The ideal PT?

[jarre]

If one were going to design a custom power transformer for a tube rectified (5AR4) rev. D board ST-35 clone, what would the secondary windings look like?:

3xx-0-3xx v @ 180ma (?) Also, why does the PA-774 spec refer to DC rectified current for the 180 ma spec.?

6.3 v @ ? A (single 6.3 tap or double better)

5 V @ ?A for 5AR4

The PA-774 is 330-0-330, but ideally how much more would you want for a 5AR4? I don't know what the forward drop across the 5ar4 is.

This transformer also has 2 6.3 v taps at 2.5 a each -- is this required for the design of the board, or would there be any advantage to a single one with enough current? 5 a? Don't plan to use a neon or other AC pilot light -- will take something off the 5v rectifier filament or 6.3v windings for this and use an LED with proper rectification circuit.

What is the filament current requirement for a 5AR4, and how much extra current capacity makes sense, if any at all?

If somebody could tell me where I might be able to locate this information for myself, I'd appreciate it. As I've said, kind of new to the tube design thing.

Any recommendations on decent books? Is the tube audio design book that Triode sells a good place to start?

thanks.

[Brik]

The good place to look for the information would be:
http://www.diytube.com/phpBB2/viewtopic ... highlight=

The PA-774 AC output is closer to 300-0-300 despite what the Triode website says. This will yield B+ of 360V DC
in an ST35 with solid-state rectification.

The 6.3V heater current rating should be at least 4 Amps total:
4 * EL84 : 4 * 0.76A =~3.0A
http://www.nj7p.org/Tube4.php?tube=el84
1 * 12AX7: 0.3A @6.3V
http://www.nj7p.org/Tube4.php?tube=12AX7

1 * 12AU7: 0.3A @6.3V
http://www.nj7p.org/Tube4.php?tube=12AU7

5V winding should have the current capability greater than 1.9 Amps:

http://www.nj7p.org/Tube4.php?tube=5ar4

I use a Hammond 272HX with a 5AR4 followed by Cap(47uF/500V)-Choke(5H/200mA:Hammond 193H)-Cap(2*100uF/450V) and get B+ of about 360V. This power supply should be more than enough for an ST35.

/b

[jarre]

Brik,

Thanks,

I've been doing some poking around and came up with some numbers that should work:

350-0-350 180 mA (leaving approx 20 v forward drop across the 5ar4, and 2 6.3 2.5 a filament windings, plus a 5v 2A filament winding for the rectifier.

Capacitor value will be 34 uF 700v (2 blackgate NX 68 uF 350 volt in series -- just because I already have them, and they're not too shabby either), and the c354 choke.

Should work well, but may need a bit of tweaking, and perhaps a buffer resistor in series with the cap, but that experimentation will have to wait until everything is assembled and working and real world measurments can be taken.

Thanks for your help and the resource.

-- J

[Brik]

Hi,

If you use cap input, 350-0V-350 and 5AR4/C354 might yield > 420V DC in an ST35. You'll have to use choke input with this transformer. You'll get less hum//lower peak current for 5AR4 this way.

Interesting discussion on this very subject in
http://www.harpamps.com/micKtubes/5AR4-data.html

/b

[erichayes]

Hi All,

Some thoughts . . .

First, I've been running the 1773s with 380 volts on the plates and the cathodes grounded for over 7 years and have never had a 6BQ5 failure. I don't know about the others, but the two that my daughter and I have are running 24/7/52 except for occaisional power outages of less than an hour. 6BQ5s are very conservatively rated, so don't be afraid to push them some.

Second, I'm a hard line advocate of choke input supplies for anything running other than class A because of its superior regulation characteristics. The problem with choke input networks is that they have to have a certain amount of current flowing through them for the choke to work. The C-354 is about a quarter the value it needs to be, if you use the 5AR4 (about an eighth if you went with silicon). A swinging choke of about 4~12H would be ideal, if you can find one, but a 5 or 6H fixed would work OK.

Third, I don't recommend the technique of 'tuning' a choke input filter by putting a small value cap on the rectifier side of the choke to get the desired output voltage. Doing this merely makes it a bad cap input filter with less regulation than a legitimate cap input filter using a 40 or 50 µF input cap.

Fourth, this being a DIY forum, you're not really going to learn (or believe) anything until you do it yourself.