Preamp PT

[ampgeek]

Sorry mate!

Here is the dedicated spec. sheet.

Disregard the secondary CT and run the 5V primaries through a bridge rectifier. You will end up with 10 VDC to 14 VDC depending on loading.

Regardless, not enough for running 12VDC reliably on the filaments.

The last B+ PS board I made for a pre-amp used a White Cathode Follower (WCF)/balanced transformer output and included a tube driven reverb circuit. The only way that I could see to do it "right" was to use two, independant VVRs. One for the WCF and one for the rest of pre-amp starting at the reverb node. A B+ voltage divider elevated the heater CT. So...no DC filaments incorporated on this one.

The rest of the filter caps went out on the turret board in typical fashion.

I would mount the mosfets off of the PCB the next time.

Homegrown, 1/32" thick PCB using the toner transfer/laminator approach works really well for my low tech, totally one-off needs (read:hack!)

Too "far" over your head?!? Nah. My theory is that we "adicts" remain a constant level underwater. We just grow taller with every new success and...failure.

Have you finalized pre-amp spec and/or design?

Cheers,
Dave O.

[Smokebreak]

Thanks Dave.

Here's my question : Looking at this datasheet http://www.mouser.com/ds/2/177/5c0013-48724.pdf

Which filament xformer would I use, if I wanted to run regulated 12VDC supply for 3 12AX7s? I'm trying to figure out if I need to be looking at the parallel or series number for rectification to get to ~16VDC, that I'll regulate at 12VDC.
I'm using Banika's MkIIC+ layout, that's the plan for now.

[ampgeek]

The 229B12 with secondaries in series and a FWB rectifier will get you there with some margin (900 ma available vs 450 ma needed). I think that you will be at ~16.4 VDC pre-regulation.

The LM7812 regulator is good for 1A so plenty of capacity there too.

Rock on!
Dave O.

[Smokebreak]

Thanks a bunch for the help Dave!

I ordered an Antek toroidal PT last night and got my Mouser order today, including this neato Hammond box.

I drew up a power supply for the MKIIC+ preamp, and will try regulated 12VDC heaters. I'm really looking forward to this build, as nearly every corner I've turned so far has been a "first"..should be interesting!

[ampgeek]

My pleasure Jeremy!

Double check the tube socket wiring on your schematic. All pin fours will want to be wired in parallel and all pin fives will want to be paralleled.

Rock on,
Dave O.

[Smokebreak]

Like this? Updated ^^^

[ampgeek]

Thats the ticket!
Dave O.

[DonMoose]

I think that you will be at ~16.4 VDC pre-regulation.

The LM7812 regulator is good for 1A so plenty of capacity there too.

Rock on!
Dave O.

(16.4 - 12)V * 1A gets you in the neighborhood of 4.5W to dissipate in your regulator. A TO-220 is definitely gonna need a heatsink for that if you want long life. Since your load is pretty constant, and you'd kinda like a slowish startup, I'd think about putting an RC in front of the regulator to take some of the heat.

According to http://www.fairchildsemi.com/ds/LM/LM7805.pdf, you have a dropout voltage of 2V @1A, so a 2-ohm/5W resistor and a 10uF to 220uF cap would do you nicely - get your dissipation down to 2.5W or so in the reg and the other 2 in that resistor.

Hope this helps!

[Smokebreak]

Thanks Don. Since I'm running 6 triodes w/ 12V series filaments @ 75mA each, or 150mA per 12ax7(3), would that change the math, as the regulator only needs to handle ~450mA?

[DonMoose]

Thanks Don. Since I'm running 6 triodes w/ 12V series filaments @ 75mA each, or 150mA per 12ax7(3), would that change the math, as the regulator only needs to handle ~450mA?

It would cut your power by half, and 2.5W is pretty reasonable for a well-ventilated TO-220. I'd probably still do the resistor thing to spread the stresses, but I like bulletproof.

[ampgeek]

Nothing wrong with bulletproof!

Just as a point of reference, I have run an LM type regulator in a 6.3VDC filament system at 4.8W dissipation for a couple of years using a simple clip on type heat sink. This is in a cage type guitar amp that gets plated about 8-10 hrs/week. The sink does get warm to the touch but definitely not burning hot.

Rock on!
Dave O.

[Smokebreak]

After a failed first attempt, my second produced a successful etch using vinegar, peroxide, and salt. It took an hour for the etching, but worked out well! I managed to mangle the traces at the bottom, a bit, cutting the board, but it was before the etching so I used a sharpie for etch-resist. Those are the traces for the relay circuit, which I'm not using anyways

[ampgeek]

Looks great Jeremy. Well done!
Rock on,
Dave O.