So I threw this together for lack of a better thing to do....

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CurtZHP said:
That's good to know.  I figured there had to be an alternative.  Those Sowters gave me sticker shock.
I wonder if OEP has something viable.  I've been using their input transformers in mic preamps and really like them.

I figure I'll go ahead and finish this thing as is and leave room for the transformers to be added once I find them.

I don't think OEP do anything that big but I may be wrong. You could always use one of the but Carnhill 10K:600 transformers, they are very reasonably priced.

Cheers

Ian
 
ruffrecords said:
I don't think OEP do anything that big but I may be wrong. You could always use one of the but Carnhill 10K:600 transformers, they are very reasonably priced.

Cheers

Ian

For grins I took a look at Edcor's site, and apparently they will make the transformers you mentioned earlier for about $20 a piece.  The catch is that I'd have to wait about 6-8 weeks for them.

Upon further research, the OEP's don't appear to have sufficient power handling capability.  I'll check out the Carnhills.

In the meantime, cleaned up the wiring job and made some recommended replacements along the way.  Fired it up for a final smoke test and gave it a listen.  To my ears it passes sine waves pretty cleanly with ample juice to drive my AKG cans.

My only concern at this point, and it may be nothing to sweat about, is how hot the 12BH7s get to the touch.  Like, to the point where touching them is a bad idea!  I measured with a temperature probe and got about 150 degrees Fahrenheit.  When I was building my last tube project, I had a 12AU7 in a PP stage that was cooking at around 140.

Tried to find out, as I did on the previous project, what to expect as far as this is concerned, and the internet was largely mum.  Found out some guitar amp output tubes can reach temps in the mid-200's, but not much else.

Cause for concern?

 
EmRR said:
C9/10 into low-Z headphones should be very large, they're scaled now for 10KΩ.  330mfd or so for Sony's I think.    I see 470mfd in some.   


Silly question....
Is a cap that large meant to be a substitute for an output transformer?
 
So, I decided to build this thing for real.  Cleaned up the wiring and implemented a few of the suggestions here.  Had some really nice cabinet-grade birch plywood, left over from another project, which I used to build a nice little cabinet for it.

This raises a question...
If the "chassis" is wood, how does one go about connecting the earth ground from the power inlet?  I tried just connecting it to the 0V ground, but this proved to be a bad idea.

It seems to have developed a hum, which I need to hunt down and address.  Connecting the earth ground to 0V makes it worse.

As for the initial hum, the only real change I made to the circuit was the final output capacitors.  I took out the 1uF poly caps and replaced them with 470uF/200V electrolytics.  (Hoping to eventually put the 1uF's back, followed by Edcor output transformers, the way you're supposed to build these...)  But, I can't imagine that would suddenly make it hum.

The hum is present regardless of volume setting, so I can rule out any silliness with the input jacks and pot, but that leaves about 90% of the circuit to pick through!
 
Took a look at it this morning.
Perfect 60Hz hum on the output.  Ground loop?

Put the original 1uF output caps back in, just to get me somewhat back where I started.  No change.  Not surprising.

Monkeyed around with the input jack and volume pot grounding and got nowhere with that, so I took it all out for now.  Shorted the input to the first tube stage for both channels.  Hum is still there.  Pulled first stage tube.  Hum is still there but not as loud.  Tried a different first stage tube.  No change.

The only other thing that changed from the original build to this one is that the wires from the input jacks/pot to the first stage and the wires from the output to the headphone jack are now Belden 8451 shielded cables (shield as ground, black as left, red as right).  Previously, they were just individual wires.  Sorely tempted to go back to that scheme.

Carefully studying the schematic, trying to find potential ground loops.  This is what bit me in the butt on my last build, but that was a little more complicated.  More opportunities to get in that sort of trouble.

 
scott2000 said:
If you don't mind , what model of power transformers are you using?

Don't mind at all...

