Need help laying out power supply with small PT

[markmalin]

Hey guys, this should't be that hard I think...but I'm struggling to understand this.  I'm going to reveal my ignorance, so please be patient with me.


I'm laying out a tube pedal and chose the PT from a Fender stand alone reverb unit because I thought it would work well to supply the plate voltages for 2 12AX7's.  The primary is spec'd at 120 VAC, the secondary is spec'd to deliver 306 "V DC" (see attached).  I'm assuming that note in the PT diagram to mean, once rectified to DC, I can expect ~306V DC, correct?  I'm experimenting with my Hammond P-TF47609 primary hooked up to AC mains (120VAC), secondary into a bridge rectifier, and at the output of the bridge I see more like 356 VDC.  Is this because I don't have a load on the supply?  In other words, would it drop to ~306'ish with a load?


I was hoping I could breadboard the power supply, 3 20uf/500V filter caps and 2 dropping resistors and figure out how to get more in the range of 250VDC for my B+ voltages...so that using 100K plate resistors I'd be in range (175-190 VCD) for the plates of the 12AX7's, but this is not turning out to be as clear as I thought it would be.


If you look at Robinette's schematic below for the Fender 5F1...for example he's showing ~300 V before the choke, about 295 after the choke, then after the 10K dropping resistor he's showing 250 VDC.  If I hook up my supply and filter caps this way on my breadboard, and not using a choke, but say a 10K in place of the choke, I still see like 352 VDC for the two B+ voltage nodes.  Again...is this because I don't have a load, or is it just that the PT I bought is giving me too much voltage at the secondary?


My original pedal project used the 12 VAC to 120 VAC toroid in Doug's original design, and laid out like he did, I get nice voltages at my plates...I want to use 120 VAC as my mains input, though, not a wallwart, so I figured a small power transformer like the Fender Reverb unit transformer might be a good candidate, but I'm not sure how to get that secondary 356 VDC dropped to the range I need.


Again, no flames please... If there's a good schematic out there you're aware of which has a similar supply that I can model mine after, feel free to point me to the link.
 

[sluckey]

In other words, would it drop to ~306'ish with a load?

It will drop to ≈306 ***IF*** you load it to 27mA. But your two 12AX7s will only be a light load of maybe 4mA to 6mA. That's OK. Just means you will need to use fairly large dropping resistors to get the voltage where you want it. I would just build your circuit and have a variety of 3 watt dropping resistors on hand to experiment with until you get the desired voltage.

3 20uf/500V filter caps and 2 dropping resistors and figure out how to get more in the range of 250VDC for my B+ voltages

That's a good plan. Resistors will likely be on the order of 22K to 47K, maybe higher. I wouldn't even bother trying to "figure" out the correct values. Just tack solder different resistors until you are happy.

[markmalin]

Thanks for the sanity check, sluckey!  Looks like I'm heading in the right direction. :)


Mark

[tubenit]

Mark,

I used a Weber RV trannie for a single ended build.  These are the actual voltages I got.  Don't know if this is useful or not?

Jeff

[markmalin]

Jeff, as always, that's super helpful!   Thanks again to both you guys.


One question, why the 220K across the second 40uf at node A?  Is that making the 1K/3W and the 220k act like a voltage divider, or did you do that to bypass high frequencies to ground there?


Mark

[shooter]

why the 220K

I use them as cap bleeders after power off, only takes ~~~ 3min to get the caps reasonably safe, <50vdc, without it they may take 8-10min to bleed down

[markmalin]

Nice!  Thanks, shooter.  I'm going to have to start doing that.  It's amazing how long a fresh Sprague Atom can hold at 300 VDC.


One other thing - why 40uf at the first two nodes and 20 at the others?  Just more filtering for the higher voltage DC?  I ask because I've chosen 20uf/450's for my pedal because of the relatively small size...(and Illinois ...cough cough... instead of Sprage Atom because of the size and cost)

[shooter]

Just more filtering

I shoot for <10mV AC ripple in the pre-amp section.  The 1st PS tap goes a long way in reaching that goal.  Since I usually build SE, there is no "cancellation" like PP offers, so to keep hum low, good PS filtering is essential

[Willabe]

One other thing - why 40uf at the first two nodes and 20 at the others?  Just more filtering for the higher voltage DC?

The 1st filter cap stage see's the most ripple.

Each stage of filtering leaves less and less ripple on the DC. So you don't need as much B+ filtering the further down stream you go.

And by the time you get to the preamp stages, they draw less current from the B+ filter caps. Ripple is the charging and discharging of the filter caps. Small current draw, smaller ripple.

These 2 things together, leads to smaller caps later down stream for the B+ filtering. Some amps use only a 10uF or 8uF for the last filter cap.

[markmalin]

Thanks, Willabe.