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6DJ8 (ECC88) Tube Headphone Amp Project
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Author:  laurie54 [ 08 Feb 2019, 14:08 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Just what I was looking for. Thank you Bruce.

Author:  Renaissance Man [ 11 Feb 2019, 00:05 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

I have built this amp according to the schematic and have quite enjoyed the results. I did some experimenting to flavor the sound more to my taste. Since it is such a simple circuit, very small changes can be readily heard.

The biggest change in sound quality came when I ran the ground of the CCS to a -6 V DC supply. The sound was much more clear and detailed. I am thinking that the LM317 is running in a non-linear area with the cathodes at the typical 2.5 V or so. Connecting the LM317 to a negative voltage allows it to work in a more linear region since the voltage difference is now around 8.5V instead of 2.5V. Or course this adds complexity to the amp, but I think the results are worth it. I ended up adding a separate 6V transformer and rectifier to build a separate CRCRC filtered supply for the CCS.

Author:  Renaissance Man [ 11 Feb 2019, 00:21 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

I almost forgot, since I didn't need the cathode to have as much voltage to make the CCS work, I also lowered the B+ to 125V and the current to 20mA per channel. The tube is still operating in a linear region, it is just is not as close to maximum dissipation now so it should last longer.

Author:  gofar99 [ 11 Feb 2019, 08:45 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hi, that is fine. I have not had any tube failures in this circuit yet. But it is running close to max dissipation. The alternative to a negative supply is to apply an equal positive voltage to both grids like in the Poddwatt DMBs. The LM317 ought to be a TO-220 type though as its dissipation is more but no heat sink is needed. This will keep the 317 from the non-linear region below +4VDC. It will allow for more output. But the level in mine is really much greater than needed. Up to 50mw of so is super clean. Down below 0.4% distortion. You can get a lot more power but the low gain of the amp means it will need more signal than many sources can provide. This is after all a super simple, low cost design. I have a big guy in the works at present that can deliver over 1 watt and will do desktop speakers as well as phones. The initial one ought to be ready this week.

Good listening
Bruce

Author:  Nim [ 23 Feb 2019, 11:59 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

It's been a while since my last post, but I've finally reached the wiring stage. I mounted the power supply components on stripboard, with virtually all of the unneeded traces removed.
I lacked the tools to deal with the 8mm aluminium front panel though, so now the controls are mounted on top of the case, as are the tube sockets.The "drill press"(a drill in a holder) was anything but precise, so it didn't end up looking as neat as I planned it, but I'm fairly pleased for my first time drilling into aluminium.
Image

The boards are all mounted on metal standoffs and isolated by two polyamide washers. The output transformers are just mounted on two of the polyamide washers.
Image

I now have a new set of questions regarding how I best wire this all up. I'm used to dealing with keyboards and usb power, so this if fairly new for me:
  • The case is made up out of 6 different pieces of aluminium, 5 of which are anodised. How do I best go about getting a chassis ground hooked up without causing a galvanic corrosion mess?
  • Can I hook up the 1uf capacitor on the output transformers directly between pins 2 and 8 on the transformer, basically using pin 8 as the ground point for each channel?
  • Could the ground lug/the x-2 RC filter on the line filter be used as the gathering point for the power/signal and chassis grounds, or is the chassis ground point a better option for that?

I went through the article on grounding audio gear, but I've not figured out how to apply that to this specific case yet, so I hope these questions aren't too silly.

- Geoff

Author:  Bradley [ 23 Feb 2019, 14:10 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Nice work. 8-)

Re: chassis ground, I would drill a hole, mount a screw with washer(s) and nut, scraping some of the anodization to allow washer to make contact on the chassis inside. Then either wrap wire where it will fit between nut and washer(s), or use an eye connector.

Author:  ILoveHiFi [ 23 Feb 2019, 16:11 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Markings on the board will deffnitly help precision, also a punch which will help the drill center to the hole.
Galvanic corrosion is really when you place really disimlar metals togehter with the correct combination.
In this case in won't happen as anodising isn't really a metal and just colour.

Chassis ground is the better point for gathering ground. As the line filter will have transients that pass through, causing extra voltage drop and noise to appear in amp.

Not entierly sure but the 1uF on transformer I think is supposed to dampen the wiering resistance to b+.
This means should be connected from pin2 to pin8 to dampen wirering reistance.

Having pin 8 as ground point for each channel does sound good but not sure if thers better ways to do it, so this means your sperating left and right grounds, kind off.
That means you need ground from mains directly to the two transformers of pin 8.

EDIT: as the transfomers are quite close togehter I recomend connected a bare copper wire between pin8 and having the middle grounded to mains.

Author:  Nim [ 23 Feb 2019, 17:07 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Oh these holes were punched, the press was just that clapped out. It had a serious case of the weeble-wobble.
The corrosion I'm worried about is when using plain old washers/copper wire. If I want any conductivity I'll have to strip the anodised layer and the oxide layer, but that means dealing with the nasty reactivity of actual aluminium.

Author:  Bradley [ 23 Feb 2019, 18:02 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

A dab of CorrosionX should resolve any concerns.

Author:  ILoveHiFi [ 23 Feb 2019, 18:24 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

steel or stainless steel washers are not a concern to aluminium galvanic corrosion. I never encountered any with my amp builds on alloy boxes

Some mains rated 220v wires are 0.75-1mm thick copper wire and is bare.

Simple flat screw driver can be usefull in removing the anodised layer.

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