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Bruce Heran 6DJ8 Headwatt Amp Build

post #1 of 24
Thread Starter 

I'm working my way through a build of Bruce Heran's 6DJ8 headphone amp, the schematic is available on diyaudioprojects. I decided to build it because I had most of the parts lying around and the transformers were cheap, so my overall investment is pretty minimal. I've built a number of Bottlehead amps, but this is my first build from just a schematic. The casework is somewhat slapdash, as I decided to try polishing an aluminum enclosure I had lying around. I discovered that polishing aluminum is a pain, it came out a bit mediocre.

 

In any case, I've finished up the power supply and things seem to be working well. While I absolutely hate building on perf board the heater DC is rock solid at 6V. From what I understand, without a load the HT doesn't mean much right now. The only thing I want to do is add a 200k, 2W bleeder resistor. An hour after removing power I still had 75V on the capacitors. I twisted the B+ supplies just to keep things tidy.

 

Next I want to wire up the input, then the tubes, and finally the output transformers. The switch between the pot and headphone jack is to select impedance.

 

 

 

 


Edited by Evan99 - 12/14/16 at 9:13pm
post #2 of 24

Looking very nice!  I dig the turret board and bus bar combo.

 

Unloaded, the B+ won't mean much.  It should be around 1.4 x the VAC output of the power transformer (minus drop across rectifier diodes).  Once you connect a load, the current drawn through the filters by the tubes will drop volts according to the resistance and DCR of the filter components.  Simple Ohm's Law.

 

One note because I can't quite see what you've got going on at the power inlet: make sure the mains AC goes to the fuse before the switch, not the other way around.  Otherwise even when the fuse blows you'll have a long run of AC power that is still live.  In a metal chassis especially that can be a problem.  That might be what you did, I just can't quite tell from the pic.

 

Polishing aluminum?  No thanks:)  

post #3 of 24
Thread Starter 
The 1.4x is right on, 220 volts coming off the transformer and 300 after the filter.

You added an extra nights work for me. This is the first time I've used a separate inlet and fuse holder and I had wired the switch before the fuse because the hard points were easier to solder to. I'm fixing that tonight, thanks for the good catch, it's definitely safer.

I wish polishing wasn't so difficult. I love the chrome of the McIntosh MC amps, but I don't want to work with steel. I have everything to build Pete Millet's DCPP, but I'm trying to figure out how to finish the top plate.
post #4 of 24

Sorry :)  On one of my early amps a more experienced builder corrected me on the exact same wiring mistake.  I'm just passing on the old timer know-how.  

 

I hear you on top plates.  I build mostly out of oiled wood aprons/frames and raw aluminum because I like the interplay of textures.  Beyond that, I don't know what I would do.  I don't like polishing and aluminum is so soft that I just worry it will scratch in the first week of use.  I've thought about powder coating but haven't tried it yet.  Hammond steel chassis are available in black and the finish is pretty nice.  Here's one I did with some oak end panels (phono preamp):

 

post #5 of 24
Thread Starter 
I do appreciate the tips. I've done a number of kits and PCB's but planning the layout from scratch is new for me. So far this has been a pretty good "advanced beginner's" amp to build. The bus bar/tag strip was a first time thing I thought I would try, like it so far.

I've generally stuck with raw aluminum. Spray painting has come out looking nice, but not especially durable. Powder coat will probably be my go to next. I've even considered putting the DCPP inside a Par Metal case, kind of like an Audio Note amp. Keeps it safe from future kids, but I just hate not seeing those tubes and big iron blocks.

Do you have any push pull EL-84 speaker amp schematics you like? I've also been curious about single ended EL-84 headphone amps. I find that tube to be interesting.
post #6 of 24

I really like the design and philosophy behind the Baby Huey push-pull EL84 but I haven't built one (yet).

 

In terms of headphone-specific, the EL84 in triode is probably a really good candidate for single-ended transformer coupled use.  Right amount of power without being to obscene, easy drive requirements, modest size.  The idea of EL84 driving an EL84 is so fun that I know I'm going to try it eventually, too.

 

I built an RH84 a while back and that was a very nice single-ended design.


Edited by Sodacose - 12/16/16 at 11:57am
post #7 of 24
Thread Starter 
The Baby Huey and the Tubelab SPP are my top two contenders. I guess it depends on if I want to build P2P or PCB. And some day when I'm feeling crazy I might just build the old Mullard 3 watt.

