gain but a bigger bottom end that isnt flubby. I cant really push my low end now becuase it gets too loose.
glpg80
Well-known member
More aggression in the amp modded world is a game with mid frequencies, not low end frequencies. You have to stop thinking like a guitar player that secretly wants to a bass player (many do) and know that more low end with vacuum tubes is a dangerous game of instability due to blocking distortion and flub city.
Don’t be afraid to mess with different smaller values. I use smaller than 4700pF. 46&2 uses 2200pF. Bigger isn’t always better.
I understand but I have many amps and many friedmans that have more umph than this does. Thats what I am trying to fix here. This low end is more muddy vs punchy. I have a be100, dlx, and ss100. Even the ss100 has more punch and that has a more mellow circuit in the low end I think. I never played the JJ but I assume that has the most low end. I could be wrong.
So all things equal and in terms of frequency effected. Changing the cap from .0047 to .0068 lowers to what frequency? Because if .0047 is around 300, I don’t like adding that frequency.
scottosan
Well-known member
Then disregard everything I’ve suggested, our definitions of aggression are way off. If your other friedmans are production, I suspect they have more refined and robust filtering that is going to attribute to low end.
FourT6and2
Well-known member
Maybe I missed it, but what amp are you talking about? Lots of other things in a circuit and they all add up. 6800pF depth cap is quite high. I would usually not go higher than 4700pF in a typical Marshall-eque hot-rodded amp. But it depends what els else is going on in the circuit. I've never played a Friedman that was aggressive enough for my tastes, for example.
EDIT:
Is it the Ceriatone AH100 you posted earlier? If so, there are a bunch of things you can do to make that thing sound better/more aggressive. But yeah, lowering the depth cap value is one of them.
Last edited:
SpiderWars
Well-known member
Pretty sure that's a Tantalum cap. I never use them, they are typically very cheap and low voltage and not highly regarded as far as audio. But in some places it doesn't really matter and the Mesa Mk IIC+ has several tantalum caps (for example) and they sound great. And some spots sound better with with a cheap cap (like the cheapo ceramic disks in the Depth).
psychodave
Well-known member
This is a trick to do to stock JCM800 amps. Just gets rid of some of that ultra high end that isn't pleasant. Again, different cap types make a difference here as well.
I’m ordering caps. .0047 and .0068. Can you send a link to what I should get? The last link Posted, that company doesn’t have .0068.
And what about a push pull 1meg. I assume audio but I saw someone say linear too. I’d like an alpha push pull 1 meg audio but I can’t find any.
Then what resistors in the nfb circuit in the board? Current is 47k but I’d like a 33k and 100k to try.