Triad VPT series toroidals.  I use them on other tube gadgets I've built with no issues.  Used to use Amveco, but can't find them anymore.
 
Well, this is interesting....

Decided to consult The Valve Wizard and read his page about heater supplies.  He says:

"Rule number one is that the heater supply must have a DC connection to audio ground. This may be a direct connection or an elevating circuit (see shortly). This is equally true for AC or DC supplies. Leaving the heater supply floating will result in almighty hum due to primary-to-secondary transformer leakage current, and is a common beginner’s error."

Looking at my build, the heater supply, which happens to be 12VDC has its very own ground, separate from the 0V ground for the rest of the circuit.  The schematic is a bit deceiving in this regard, because it shows the same symbol for heater ground and 0V, with no distinction being made between the two.  This suggests that they are connected, but in reality on the bench they are not.

Could this be the issue??
 
mjrippe said:
Absolutely.

Ran a little jumper from the heater ground to 0V.  So far, without the input jacks or volume pot connected, no hum!

Going to reintroduce the input hardware and, hopefully, it continues to behave.  I guess my original build was correct in this regard, and I just missed that detail.
 
Just a note about Edcor,  I ordered from them before and the website also said 6-8 weeks.  But then I had them at my door in about a week.  :)
 
Resurrecting this thread because I rebuilt this thing.

Added a pair of Edcor transformers on the outputs and bought some new tubes.  Audio sounds good through it, but the hum is back.  Could be sloppy wiring between the circuit and the output transformers.  Or the fact that the output transformers are too close to the power transformers, with nothing between them but a thin sheet of metal (part of the chassis).

Time to fire up the test gear and track it down!

 
Popped it apart while it was running and was able to physically move the output transformers away from the power transformers.  Hum faded away to just about nothing when I did that, so that must be the culprit.

The chassis is a very small box.  About seven inches square and about 3 1/2 inches high, so not a whole lot of room inside.  The tubes and output transformers are mounted on top, out in the open, to save space inside and to help the tubes dissipate heat.  The output transformers ended up right above the power transformers, because there really wasn't anywhere else to put them.

I suppose I could build a bigger box and put some shielding around the power transformers.  Or put them in a separate box with an umbilical between that and the amplifier.

It didn't hum before I added the output transformers.  I guess that either made it more sensitive to that sort of thing, or the transformers need shielding, or I got sloppy with the additional wiring.

Maybe I just need to change the whole layout.


 
CurtZHP said:
A tiny bit on the noisy side (
That is not surprizing. You have lots of gain, that is probably not required. A typical headphone amp driven by a HiFi source (typ -10dBu) requires less than 20dB gain for driving headphones VERY loud.
Here you have 33dB in the first stage. You may want to reduce gain by removing the cathode caps.
 
abbey road d enfer said:
That is not surprizing. You have lots of gain, that is probably not required. A typical headphone amp driven by a HiFi source (typ -10dBu) requires less than 20dB gain for driving headphones VERY loud.
Here you have 33dB in the first stage. You may want to reduce gain by removing the cathode caps.

I might do that.  But there is definitely an increase in hum with all the iron sitting right on top of each other.  (By the way, the output transformers are not shielded at all.)  This is even with no input and the volume pot all the way down.

This morning I took all the guts out of the box and laid the whole thing out on the bench.  Situated the OPT's a good 6-8 inches away from the power transformers, and that was enough to improve things.  The only problem is that this layout won't fit in the existing box.  Not a real big deal.  I have enough scrap lumber to build another one a little bigger.  Holiday weekend here in the US, so I've got nothing better to do today!

 
So, here's a really stupid question....

The chassis for this gadget is made of wood.  The top panel, to which are attached most of the electronics, is acrylic.

There's no metal chassis to attach chassis ground from the power inlet.  Is it OK to just attach it to power ground?  I'm thinking yes.
Wanted a second opinion .


EDIT:  Disregard.  The acrylic is not working out as planned.  It'll be metal after all.
 
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