Modified Paramour 2 (SR-45) as headphone amp?

[Mikey]

I haven't tried that yet, but will do so!

[Paul Joppa]

That 1.41 number is quite unrealistic. It is the maximum voltage, at near-zero current. In teh real world, most power supplies will get around 1.15 times the AC voltage unless the power transformer is grossly oversized.

With a cap-input filter, it's the peak voltage that charges the capacitor. The peak happens when the current also peaks. I did a quick model in PSUD, and the peak current (for a 45 drawing 1.5 amps) is about 5.5 amps. So you must find the diode drop at 5.5 amps, plus the resistance drop. The winding resistance including primary is about 0.082 ohms , so there's another 0.45 volts lost on top of the two diode drops.

Complicating matters is that the high voltage is also cap-input, so the peak current losses in the primary includes ALL the power supply; effectively it clips off the peaks so that you actually get a bit less than what PSUD would suggest.

Open circuit voltage is about 2.86 volts with a 120v power line. You can ue these numbers in PSUD, which will tell you that you'll get 2.55 volts with 715mV pk-pk ripple assuming a 10000uF/0.02-ohm ESR capacitor. My experience is that you will not get that much. The 8-amp rated Schottkys are available in the same package and have a smaller voltage drop, which might get you there. OR, if you can count on your power line voltage being high, you may not need them.

[Mikey]

Well, I couldn't resist testing the boards I built....after all, it only took about ten minutes!
As expected, the DC voltage at the filament of the 45 tube was low, 2.08 VDC to be exact.

I'll probably install a separate filament transformer in the near future.

Mike

[Paul Joppa]

I did a quick simulation, and the 8-amp Schottkys will get you another 0.2v, not enough.

[Mikey]

On the other hand, you can tuck a 5V/4A power transformer under the hood, bridge rectify that, then select an appropriate dropping resistor (or regulator) based on your results (that's what I'd do).

-PB

I'm bringing this thread back from the dead!   
I'd like to revisit DC heaters for the 45 tube and see how it pans out.

I looked at the Hammond site for an appropriate power transformer, and three candidates stuck out:

http://www.hammondmfg.com/166.htm

166MS - 5.0 VAC C.T., 3A
166M6 - 6.3 VAC C.T., 3A
166N6 - 6.3 VAC C.T., 4A

Would any of these fit the bill?  If not ideal, is there a better choice?

Mike

[Paul Birkeland]

The 5V/3A unit should do the job! 

[Mikey]

The 5V/3A unit should do the job!

Thanks Paul.

I shopped around for that transformer this morning, but nobody had a pair in stock.
The 6.3V/3A was hard to find as well, so I ordered a pair of the 6.3V/4A transformers.
That one must be a more common part number, as everyone had them in stock, and they were $8.00 cheaper apiece.
Guess I'll need a slightly bigger dropping resistor.

So, once these transformers arrive, I'll abandon the 2.5V winding on the PT-2 transformer.
I'll mount them somewhere under the chassis plate, and wire them to my existing 'perf board' filament supply boards.
Then I'll wire the boards up to pins 1 and 4 on the tube socket.  But then...

1.  What should I do with the center tap on the secondary side of the 166N6 transformer?  Ground it?  Leave it disconnected?
2.  Should I leave the hum balance pot intact, or pull it out of the circuit?

Mike

[Paul Birkeland]

You should leave the center tap floating. I would also retain the hum pots, as there will still be residual noise with the DC supply that can be nulled out.

-PB

[Mikey]

Thanks Paul, that's what I'll do.

Mike

[Doc B.]

Yo Mikey! Doing a track day this year?

[Mikey]

Yo Mikey! Doing a track day this year?

Way more than one!   
I've got a membership to this place:

http://www.nyst.com/

If you Google the place, you can probably see some beter aerial shots of the track.
Super grippy pavement, lots of elevation changes....a real blast to ride!

[Doc B.]

Very cool. Out here the hot new track is called The Ridge, though it has been getting some flak lately for being kind of arbritrary with regulations. Might give it a try some day after I get my old street riding skills back. I'm suffering from old age at the moment, messed up my knee tucking on these dadgummed sportbike pegs. Don't recall that being an issue when I was 21.

[Mikey]

Oh man, that place looks really nice!
How close it it to your house?

I can wholeheartedly recommend track days as a great way to hone your riding skills.
The coaching and tutoring you get from the riding instructors is very beneficial, and
much of it translates to street riding as well.


I've been getting spoiled lately, with four tracks within three hours of my house.  The
latest and greatest facility will be opening in the spring, and is less than 1/2 hour away!

[Doc B.]

The locals laugh because it is in Shelton, which is sort of the Appalachia of Western Warshington. It's about 45 minutes away, not too bad.