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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 28 Jul 2018, 15:25 
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Joined: 05 Feb 2018, 11:49
Posts: 10
I'm finally getting around to posting an update. 8-)

Here are my circuit modifications for using 6CG7 tubes with both headphones and small speakers:

Amp
- 330R input resistor
- 47R CCS resistor or quality 100R trim pot (recommend Bourns 3296)
- Edcor GXPP10-10K output transformer with 6 Ohm secondary
- 4.7uF film capacitor B+ to ground near output transformer
- 6.8R 5W non-inductive wirewound headphone positive to common (recommend Mills)

Power supply, LC filter version
- C2: 220uF 350V (recommend Nichicon PW)
- C3: 100uF 315V (recommend Nichicon PW)
- R1: 0.45mH/16R choke :wizard:
- R2: 500R 5W

Power supply, RC filter version
- C2: 220uF 350V (recommend Nichicon PW)
- C3: 100uF 315V (recommend Nichicon PW)
- R1: 100R 5W
- R2: 47R 5W

Loaded B+ is 200-210V depending on the specific tube. To achieve such a high voltage with this power transformer, filter resistance has to be reduced to almost nothing, so I increased the capacitor values to help compensate. Measured ripple is less than 4mV for the RC filter version.

Late into my design, I lucked out finding some 0.45mH chokes that measure approximately 16 Ohms and are only 43x33x35.5mm LxWxD. I ended up hot gluing them to the inside of my power supply chassis since I could only squeeze them in after the fact. Ripple is practically immeasurable for the LC filter version, nearly a straight line on my scope's 2mV setting.

The chokes are a very nice upgrade; however, either configuration will allow the amp to perform well sonically. The trick is simply keeping ripple low while keeping B+ over 200V if possible, for 6CG7 tubes.

In the amp stage, reducing the input resistor allows slightly more input signal to pass. I installed trimmer potentiometers in place of the CCS resistors and while adjusting their values, measured distortion at the speaker terminals. Approximately 47 Ohms provided the lowest THD and more current has not proven necessary to drive my speakers without distortion. I am using a pair of EPI M60 which are 2-way speakers, 6-inch woofer, DCR 5.5 Ohms (8 Ohms nominal), and ~90dB sensitivity. I decided to leave the trim pots in place since I used high quality Bourns 3296 types and they provide some flexibility to calibrate for other tube types able to run in this voltage range.

The output transformers are Edcor GXPP10-10K with 6 Ohm secondaries for running speakers. I measured a slight benefit increasing the local bypass capacitor value on the output transformers with the RC filtered supply since it is a less efficient filter at these resistor values. The LC filtered supply makes raising this cap value mostly unnecessary.

A well-matched 6.8 Ohm non-inductive wirewound resistor is installed from each headphone positive to common at the jack. The load presented to the tubes is between 5 and 6 Ohms across the 32-600 Ohm headphone range. The jack has NC contacts to switch the resistors out of circuit, and speakers in circuit, when headphones are disconnected. 2-5W resistors should be more than fine, but I had Mills MRA-10 10W available. Metal film is a less expensive alternative here.

Headphones are plenty loud by 2 o'clock on the volume knob. Speaker levels, given 1-2V line level input, are what I would call night time listening volume, or quiet daytime listening volume, sufficient for my musical needs. Best for small speakers in a near field monitoring configuration. The tuner I am using has a variable output in addition to the line level output, which provides slightly more volume—I measured better than 60dB at 1 meter playing rock radio. The amp will easily handle 4-5V input, so professional gear or a line buffer/preamp will work fine into the inputs, and give more output. Distortion with line level input is very low at less than 0.039%.

So, how does it sound? Marvelous! Thanks again to all participating in this thread and of course, our fearless leader, Bruce.


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PostPosted: 28 Jul 2018, 21:09 
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Joined: 04 Jun 2008, 20:59
Posts: 3787
Location: Arizona, USA
Hi, Nice build and good information. For such a simple design and one that I nearly was not going to make into a project for diy use it is an amazing device. Variations are certainly possible and only increase the possibilities for user customization. I had considered originally that 6CG7s would be good candidates for the amp, but while I had some I found they were sort of scarce and getting costly. Thus the 6DJ8 was used. Good gain and reasonable current capability. Nearly any dual triode or pair of triodes will work and really even pentodes would as well.

