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12b4, 6ca4 build questions....

saltwater

saltwater

Super Member
Ok well i received some items form jaymann! (Thanks a million!) and making a list of what i have and opinions on what i need, never had anything to build, so i wanna build a preamp!

List of things i got:

Power cord
Volume pot 10k?
Fuses
Fuse holder
Wire terminlas/ seperators
3 12b4 sockets/ 4 12b4 tubes
on/off switch
1ft of heatshrink
1ft of skinny twisted black wire(jay dont know what guage, its in all the tube amps you builders do)

100uf 450v 105 c cap
47uf 450v 85 c cap
2- 10uf fj 630v cap
power cord locker
transformer w/ wiring diagram,
green green 6.3 volt heater circuit (for 6ca4 rectifier if i use that, dont have that tube)
black wire, black and whit 110 power in
red h.v.
red yellow c.t? h.v.
red h.v

black---) green
) green 6.3 vac
) hv
) ct
) hv
black---)

got a reemer bit up to 3/4

I got a cabinet for mounting all of this stuff prob 15 x 15 x 10

out of my stuff i got the rca jacks, courtesy of old decks, two transformers that come out of some preampthing numbers are: 171 3312 (says that on top stamped in black ink) 2 black wires coming out of them. I also have two rca 866a big honkin tubes, um various wire but none like what he sent me, 150 diff resistors, got the mega pack from radio shcak... some other odds and ends, oh and silver solder, is this ok to use, its like .015 or .15 thick, really like hair almost...

I got this stuff intented on building a 12b4 preamp w/ a 6ca4 rectifier, and i m gonna do it, im having trouble finding a schematic that will work on my budget (cheap)..... any ideas. thanks jay agian!
 
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If you wanted to do a CCS load like Douglas suggested, these are the semiconductors to use. I got a few here I'm planning to try out. Good stash you got there, way to go jaymanaa.
 
hey-Hey!!!,
Looks like you need power Iron. So, will you run resistive or CCS/mu-follower plate loads? Resistor is simpler looking, but the CCS has some distinct advantages. In no particular order, tolerance of more PS ripple and lower output Z/more drive. Get that answered, and we can go hunting some plate iron, or perhaps make a compramise that will allow switching back and forth.
cheers,
Douglas
 
Guys this would be my intro to tube building all this ccs, mu stuff is making no sence, except the iron.. haha dont laugh at me...
 
Good read, eh? Most of what applies your circuit is in the first half of the page. The Supetex DN2540 is a depletion type MOSFET, so the circuit would be the "Cascade Depletion Mode CCS" on the gpimm page. If you look here (at the very bottom plot), you'll also see that he did some testing, and it turns out that simple circuit is one of the best performers too. All you need is 2x DN2540s and a resistor (potentiometer in this case to trim for varying device characteristics).
 
hey-Hey!!!,
For the Cascode circuit( the only one that makes sense, singles don't sound as good ), use DN3545N3 on the bottom, and DN2540N5 on top with a heatsink.
cheers,
Douglas
 
so this potentiometer, which one do i need?

Variable Resistor (Potentiometer)
Resistors are one of the most common electronic componens. A resistor is a device that limits, or resists current. The current limiting ability, or resistance is measured in ohms, represented by the Greek symbol Omega (W [if you see a "W" here instead of the Omega, then you don't have the necessary fonts installed]). Variable resistors (also called potentiometers or just "pots") are resistors that have a variable resistance. You adjust the resistance by turning a shaft. This shaft moves a wiper accross the actual resistor element. By changing the amounts of resistor between the wiper connection and the connection (s) to the resistor element, you can change the resistance. You will often see the resistance of resistors written with K (kilohms) after the number value. This means that there are that many thousands of ohms. For example, 1K is 1000 ohm, 2K is 2000 ohm, 3.3K is 3300 ohm, etc. You may also see the suffix M (megohms). This simply means million. Resistors are also rated by their power handling capability. This is the amount of heat the resistor can take before it is destroyed. The power capability is measured in W (watts). Common wattages for variable resistors are 1/8W, 1/4W, 1/2W and 1W. Anything of a higher wattage is referred to as a rehostat.
 
saltwater said:
so this potentiometer, which one do i need?
Click to expand...

Well, rather than just telling you, I'll show you how one is picked. :thmbsp:

Not knowing the B+, first thing would be to look at the 12B4 curves.

