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PostPosted: 20 Apr 2018, 06:45 
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Joined: 05 Feb 2018, 11:49
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:geek: Good news, everyone!

After months waiting, I finally received transformers from Edcor to finish my electrical build. The only modification at this time is pinout conversion for the power transformer, as I am using the shielded version with the 8.5V heater tap. As most have previously reported, the noise floor is dead silent and boy does this amp sing.

Trying out the stock build, my favorite NOS 6CG7 tubes sound excellent through my Sennheiser HD-595; very dynamic with accurate highs. Bass is of course tight on these tubes almost to the point it sounds mildly diminished. Once I get the voltage up a bit by changing the dropping resistors, they should sound slightly more linear. I took a quick photo of the open air build since the chassis has not yet arrived, and made some quick THD measurements. These are barely scientific since the entire amp lacks a proper shield, but I hope reasonable enough for curiosity's sake. Impressive as-is, and something to revisit once I get everything bundled up.

JSC Voskhod 6N1P-EB tubes from The Tube Store (no affiliation) also sound very good while slightly less dynamic to my ears. Much improved bass response compared to the 6CG7 in the stock config. For $10/tube USD, they're one heck of a bargain. While the B+ hovered around 150V using the 6CG7, the 6N1P-EB were pulling 175V.

I did try running a set of small 2-way (6" and 1") bookshelf speakers with a DCR of ~5 Ohms and they sound fine at the lowest nighttime volumes. As expected, there is some distortion with moderate signal due to lack of current. I'm still waiting on proper 6 Ohm output transformers from Edcor, so my next step is to try for enough amperage to produce lower distortion at normal listening volumes through those.

Again, many thanks to Bruce and everyone here for a wonderful amp and fruitful discussion.


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PostPosted: 20 Apr 2018, 08:50 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Good show. You post indicates something that I didn't emphasize.....the circuit can be used with pretty much any dual triode (or lots of singles) As long as the halfs match. Power output is only limited by the tube type. All that is needed to change from one to another is current level and output loading. The 6DJ8s seemed a reasonable choice as many audio diyers have them already and they do a really clean 1/2 watt in the design.

Good listening
Bruce

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PostPosted: 03 May 2018, 17:46 
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6 Ohm output transformers arrived from Edcor. The bad news? I accidentally ordered the push-pull GXPP10-10K instead of the single-ended GXSE10-10K. Can I still use these in any configuration, or should I be placing a new order?

I'm still mulling over whether I want to add a pair of 24 Ohm Mills non-inductive wirewound resistors from the 6 Ohm secondaries to the headphone connections or switch between two sets of output transformers.


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PostPosted: 03 May 2018, 20:10 
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Hi, The headphone amp uses the push-pull transformers. It is a push-pull design. So all is as it should be. 8-)

Good listening
Bruce

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PostPosted: 03 May 2018, 22:57 
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gofar99 wrote:
Hi, The headphone amp uses the push-pull transformers. It is a push-pull design. So all is as it should be. 8-)


Yes, of course! This I knew. No idea what I was thinking.

Here are some photos using jbwebst's boards on Osh Park. As I previously mentioned, I modified the power supply board to accept Edcor's XPWR191 transformer which has a different pinout, in addition to a shield and a 8.5V filament tap.


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PostPosted: 03 May 2018, 23:08 
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And some photos of the completed power supply board (some values modified for use with 6CG7 tubes) and chassis. I'm using a 4 connector M12 cable to carry Common/B+/6V/B+ to the main board in an identical chassis, yet to be completed.


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PostPosted: 04 May 2018, 00:37 
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Joined: 17 Mar 2016, 07:26
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Location: Prêles, Switzerland
Well-done and nice work ! Interesting approach the dual-box. I would check the temperature of the heatsink and case internals after running for 2h. Mine gets pretty hot and that doesn't speak for long-life of the electrolytic capacitors.


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PostPosted: 04 May 2018, 17:53 
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Hi, Yes the Edcor 191 is the same as the 083 just with a copper wrap. I have used both types. Since I generally use either a steel or Mu metal case over them then the value of the copper shield is minimal. Still it can't hurt. I considered using the 6CG7 but decided that the limited availabilty and I didn't need that much power output deterred me. I see no particular reason that any dual triode (like a 6SN7, 12AT7 ........others) could not be used. I would want one that could do at least 10ma per triode though to insure that there was enough output for most phones. A change in the sense resistors and transformer impedance is all it would need. Possibly a B+ change for some tubes (more power output) as long as you stay within the dissipation ratings.

Good listening
Bruce

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PostPosted: 04 May 2018, 18:47 
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@cbueche Thanks for the kind words. Components seem to be hovering around 65°C max. The chassis is only slightly warm. I suspect the 45mm bottom hole I drilled is providing sufficient airflow.

I tested intermittent contact between the power inlet and the chassis, so after I making the photos I added ground wires to the top and bottom halves, and the M12 socket.

@gofar99 Indeed, the design is very flexible. Last we talked about 6CG7 tubes, I believe I was trying for ~30 mA per CCS (using a 41.2 Ohm resistor) with a B+ of ~210 V. I've been able to get the voltage up to 185-190 V by lowering R1 significantly (100-220 Ohms) but of course this yields poorer filtering. I've ordered some 270 Ohm resistors for both R1 and R2 (also have 220 and 330 on hand) and will double the size of the filter capacitors to help compensate.

With extra voltage, my beloved 6CG7s open up nicely. Like a thin veil removed; more forward and dynamic. I suspect in a stock build, increasing B+ to 175-185 V with no current modification would be the best range for 6CG7s driving only headphones.

I did not notice any significant mismatch listening to my 50 Ohm Sennheisers via the 6 Ohm secondaries, though I do plan to add parallel 6.8 Ohm resistors to each of the headphone jack's connected terminals (disconnected when speakers are in use) to create a consistent load given 32-600 Ohm headphones. A brief test sounded fine with plenty of volume control, so hopefully the final addition will measure clean of distortion at reasonable levels.

Continued thanks to everyone! I can hardly wait to get this one finished.


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PostPosted: 05 Jun 2018, 07:37 
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Joined: 02 Jan 2018, 22:26
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Hello all!
Finally I started to mount my headamp, but I found a problem... the output jacks will be in contact with the aluminum chassis, this will make that the PSU and OPTs housing/shielding will share the same ground. The input RCAs will be isolated in the rear wooden part of the cabinet, but the output and the volume pot will be tied with the transformers shielding thru the chassis, what do you recommend me to do? Isolate the output jack and volume pot? or will be ok?

Here is a photo of the mounting first stage.
Attachment:
IMG_20180605_084238416.jpg

Here is a view of the complete design: http://diyaudioprojects.com/Forum/download/file.php?id=9359&mode=view


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