Hi guys,
I posted recently wanting to rework a Stromberg Carlson amp into a bass amp. However, I quickly realized I was biting off way more than I could chew and have rethought my strategy and have decided on building a bass preamp instead, start small and work my way up!
I have some questions I need answered, attached is a schematic for the build.
1. I think I found a good transformer for the project, it is marketed for standalone reverb units. It has 250vac/25ma and 6.3vac/1A heater. I think that that gives me about 250*1.4 = 350v dc after rectifier, which I probably want to step down to around 300v for the 12ax7s. How do I figure out the size of the resistor I should use to drop the voltage before I build the thing or is that more something you meter as you go and find values that way?
1b. Kind of same as first question, with biasing (cathode biasing in particular) is that something that you do after everything is up and running and you can measure the grid/cathode or is there a way to know that value beforehand? It's frustrating that Ohm's law is so simple mathematically and yet I still don't really understand when to use it.
2. Some things I read say that I should have a center tap on the heater winding to be grounded, but that I could create a false center tap by wiring both legs with identical resistors to ground. Is this legit and again how would I determine the value of the resistor?
3. What size filter caps do you recommend for a project like this? I have a 47uf and a 22uf in the schematic, but this is just from borrowing from other peoples designs and I don't have a firm grasp as to why those values or why use 1 vs. 2 vs. 3 filter caps.
4. Please if you would, look over the schematic attached and ridicule me for any omissions or errors. It is still a work in progress. Also, please let me know and recommendations you have for improving the circuit to work better with bass guitar.
Thanks everyone!
I posted recently wanting to rework a Stromberg Carlson amp into a bass amp. However, I quickly realized I was biting off way more than I could chew and have rethought my strategy and have decided on building a bass preamp instead, start small and work my way up!
I have some questions I need answered, attached is a schematic for the build.
1. I think I found a good transformer for the project, it is marketed for standalone reverb units. It has 250vac/25ma and 6.3vac/1A heater. I think that that gives me about 250*1.4 = 350v dc after rectifier, which I probably want to step down to around 300v for the 12ax7s. How do I figure out the size of the resistor I should use to drop the voltage before I build the thing or is that more something you meter as you go and find values that way?
1b. Kind of same as first question, with biasing (cathode biasing in particular) is that something that you do after everything is up and running and you can measure the grid/cathode or is there a way to know that value beforehand? It's frustrating that Ohm's law is so simple mathematically and yet I still don't really understand when to use it.
2. Some things I read say that I should have a center tap on the heater winding to be grounded, but that I could create a false center tap by wiring both legs with identical resistors to ground. Is this legit and again how would I determine the value of the resistor?
3. What size filter caps do you recommend for a project like this? I have a 47uf and a 22uf in the schematic, but this is just from borrowing from other peoples designs and I don't have a firm grasp as to why those values or why use 1 vs. 2 vs. 3 filter caps.
4. Please if you would, look over the schematic attached and ridicule me for any omissions or errors. It is still a work in progress. Also, please let me know and recommendations you have for improving the circuit to work better with bass guitar.
Thanks everyone!