Anyone ever made the EL84 P-P circuit referenced on the Tubecad Journal site? - diyAudio
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Old 5th August 2010, 07:12 PM   #1
cjkpkg is offline cjkpkg  United States
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Default Anyone ever made the EL84 P-P circuit referenced on the Tubecad Journal site?

I am looking to make a small P-P amp and this little circuit seems to fit the bill nicely. This will also be my first P2P wiring attempt. I will however probably purchase Broskie's PS-4 psu board.

Info HERE
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Old 5th August 2010, 07:30 PM   #2
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That circuit is fairly common. If you look at the Tubelab Simple P-P, SY's Red Light District, Morgan Jones Bevois Valley, and Broskies circuit, you will find them very similar. Why? Even though most of these appear to be independently developed, this seems to be the simplest way to make a small P-P amp with good performance.
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Old 11th August 2010, 10:11 PM   #3
cjkpkg is offline cjkpkg  United States
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I will be using edor CXPP OPTs'. They have a provision for screen taps. Which screen from the EL84 would these tie to? The Plate or the Cathode? Not recommended?

Thanks,
Carl
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Old 11th August 2010, 10:52 PM   #4
cjkpkg is offline cjkpkg  United States
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here is a schematic of the base pinout...Click the image to open in full size.

From this it looks as though g1 is input and g2 is either tied to the plate with the 100R or could be tied to the screen tap of the OPT?
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Old 11th August 2010, 10:58 PM   #5
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The "screen" is a specific grid inside the tube - on the EL84 its g2.

Look here.
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Old 29th August 2010, 09:11 PM   #6
cjkpkg is offline cjkpkg  United States
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OK,

This morning I finalized one channel and fired it up for a smoke test. All went well so I put the tubes in. I must say that I am very happy with this PP amp.

Next on the docket...
1. Clean up finished channel wiring
2. Complete second channel
3. Put bias pots on the EL84's
4. Implement a CCS for EL84 cathode bias

Pics to come in the next couple days...
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Old 1st September 2010, 02:15 AM   #7
cjkpkg is offline cjkpkg  United States
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UPDATE
Both channels are done! I still need to implement a pot for bias adjustment and maybe a CCS for the EL84 cathodes...

Not the prettiest wiring, I'm sure but for my first P2P wiring job I think it came out OK...Just got to listen for a couple mins tonight before I got yelled at by the 3 yr old...

Also, a fellow board member was nice enough to point out an error in the schematic...The shared cathode resistors for the EL84s should be 200 ohm, not 400 ohm. I bought matched tubes but I still need to take some measurements to see how they are behaving...

Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
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Old 1st September 2010, 05:11 AM   #8
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Nice job!

Please post the part numbers for the power and output transformers.

Thank you.
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Old 1st September 2010, 12:05 PM   #9
cjkpkg is offline cjkpkg  United States
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XPWR007 120V 60Hz to 550 volt (275-0-275) @ 200mA,
6.3 volt @ 6A, 5.0 volt @ 3A
Power Transformer
RoHS Compliant

CXPP25-MS-7.6K 25Watt 7600 Z ohms to 4, 8, and 16 ohms
40% Screen Voltage
Classic Push Pull Transformer
RoHS Compliant

Maybe I missed it last time but they do show a 8K Z ohm OPT now...I swear it wasnt there when I ordered mine...

There is also a non-multiple secondary version that is a couple bucks cheaper too. I have a couple different speaker drivers in the stable so I figured I would go with the multiple option.
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Old 1st September 2010, 01:16 PM   #10
TheGimp is offline TheGimp  United States
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Nice job.

Why no power switch?
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