hybrid PSU for Ear+HD and transformer potting (pics warning)
Mar 17, 2006 at 5:25 AM Thread Starter Post #1 of 21

Voodoochile

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<< pics added of finished potted trafos down in post #17>>

Finally am making some headway on my custom Ear+HD. More details on the overall project to follow later, but this is the PSU portion of this trip. I disconnected the B+ in my existing amp and lashed this PSU to it; and borrowed the filament power from the amp for the PSU's rectifier heat.

I've been running a bunch of sims for a while, and finally breadboarded (plywood-ed?) it this weekend with a solid state bridge. It worked very well, so I continued on tonight by replacing half the bridge with a tube rectifier as per my end plan, and fired it up (no pun intended).

Make sure the kiddies are asleep before you try this at home...
12x4_lashup.jpg


Detail shot of rare multi-ply fir circuitboard:
12x4_breadboard.jpg


The original Ear+HD uses four FREDs and a C-R-C-R-C filter in the PSU. This dicey looking rig is a hybrid bridge made from two FREDs, the tube rectifier (+ out) and is now C-R-C-L-C-R-C. The LC filter was a later thought, after scrounging up a nice NOS Thordarson choke that was suitable. The voltage needs to be adjusted about 14 volts higher... I don't have the right value resistor on hand yet, but otherwise it's running like a champ! The B+ comes up nice and smooth, and the output looks smooth, like buddah!

More details to come, but I'm off to bed now. Been up for 21 hours now, and I have to get up again at 4:15 to go back to work.
rolleyes.gif
Damn work interfering with my projects.
 
Mar 18, 2006 at 2:17 PM Post #2 of 21

erix

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Say Vood,

So you're using the two FREDS as a full-wave and then tube recto or are you making a bridge, half of which is silicon and the other half glass?

And how about that choke - 5H? More?

Did I forget to say NICE FREEKING WORK! I especially like the plywood.
I like to see people tweak this amp all up in the place. It's made for it.

Myself, I dropped dime on a pair of custom output transformers yesterday - I'll have them next week. Stay tuned for another FrankenEar...

ok,
erix
 
Mar 18, 2006 at 4:11 PM Post #4 of 21

Voodoochile

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Quote:

Originally Posted by erix
So you're using the two FREDS as a full-wave and then tube recto or are you making a bridge, half of which is silicon and the other half glass?


The two FREDs and the dual diode rectifier are combined to make a composite bridge rectifier. The AC connects to the junctions between the FRED cathodes and the tube's plates. The cathode of the rectifier is the + rail, and the anodes of the two FREDs are the neutral rail.
Quote:

Originally Posted by erix
And how about that choke - 5H? More?


It's 7 Henries at 60mA, so it's running a bit higher. It's amazing what a difference it makes.
eek.gif


Thanks for the comments, guys. This was my bigger hurdle. I did not want to set off building it like this, only find that it was not to my liking (as compared to my present Ear+HD, which I really love).

Reasoning was originally because when I built my current Ear, I wanted to use the Black Beauty pot I had. It was just too tight to fit, though. So an attenuator is definitely out. I also wanted to seat the HP jack deeper into the front panel, almost flush. There is not enough clearance behind it to do this! Darn tight in there. Finally, I wanted to add one or two more inputs.

The inputs I could fit, but the attenuator and jack will not work in this chassis. I considered gutting the amp and recasing in a slightly larger chassis. Cool, but as usual, when you sell yourself on going bigger, then other things become a possibility. My first thought was tube rectification. After discussing this with Dr. Peppard, he mentioned to me that his original Ear amp had a tube rectifier. Also, his current production PS2 PSU has this composite FRED/glass rectifier bridge. It's a great setup, but it would really drive up the cost of the Ear, so instead it has a normal bridge in it, with the HD version having the FREDs instead. The filament transformer in the HD amp is a 14v/1.5A unit, versus the 12.2v/1A unit in the normal Ear. So it has easily enough juice to handle the tube rectifier's heater. The B+ comes up nice and smoothly, with no overshoot (unlike the FRED version), and has just enough sag that the filaments can warm up a bit as the B+ is rising.

Then the last part was the choke. I had considered this, but only in passing. When I found this compact Thordarson choke, and it happened to be nearly perfecty suitable for this circuit, I started over again with Duncan's PSU software. I also consulted again with Dr. Peppard to be sure I was on the right track. He checked out the sim, and said it looked great! I still wanted to check it out on breadboard before committing to chassis work.

