I´m a"baby" in this amp stuff and i don´t fully understand some things , so , i have a few questions to learn a little more ...
- Preamp Cathode resistors and Caps , why are they there and their impact in sound ? I usually see electrolytic on those places , but some amps uses others like poliester , polypropilene , metallized polypropilene and metallized polyester ... why ?
- Preamp valves have low voltage in cathode , i believe 12AX7 can handle 100V max. , but in my 18Watt i measure only 1.2V max !! So , i don´t need to use a 450V cap , i believe 100V is a good one ... i think !
- If i don´t use a 18,2K resistor and use a 18K instead , will i lose the "mojo" ? Yes , i know that i can´t blow an amp with that kind of difference ! Hehehe...
- In the TWreck presence pot ( 5K linear ? ) , we have a 0.1uF connected to the middle , why ? And there is a 47K resistor connected too , in some versions have a 100K ! Herrr.. the difference between them are ... ?
Sorry , they are rookye questions ...
Sorry my poor English ... i hope you understand .
Some tech questions !
Moderators: pompeiisneaks, Colossal
-
BlueDragon
- Posts: 24
- Joined: Thu Mar 10, 2005 12:00 pm
- Location: South Europe
Some tech questions !
Learning every day ...
-
loverocker
- Posts: 104
- Joined: Thu Mar 03, 2005 11:55 am
- Location: England
- Contact:
Re: Some tech questions !
> Preamp Cathode resistors and Caps , why are they there and their impact in sound ? I usually see electrolytic on those places , but ...
Well, cathode resistors help to set the gain and bias point for the triode - you need to get down and dirty with the graphs and maths to work out exactly how much... or just use your ears!
High values of cathode cap are easiest to find in electrolytic. Bet you could use others, too. They'd probably cost more, and may have no significant 
Supposedly tanatlum are a bad idea because they are more likely to fail in shortcircuit, but that's just Net scuttlebutt and I've no experience of it.
> Preamp valves have low voltage in cathode , i believe 12AX7 can handle 100V max. , but in my 18Watt i measure only 1.2V max !! So , i don´t need to use a 450V cap , i believe 100V is a good one ... i think !
Yes - that's OK. Preamp cathode caps need only be rated as low as 25V, probably even lower. Keeps them small and compact, too. No problem.
> If i don´t use a 18,2K resistor and use a 18K instead , will i lose the "mojo" ? Yes , i know that i can´t blow an amp with that kind of difference ! Hehehe...
Yup - go for it. If there's any extra mojo in that 200ohms, I'm a banana
- In the TWreck presence pot ( 5K linear ? ) , we have a 0.1uF connected to the middle , why ? And there is a 47K resistor connected too , in some versions have a 100K ! Herrr.. the difference between them are ... ?
> The cap and resistance forms the filter for the negative feedback, making a 'sort of extra Treble' action for the Presence pot. The 47k/100k resistor is the negative feedback resistor - vary to taste (see typical Fender/Marshall schematics for ideas).
Well, cathode resistors help to set the gain and bias point for the triode - you need to get down and dirty with the graphs and maths to work out exactly how much... or just use your ears!
High values of cathode cap are easiest to find in electrolytic. Bet you could use others, too. They'd probably cost more, and may have no significant Supposedly tanatlum are a bad idea because they are more likely to fail in shortcircuit, but that's just Net scuttlebutt and I've no experience of it.> Preamp valves have low voltage in cathode , i believe 12AX7 can handle 100V max. , but in my 18Watt i measure only 1.2V max !! So , i don´t need to use a 450V cap , i believe 100V is a good one ... i think !
Yes - that's OK. Preamp cathode caps need only be rated as low as 25V, probably even lower. Keeps them small and compact, too. No problem.
> If i don´t use a 18,2K resistor and use a 18K instead , will i lose the "mojo" ? Yes , i know that i can´t blow an amp with that kind of difference ! Hehehe...
Yup - go for it. If there's any extra mojo in that 200ohms, I'm a banana
- In the TWreck presence pot ( 5K linear ? ) , we have a 0.1uF connected to the middle , why ? And there is a 47K resistor connected too , in some versions have a 100K ! Herrr.. the difference between them are ... ?
> The cap and resistance forms the filter for the negative feedback, making a 'sort of extra Treble' action for the Presence pot. The 47k/100k resistor is the negative feedback resistor - vary to taste (see typical Fender/Marshall schematics for ideas).
Re: Some tech questions !
A good place for info is Randall Aiken's site: www.aikenamps.com. He has a tech page full of info. Also try the TUT series of books (The Ultimate Tone) from www.londonpower.com. They are great! Heck, even Dan Torres' book is good for beginners, but there is some misinformation in there.
-erigm