ruffrecords

Re: So I threw this together for lack of a better thing to do....
« Reply #20 on: May 08, 2020, 05:24:51 PM »
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
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'


CurtZHP

Re: So I threw this together for lack of a better thing to do....
« Reply #21 on: May 09, 2020, 04:10:21 PM »
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?

Electrons don't read schematics.

EmRR

Re: So I threw this together for lack of a better thing to do....
« Reply #22 on: May 09, 2020, 04:27:10 PM »
Yes, a 12BH7 should burn you if it's doing any work. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

CurtZHP

Re: So I threw this together for lack of a better thing to do....
« Reply #23 on: May 09, 2020, 04:29:06 PM »
Yes, a 12BH7 should burn you if it's doing any work.

Good to know!

Keeps the coffee warm while I listen!

Electrons don't read schematics.

CurtZHP

Re: So I threw this together for lack of a better thing to do....
« Reply #24 on: May 10, 2020, 07:41:48 PM »


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?
Electrons don't read schematics.

CurtZHP

Re: So I threw this together for lack of a better thing to do....
« Reply #25 on: May 28, 2020, 10:51:14 PM »
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!
Electrons don't read schematics.

CurtZHP

Re: So I threw this together for lack of a better thing to do....
« Reply #26 on: May 29, 2020, 11:22:46 AM »
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.

Electrons don't read schematics.

scott2000

Re: So I threw this together for lack of a better thing to do....
« Reply #27 on: May 29, 2020, 11:33:18 AM »
If you don't mind , what model of power transformers are you using?

CurtZHP

Re: So I threw this together for lack of a better thing to do....
« Reply #28 on: May 29, 2020, 11:41:42 AM »
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.
Electrons don't read schematics.

CurtZHP

Re: So I threw this together for lack of a better thing to do....
« Reply #29 on: May 29, 2020, 12:11:54 PM »
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??
Electrons don't read schematics.


mjrippe

Re: So I threw this together for lack of a better thing to do....
« Reply #30 on: May 29, 2020, 12:44:14 PM »
Absolutely.

CurtZHP

Re: So I threw this together for lack of a better thing to do....
« Reply #31 on: May 29, 2020, 12:54:35 PM »
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.
Electrons don't read schematics.

john12ax7

Re: So I threw this together for lack of a better thing to do....
« Reply #32 on: May 29, 2020, 01:10:25 PM »
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.   :)

CurtZHP

Re: So I threw this together for lack of a better thing to do....
« Reply #33 on: May 29, 2020, 01:26:01 PM »
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.   :)

Good to know!  Thanks!
Electrons don't read schematics.

CurtZHP

Re: So I threw this together for lack of a better thing to do....
« Reply #34 on: May 29, 2020, 02:55:43 PM »
Slapped the inputs and volume control back in, and all is well.

Might finish this thing this weekend! 
Electrons don't read schematics.


 

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