Hi I am not a complete newb but by no means a seasoned hand, I've built 2 tubelab amps but want to delve into point to point wiring on my next project. I have a Decware Se84c breadboarded just to play with as the design seems simple and I have all the parts needed for it. Not sure if I'm going to build it though, right now it is just for experimentation.
Anyway my question has to do with the transformer hookup in the power section. The transformer I have on hand does not have a center tap on the 6.3v secondary as called for in the schematics ( see attachment)
What I'd like to know is can I use an artificial center tap using two resisters on the 6.3v line with this design or do I need to change something else to make it work?
The tranny has a center tap on the 300v secondary, but since it and the 6v center tap are shown tied together to ground in the schematic I wasn't sure if using resisters would be ok.
Any thoughts from you old hands with more experience building these wonderful classic tube amps would be greatly appreciated. I know answering newbie questions can be annoying at times, so I can't thank you guys enough. 😀😀😀
Anyway my question has to do with the transformer hookup in the power section. The transformer I have on hand does not have a center tap on the 6.3v secondary as called for in the schematics ( see attachment)
What I'd like to know is can I use an artificial center tap using two resisters on the 6.3v line with this design or do I need to change something else to make it work?
The tranny has a center tap on the 300v secondary, but since it and the 6v center tap are shown tied together to ground in the schematic I wasn't sure if using resisters would be ok.
Any thoughts from you old hands with more experience building these wonderful classic tube amps would be greatly appreciated. I know answering newbie questions can be annoying at times, so I can't thank you guys enough. 😀😀😀
Thank the function of both center taps is completely different 🙂
The HV center tap passes the full amplifier current all the time.
Since it uses a full wave rectifier, half windings take alternate turns feeding the diodes, but center tap works the full cycle at whatever current the amp pulls.
So you need a real, copper wire center tap.
Now filaments are balanced; all current sent by one winding comes through the other and viceversa; the center tap is just a ground reference (to avoid excessive hum), does not pass any current at all and so can be simulated by a couple of resistors.
> The transformer I have on hand does not have a center tap on the 6.3v secondary as called for
The 6V heater winding can be CT-ed with a couple low-value resistors. 100r is traditional in guitar amp, but not critical (if you have an overstock of 68r or 270r, that will work, but match better than 10%.)
I do not like the way that drawing glibly runs High Voltage CT to a ground symbol. That wire must go _TO_ the first filter cap return, -then- to chassis or other ground scheme. There's big dump-trucks of current pulsing in that wire, and you want it to go right to the main cap, not round-about.
The 6V heater winding can be CT-ed with a couple low-value resistors. 100r is traditional in guitar amp, but not critical (if you have an overstock of 68r or 270r, that will work, but match better than 10%.)
I do not like the way that drawing glibly runs High Voltage CT to a ground symbol. That wire must go _TO_ the first filter cap return, -then- to chassis or other ground scheme. There's big dump-trucks of current pulsing in that wire, and you want it to go right to the main cap, not round-about.
Attachments
Thanks for the explanation, I'm still learning to understand schematics and while I can easily follow them to build the circuit I don't always understand how they all work.
Hi All
The SCHEMATIC is the connections of all of the parts in the circuit. This is called a NETLIST during making a PCB. The ART or REAL DESIGN is how you connect the parts on the PCB or WIRE them to make the circuit function as required.
Many times the schematic is “sort of a wiring picture”, not the REAL DESIGN. I started out in RF, Microwave, Coaxial switches, Antennas and the schematic are very poor to show the REAL DESIGN.
PRR and JMFahey are showing their talents of the REAL DESIGN.
The SCHEMATIC is the connections of all of the parts in the circuit. This is called a NETLIST during making a PCB. The ART or REAL DESIGN is how you connect the parts on the PCB or WIRE them to make the circuit function as required.
Many times the schematic is “sort of a wiring picture”, not the REAL DESIGN. I started out in RF, Microwave, Coaxial switches, Antennas and the schematic are very poor to show the REAL DESIGN.
PRR and JMFahey are showing their talents of the REAL DESIGN.
What PRR described and showed is different from 'everything goes to a star'. The connection from the high voltage CT must go straight to the first (reservoir) cap and not go anywhere near the star. Then if there are smoothing caps they can come next. Then finally if you are using a star the smoothing cap can connect to the star.riven67 said:
You design a town. Streets. Big dump trucks, speeding sportcars, kids on bikes, old folks on canes. Do you plan all the streets "star", trucks and bikes and canes converging on a point? Or have separate Industrial Road, Speedway, Bike Path, Memory Lane, with only incidental cross-connections? (I dunno... I live on "All of the above" road.)
No talent. No design. Dim memories of troublesome builds.
if using direct coupled stages, chances are you will need to dc bias the filament, does not need to be with a center tap, connect one side to a dc source of about 50vdc and using a 10k 1/2 watt resistor, a 10ufd/100vdc bypass cap to ground gets you there...
the underlying principle here is that no element of a tube should be left hanging, it should connect to something...
the underlying principle here is that no element of a tube should be left hanging, it should connect to something...