12b4.JPG

A nice operating current to me looks like about 15mA. The curves look evenly spaced and it will keep plate dissipation under 5.5 watts under most circumstances. I would assume about a 250-300 Volt power supply, so your plate voltage would ideally then be 125 to 150 volts at idle. This also determines your 12B4 cathode resistor. If your B+ is 300 and Ip = 15mA, then from the datasheet we see we need about 22 volts from cathode to grid. Therefore out cathode resistor will approximately be 22/0.015 = 1.5K ohms. It should be able to handle 0.015^2*1.5k = 0.34 watts, so a 1 or 2 watt resistor would be good here.

Now on to the CCS. At our operating point of 15mA, it's hard to see what Vgs is required (which is also another good reason to have adjustability). The lowest curve is around 50mA at Vgs = -1.5V volts, so I'd ball park at around Vgs=-2v for Id = 15mA. There will always be more than 100 volts across the CCS, so we don't need to worry about the non-flat portions of the curves.

DN3545.JPG DN3545_2.JPG

So, then that resistor in the Cascode CCS will need to drop about 2 volts at 15mA, hence R = 2/0.015 = 133 ohms. I would use an 82 ohm,5% resistor and a 100 ohm 15-turn trimmer potentiometer in series.

If you want to wait, i think i have everything i need to prototype this circuit. I could tell you for sure if everything will work, but it will be by tonight or tomorrow before I can get to it.
 
thanks for tutorial, i can already see that this is going to be an ongoing thing, im gonna have to learn it all! can i get everything im gonna need from partsexpress? or another chain i dont know... vgs short for voltages?
 
saltwater said:
thanks for tutorial, i can already see that this is going to be an ongoing thing, im gonna have to learn it all! can i get everything im gonna need from partsexpress? or another chain i dont know... vgs short for voltages?
Click to expand...

I don't see multi-turn potentiometers from parts-express, so you might have to get them from elsewhere, digikey, mouser, etc. You may be able to get away with a single turn pot, but it will be touchy when adjusting. If your getting the supertex devices from mouser, you can get the resistors and potentiometers from them as well. If you want help selecting parts, lemme know.

Vgs is short for the voltage from gate to source. Just like a tube, a depletion MOSFET is usually operated with its control grid at a lower voltage than its cathode, or source, which is why the value is negative in the datasheets.

MOSFET - TRIODE analogy
gate <-> grid
drain <-> plate
source <-> cathode
 
ok will do, i must say though i need someone to take me under thier wing, i dont know what pin is what on the bottom of the tube(12b4), i can solder and use a digivolt meter, woodworking, i can do it all but electronic stuff is new to me.

Supertex devices?
 
saltwater said:
ok will do, i must say though i need someone to take me under thier wing, i dont know what pin is what on the bottom of the tube(12b4), i can solder and use a digivolt meter, woodworking, i can do it all but electronic stuff is new to me.
Click to expand...

This is a great project to learn by, and there are plenty of people here to help, just be careful with the high voltages, we don't wanna see you get hurt. :thmbsp:

Frank's tube datasheet archive has pretty much every datasheet I've needed, save a few. http://www.mif.pg.gda.pl/homepages/frank/

For a pinout of the 12B4: http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12B4A.pdf

saltwater said:
Supertex devices?
Click to expand...
Supertex is the manufacturer of the depletion MOSFETs DN2540 and DN3545.
 
oh ok, thanks for the links, that took alot of ease off my mind! so this is a total newb question of sort, but the whole cascode circuit is this the power supply to the tubes? dont laugh! please
 
One thing to understand is that tube pinouts are as seen from the bottom of the amp (where you would be connecting wires). If memory serves, the power TX I sent was ~500vct (volts center tapped). In other words, 500v across the outside "tails" and 250v from each tail to the center tap (which will go to your chassis ground). I usually start at the power supply first (but that's just me). It's a pretty easy spot to get some experience hooking up sockets, reading you meter, understanding rectification (changing AC to DC), etc..... Have we figured out how big the chassis is going to be? I can look through my AL stash and see what I have. I also have lots of multi turn pots that I don't have any use for, do we know a value yet? If I missed the answers to these questions somewhere above, excuse me, I'm on pain meds.:D
 
saltwater said:
oh ok, thanks for the links, that took alot of ease off my mind! so this is a total newb question of sort, but the whole cascode circuit is this the power supply to the tubes? dont laugh! please
Click to expand...


No, I think they're referring to a "Constant Current Source" (But maybe not) which I will learn about here as well. You may be able to find some reading on it at the Bottlehead website as they use them I think.
 
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