I currently have a completed Decware Zkit 1 that I thought of modifying to run headphones. The problem right now is I get a lot of hum. I built it on a tiny chassis so I would need to re-case it. Also thought of building a DC heater supply, and probably a beefier HT supply. Or look for a different circuit. I'm actually really surprised there are so few EL-84 headphone amps out there.

Sowter makes a transformer for SE EL-84 with a 5K primary to 40/150/300/600. The pound is weak right now, but a pair is still $400. I don't know how far I want to go in that rabbit hole.
post #8 of 24
Thread Starter 
I wired the inputs to the pot and placed the LM317. I also wired the heaters up. The .1mf capacitor just bridges pins 4 and 5 right? Is that referred to as a snubber, and what is the reasoning?




Edited by Evan99 - 12/17/16 at 8:07pm
post #9 of 24
Thread Starter 

Well, it's alive! Due to the near sub zero temperatures I had a little extra indoor time to work on projects today. Fired right up. My one concern is that I am getting 159.7V on each of the plates and the datasheet rates the 6DJ8 for 130V. Do I need to replace one of the HT supply resistors with a higher value? Should I not use the amp at all until I change that? What is the method for calculating the ideal resistance value?

 

I tested the amp with an old pair of earbuds I had lying around and things sounded pretty good. So I plugged in my HD-650's and played a few clips of songs before family obligations pulled me away. The amp sounded really good. It had a nice open sound, with solid bass, and good detail. These are just very preliminary impressions, however. I couldn't hear any background noise with the earbuds, though I wasn't in the quietest of rooms. It is a slight adjustment to use an amp with such a relatively small amount of power. I was able to use much more of the volume knob than I am accustomed to using compared to my Elekit TU-8200 making 4 watts with KT-88 tubes.

 

 

post #10 of 24

Good to see another HeadWatt living.  Hope you enjoy yours as I do mine!

 

Me

post #11 of 24

Looks great, Evan!  Really tidy amp you built.

 

I have seen more than one design and recommendation to run the 6DJ8 over 130V on the anodes.  It was originally designed for cascode operation and I wonder if that spec isn't some carry over from that application.  Even Bruce's original design specs 135-150V on the anode (though some would be dropped across the primary of the output transformer and the cathode bias).  That said, yes you can just adjust one of the CRC resistors to drop another 10-20V.

 

The snubbers help shunt any residual AC noise on the DC heater line.  

post #12 of 24

That's one tidy looking amp. I dig the polished Aluminum.

 

What is the part number for the LED used in power supply? 

post #13 of 24
Thread Starter 
Tubey,
Thanks for the compliment. Personally, I think my tube builds look messy, but I guess there is some level of birdsnest with point to point.

I like polished aluminum, but I can't say I would try that again. Like Sodacose said, it just scratches too easily. I've since found this chassis and thought it would be cool to build in


I actually did not install the power LED. Bruce recommends it since the tubes don't glow very much and it can be hard to tell if the amp is on. With my power switch on the front I figured it was simple enough. But I did buy this LED from tube depot, it looked like the easiest one to mount.

https://tubedepot.com/products/red-snap-in-panel-mount-led

Are you starting a Headwatt project? With the holidays and family time I haven't gotten a ton of hours listening, but I do think it sounds great and the overall build cost was very reasonable. It was a really nice project for my first schematic only build (as opposed to a kit).
post #14 of 24
Quote:
Originally Posted by Evan99 View Post


Are you starting a Headwatt project? 

May be. Either this or the Torpedo. There is no capacitor in signal path so I'm finding this design very attractive. Thinking about using Russian 6N1P EV tubes which are rated for 250V anode, so have to use different power supply with B+ of 245V. 


Edited by tubey1 - 1/8/17 at 1:32am
post #15 of 24
Thread Starter 
The Torpedo has been a tempting amp to me. I had seriously considered buying the T3 kit when it came out, but i just don't like the look of the amp. I know the whole point is to be long and thin, but it just looks odd to me. And now that the T3 is available only as pre-built I can't justify the price. The T1 kit could be a nice project, pretty straightforward build, and the Cinemag option is cool. I guess it depends on if you want to build from a kit on PCB or point to point schematic.
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