16 - 18V tap on Stereomour Transformer

[rlyach]

I updated the circuit with the 80SQ045N (Vf=.26V typical at 25C). I also used a 12,000uF cap and updated the transformer data. The results look too close to call. The DC output of this rectifier on the filament winding with the filament of the 12AT7 drawing 300mA and the stepper motor drawing 400mA is ~13.2V. If I run the simulation unloaded with 4mA standby I get ~15V. The large capacitor will handle the initial surge to get the motor moving. You will have 54ms of surge current before the voltage drops to 13.2V. I also looked through the documentation and could not determine if the stepper motor has a grounded chassis. If it is, this wont work. You could order the optional $10 120Vac to 15VDC power supply, put a metal shield/box around it, and mount it far from the tubes and transformers. 

[Paul Joppa]

Three points:

1) the 12AT7 draws 150mA at 12.6 volts

2) All these calculations were made assuming a 120v power line; for reliable operation you would want 16.5v under this condition so that +/-10% on the power line will still give 15-18 volts. Assuming the spec means the voltage must never ever under any circumstances for any length of time fall below 15.000 volts. Which I doubt - see #3 below.

3) There is no spec for ripple on the power supply - you'd have to ask John at Bent Audio about that.

[rlyach]

Paul,

I am confused. My datasheet for the JJ ECC81 says 300mA or 150mA @ 6.3V or 12.6V. I assumed that this meant 300mA at 12.6. and 150mA at 6.3V. This is for my edification and nothing else.

[Doc B.]

P=IV

[rlyach]

Oops... I was thinking resistor instead of light bulb (I=V/R).

[Paul Joppa]

Oops... I was thinking resistor instead of light bulb (I=V/R).

Haha! TWO resistors, series vs. parallel! Yeah I've done that too!

[drewh1]

OK, I am going to order the power supply and shield it. - I also think it might make sense if I want to move the Goldpoint to my next pre-amp (when I build the Paramounts).  Ok this is probably a stupid question, but if I build the Paramounts, can I convert the Stereomour to act a s a pre-amp?  Man there is so much I do not know about this stuff.  Glad you guys are so helpful.

drew.

[chard]

 In response to question about your stereomour as a preamp.  Some people go Bi-amp. One amp for mids/treble and another amp for base. Each amp has their own speakers. I don't know anything about this solution or the inherent problems. Since you already have two amps this may be a nice solution. Some people who go the Bi-amp route use open baffle speaker designs for both the bass amp and the mids/treble amp. They can get away with an open baffle design for the bass speakers because they can use very large sensitive 12"-18" woofers because they don't have to worry about anything but the bass frequencies. This design gives you independant control over the bass frequencies and the mid/treble frequencies. Open baffle speaker designs my present a more natural acoustic sound as the box that defines it is the room not the speaker cabinet. I think Bi-amps have a flow chart as follows. Others can chime in if I am wrong or give more details.

                                      / power amp-filter-only bass frequencies to woofers
2 output music source                                   
                                     / power amp-filter-only mid/treble frequencies to drivers


                                                                                / power amp- filter - only bass frequencies passed to woofers
1 output music source-line amp ( BeePre or other)
                                                                                / power amp- filter- only mid/treble frequencies passed to drivers

[rlyach]

Back to the tube filament wiring issue... My confusion with the filament current arose because of my ignorance of the difference in wiring between 6.3V and 12.6V. I always thought that the filaments were wired in series, using pin 4 for one connection and pin 5 for the other. I thought pin 9 was optional. Thus, my reasoning was that if I put 12.6V across pins 4 and 5 I will get twice as much current when compared to 6.3V across pins 4 and 5. After some research, I have discovered my mistake. The worst part is I now realize that I have miss-wired an earlier project, using a 6.3V supply for a series wiring of the heater, which will only give 75mA of heater current. The funny thing is that the 12AX7 worked in this configuration. I will have to go back and correct my mistake. The reason I am relating all of this is so that others who are new to tubes will learn from my mistake. Please confirm the attached diagram. Also, I believe that the 12.6V heater ground connection is optional.

[Paul Birkeland]

Also, I believe that the 12.6V heater ground connection is optional.

Yeah, usually you will see the AC winding center tap grounded or biased up if heather/cathode insulation ratings are an issue, it's actually fairly uncommon to see the arrangement in the Steremour. 

[rlyach]

Drew,

Now that I have everything straight... I re-ran my simulation using the proper filament model. I also updated the transformer model to match Paul's unloaded performance. The results are a little more encouraging. The unloaded output voltage is 16.4V and the loaded case is 14.4V. This might have a chance at actually working. The only question that still remains is the state of the ground connection within the stepper motor. Bent Audio should be able to answer this question. If the motor is capable of floating you might want to give this a try.

[drewh1]

that's great to hear, thanks Randy - You have been really helpful.  I really like that site you did your simulation on. That will be very helpful to me as I learn more.

I'll double check with John Chapman at Bent Audio on the stepper motor, he has been very responsive. I'll have the unit next week and keep you posted. There might be other couch potatoes out there that want to integrate a remote volume control.

Also putting any speaker build plans on hold, things are starting to sound really good, bass is improving and Roy at Green Mountain Audio is sending me some updated Xover's to try out in my speakers.

[rlyach]

Drew,

I know this is not optimal, but I handle the "remote" issue by using iTunes to send digital data to my DAC. I leave the Stereomour volume all the way up and use iTunes to control the volume. iTunes scales the data to reduce the volume. I know this introduces scaling errors, but for most of my listening it works well. When I want to listen critically I set iTunes to full scale and use the step attenuator to control the volume. With this setup I can use my iPhone, iPod or computer to play music and I always have a "remote" with me that controls song selection and volume. Good luck with the Bent Audio control.

[Paul Joppa]

With respect to grounding the center tap ... it is optional with respect to making the cathode hot enough to emit electrons, but the Stereomour has other functions that glowing ...  :^)  in this case, the 12.6 volt winding in the transformer also serves as an electrostatic shield to prevent power line noise from affecting the 2A3 filaments. To do this, the winding must be grounded, and for it to couple as little as possible itself to adjacent windings it should be a center tap ground. The winding itself does not have a center tap since it's just one layer of wire, so the CT on the 12AT7 filament provides a convenient place to do it.