Good listening
Bruce

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PostPosted: 28 Jul 2018, 22:27 
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Joined: 05 Feb 2018, 11:49
Posts: 10
Thanks for the kind words, Bruce. It really is a great circuit.

I realize now that I mistyped the choke value, they are 0.45H (almost half a Henry). Pretty amazing for such a small package and low resistance.


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PostPosted: 19 Aug 2018, 01:55 
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Joined: 30 Nov 2014, 06:38
Posts: 4
riverty wrote:
I'm just gonna leave this here.
dead image
Come see more at: dead link
Special thanks to Bruce Heran and Matt over at Cascade Tubes.
Enjoy...

Didn't find a way to edit my previous post so quoting the old and adding the new:

I'm just gonna leave this here...

Image

Come see more at: https://sevenhifi.com/6dj8headamp/

Special thanks to Bruce Heran and Matt over at Cascade Tubes.

Enjoy...

Image

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PostPosted: 22 Aug 2018, 04:53 
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Joined: 09 Aug 2018, 03:15
Posts: 11
Location: Forsbacka Sweden
So, this project is growing now. I have started with the power supply parts. Made an PCB and started it up. Its working and the missing parts is some Sprague Atoms for the final stage. In my opinion this is going to be easier when this part is done. Hope I got it all right and noice and hum is eliminated.I suppose its perfectly allright with the high voltage before a decent load is attached?


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PostPosted: 25 Aug 2018, 15:11 
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Joined: 04 Jun 2008, 20:59
Posts: 3787
Location: Arizona, USA
Hi, 282 is about right for an unloaded power supply.

Good listening
Bruce

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PostPosted: 01 Sep 2018, 19:41 
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Joined: 01 Sep 2018, 19:08
Posts: 1
I am new to the sight and have just finished wiring my first
project, the 6DJ8 Headphone Amp. My amp is not putting out
any sound from my ipod input and my voltage readings have changed
since I first turned it on. Not sure what to do next. When I first turned
it on, the tubes lit up, 6 volt supply working ok, and I was getting voltage readings of 162 vdc at
the right and left outputs of the power supply, also 156 vdc at the plates of both
tubes. Before doing this, I had put 75 ohm resisters at both outputs. After turning
off the unit and hooking up the ipod and headphone, I switched it on to find
no sound at all. Checked voltages again, I now have 290 vdc at the power supply
and at each plate. Not sure what happened. I was worried that the high DC at
the plates would distroy the tubes. The tube specs say 130 vdc max. My tube
tester shows good emmision on both tubes, so I think they are ok.
Also, I did not use the 120 ohm/x2 capacitor setup to mains ground. I thought
I could float the DC but maybe I was wrong. Any suggestion on what to check
first?


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PostPosted: 03 Sep 2018, 10:24 
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Joined: 04 Jun 2008, 20:59
Posts: 3787
Location: Arizona, USA
Hi Everyone, this was answered in detail by PM. Initial thoughts are the cathode portion of the circuit is not functioning, probably because of a grounding issue or LM317 failures. Since both channels failed, most likely a grounding issue. The reason for the X2 and parallel resistor has been explained elsewhere in depth and is needed or something like it (sometimes rectifiers in that location) for the metal portions of the enclosure to comply with electrical codes and act as an EMI shield.

Good listening
Bruce

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PostPosted: 04 Sep 2018, 06:27 
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Joined: 09 Aug 2018, 03:15
Posts: 11
Location: Forsbacka Sweden
My metalbox is just days away right now.. and I got some sparetime so this is what I did:
viewtopic.php?f=11&t=6332
Hope to use it with the amlifier in the near future. Also own a pair of B&W P7. I will try to use the 75 to 150ohm version for both headphones. The P7 are low. Just under 30ohms and the HD600 are at 300ohms. But what is written about it earlier in this tread it will work fine anyway.

Regards Kalle G

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PostPosted: 12 Sep 2018, 07:12 
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Joined: 09 Aug 2018, 03:15
Posts: 11
Location: Forsbacka Sweden
So... this is the output box of the Amp. I have a separate one with powersupp. I have started wire it up and i think it will be nice. I needed some real long extenders so fit the top of the box and still be able to put everything in place. I hope it will be fine with separate metal boxes, so you can put output transformers away from main power transformer. Also, less heat issues in the tube box sins I think the heat will be in the powersupp box.


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