• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Another 12B4 Beginner's Build

The Reverend

The Reverend

Just terrible.
Hello,

I am working on a 12B4 line stage based upon the circuit and PSU attached. I am trying to keep the costs way down, and I was recommended this circuit over at diyaudio.com. I was just hoping to create a thread to get some feedback on parts selection and troubleshooting when problems inevitably arise.

Onto my first questions:

At what locations are bypass caps necessary on this circuit? How does one go about selecting decent bypass caps? What makes an appropriate cap for this application?
 

Attachments

  • brett-12b4.jpg
    brett-12b4.jpg
    33.1 KB · Views: 104
  • 12b4-psu1.jpg
    12b4-psu1.jpg
    79.3 KB · Views: 73
Register to hide this ad
BTW, I am thinking of changing the cathode bias cap to red LEDs. I have read that I should use a 10000uF cap and 4 LEDs for some reason, but I have also read that I should just throw out the cap and use 11 LEDs. I don't know how people are coming up with this stuff!
 
Here are a couple of parts to start with:

For the coupling cap, I want to keep things cheap for starters, so I will go for a Solen 3.3uF/630V from Tubesandmore (C-FS3D3-630).

The cathode bias will either be Illinois 100uF/450V from tubeandmore or 11 cheap red LEDs from ratshack.

Does this sound good so far?
 
All i can sasy is good luck! Bricktop is gone for weekend, and so jaymanna and pakprotector is up to pitch, but they know thier shite! Have you seen the shematic that bricktop drew up?
 
hey-Hey!!!,
I have used red LED's as cathode biasing elements for a while now. I have never found the need to put a capacitor in parallel with them. If there are other bits yu would buy from digikey, get their HLMP-6000's for ~30 cents each. You'll need 5 or 6( perhaps 7 or 8) of them in series, and they're good to 30 mA continious.

I make the bias point at about -12 volts at the specified current draw. With a resistor, it will be quite colse, with LED's, less so, since their voltage is not dependant on current.

I'd rather a different PS, but yours is quite a bit ahead on the cost, and it will serve.
cheers,
Douglas
 
I am open to suggestions on the PS, I just took it for the low price.

Also, I am trying to purchase the parts from just Digikey and tubesandmore.com (excluding the tube, which I am going to buy from e-bay).
 
The Reverend said:
I am unclear what constitutes an appropriate TX from this schematic. Would something like the linked file be appropriate? http://www.tubesandmore.com/scripts...evclients/cemirror/ELEVATOR.FXP?item=P-T270FX
Click to expand...

Hi there Reverend. That power tx is quite a bit overkill, it will give you a very high supply voltage, and likely higher than predicted since they usually are designed for 115VAC. You won't need a whole lot of supply voltage or output voltage swing for a line-stage. I would suggest something like the hammond P-T369EX (190-0-190 VAC@65mA, 6.3V@2.5A), with something like the attached schematic. You could easily add another RC stage without much extra cost. How much filtering you need will be determined by your plate load -> more filtering with resistor plate load, less with CCS. For $10-11 more, you could get a choke: P-T155H (5H, 50mA) or P-T154G (9H, 40mA), which would be a nice improvement.

The schematic I wrote up there needs an update. Douglas (PakProtector) suggests using a 510ohm or greater resistor for Ra and Rb, which is a good idea to keep the MOSFETS from oscillating. I better fix that and update it. Also, you can use any potentiometer/resistor combination to yield 180 +/- 50 ohms for the DN3545 source resistor. I just used what parts were available when making the schematic.

Good luck:thmbsp:
 

Attachments

  • 12b4ps.JPG
    12b4ps.JPG
    9.4 KB · Views: 65
Cool, I will add a choke. Can I just place it in series with the load? Or should I replace R1 with a choke of similar resistance?
 
Keep the first stage RC filter on the previous supply, and add an LC filter like in the original schematic.

Tweak the first resistor to get your desired power supply voltage while making sure that the steady state RMS current in the transformer does not exceed the ratings. (65mA) The lower it is, the cooler the tranny will run. Then make sure to buy a resistor that exceeds the power rating. Look at the steady state RMS current in PSUD for this as well.
 
Since I am new to this, I am not 100% sure what my desired supply voltage is, I know I want to have the tube running in its most linear region, so I suppose I should look at the specs on the 12B4.
 
BTW, for a little background I have built a couple of other small projects and I am currently working on one as we speak. I started with a CMOY headphone amp and promptly moved to a SOHA tube hybrid (12AU7 based) which I am both updating and using as my pre as we speak. The CMOY was all protoboard and the SOHA is PCB based, so this will be my first p2p build.

I have also taken a decent amount of electronics (1 semester of engineering circuits, 2 quarters of upper division physics electronics), but I am really not up to speed on all of the tube lingo that gets tossed around. I suppose I can remedy that by reading through the NEETS module 6 about tubes, though.

Thanks for all of the help (I am sure I will ask a few more questions as I go!).

Best,
Colin
 
Assuming I use your (Bricktop) schematic, can I use a 5Y3GT instead of the 6CA4? I have an old one and would like to try and save money. Of course, I have no idea if the tube works or not.

I wont use it if it is a poor choice for this application for one reason or another.
 
The Reverend said:
Assuming I use your (Bricktop) schematic, can I use a 5Y3GT instead of the 6CA4? I have an old one and would like to try and save money. Of course, I have no idea if the tube works or not.
Click to expand...

It'll work, but you'll need to get a different power transformer: one that has a 5V and 6.3V or 12.6V filament winding. Something like the P-T270CAX will do fine. I did a quick supply with PSUD and came up with this, which uses the P-T154G choke, the P-T270CAX power tranny, and a few resistors and caps. It is probably a bit overkill for a CCS loaded 12B4, but it'll give you the option if you ever wanted to do a resistor load for any reason. Should be super quiet, not too expensive either...

Schematic will be different with the 5Y3 rectifier, as the heater is connected to the cathode on the 5Y3. I'm sure you've seen how amps do it.
 

Attachments

  • newest12b4ps.JPG
    newest12b4ps.JPG
    77.9 KB · Views: 24
How does this choke sound: Hammond P-T154G
9H, 40mA, 700R?

For the TX, how about P-T270CAX? Or would I need to move up to 100mA?
 
You must log in or register to reply here.
Back
Top Bottom