The latest deviation I'm looking at is possibly regulating the filament supply. Still thinking and getting opinions on that.

Seeing how the PSU has turned out, I'm going forward with the amp at last. Ordered the thermal epoxy for potting the two power transformers, and am firming up chassis layout now. It will be a bit sad when it's done, in a way. I'm anxious to finish it so I can use it, but the planning and build is also one of my favorite things to do.
 
Mar 18, 2006 at 6:21 PM Post #5 of 21

erix

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Coolness...

Whatcha using for a tube? The Dr. uses a 6X5GT on the PS2 - an octal. Yours is definately not octal! One of the EZ-types?

What kind of current can you get out of that rig?

ok,
erix
 
Mar 18, 2006 at 7:13 PM Post #6 of 21

rickcr42

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very sound ps philosophy there Marko Polo.Split bobbin power trafos to CRCRC can only be bettered by CLCRC but still about as quiet as it gets.



Is that the transformer you were formerly considering "potting" ?


oh yeah......


[size=x-small]POWER ON BOARD ![/size]
tongue.gif
 
Mar 19, 2006 at 1:43 PM Post #7 of 21

Voodoochile

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Ah, the rectifier is a 12x4. With the PS2, the output is tied to the +6.3v filament reference. Being that my amp has 12.6v filaments, it sits on the center tap of the 5751. As a whole, the PSU is rated for about 50mA. It's drawing close to that. The trafo is the lightest part in the PSU. I could get more if I wanted, but you don't seem to need it. The 12x4 data sheet has the filament ratings on it only, then it says otherwise identical to the 6x4.

Yes, that's one of the trafos I'm going to pot. I'm not a fan of the way it mounts in my existing Ear. The other one is the filament trafo, which is a Tamura 14.4v/1.6A unit. The two OPTs are grossly oversized and have virtually no work to do, so they will stay put as they are. The two power trafos are rather busy, and although you can't hear the filament trafo buzzing, you can feel a slight vibration. It is currently bolted right to the chassis panel.

When I pot it, I'm going to bed a pair of bolts alongside, between the trafo and the pot, rather than through the trafo's frame. Epoxy is hard, but not as hard as iron bolted to steel. Plus, a thin asphalt pad between the potted unit and the top plate.

Power on board? I'm never going to live that down!
Millett on board... I had PPA on board for a while, but I got that one cased up.
 
Mar 19, 2006 at 2:30 PM Post #8 of 21

n_maher

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Quote:

Originally Posted by Voodoochile

Power on board? I'm never going to live that down!
Millett on board... I had PPA on board for a while, but I got that one cased up.



Kinda like smoking resistors, eh?
evil_smiley.gif


Must be very cool to have something you designed actually up and running Mark, think you'll get it done in time for the Nat'l meet?

Nate
 
Mar 19, 2006 at 2:57 PM Post #9 of 21

Voodoochile

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Quote:

Originally Posted by n_maher
Mark, think you'll get it done in time for the Nat'l meet?


I'm hopeful; it's doubtful.
plainface.gif
 
Mar 19, 2006 at 3:06 PM Post #10 of 21

n_maher

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Quote:

Originally Posted by Voodoochile
I'm hopeful; it's doubtful.
plainface.gif



Well, based on your post times it looks like you're burning the midnight oil as it is! I have a feeling I too will fall well short of my goals for the Nat'l meet but what can you do? And it's not like your current Ear is such a slouch either. YGPM by the way.

Nate
 
Mar 19, 2006 at 4:02 PM Post #13 of 21

rickcr42

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Quote:

It is currently bolted right to the chassis panel.


rubber grommets to isolate should be fine to avoid the potential to buzz.You could even use nylon hardware in combination.That or normal mounting hardware but a bit longer and a slice of luan between the panel and the trafos (I like to use some natural materials where possible and especially wood).Dissimiliar materials metal/rubber/wood/metal should clean up any transformer buzzing.

And of course you can then say :

[size=small]Transformer On Board ![/size]
tongue.gif
 
Mar 19, 2006 at 4:38 PM Post #14 of 21

Voodoochile

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Quote:

Originally Posted by rickcr42
....never heard of that fetish before.Anything like sleeping with twins ?
very_evil_smiley.gif



Ever see the movie Robots?

Luan is a good idea also. I was considering making some sort of "mounting pad" from thin plywood to act as both a drilling guide, and also as a jig to hold my bolts while the epoxy is baking. Maybe I could integrate the two, end up with a jig plus two pads.
 

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