• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

12HL7-triode SE Headphone Amp - I finally built something!

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I finally (finally!) finished putting together my first self-designed headphone amp. I think it came out pretty well.

This amp is a pure junkbox project. The goal was to make a useful toy out of parts I've collected over the years. The idea was to NOT buy ANY parts, if at all possible. The amp's built in a discarded Heathkit W5M chassis a friend gave me something like 15 years ago. I also wanted to put to work things I've learned from folks here on diyAudio. Thanks to everyone who gave me advice and instruction along the way. (I know the "strain relief" on the AC cord is laughable. I promise I'll do better next time.)

It was pretty funny how many gremlins were tripping me up today. The first time I brought the amp up on the variac it blew its fuse. Turns out there was a faulty rectifier diode. Replaced that and brought the amp up again. This time the fuse held. Now the right channel tube's heater wasn't lighting up. A little jiggling of the pins in the socket and that problem went away. Weird. I checked for a cold solder joint, but all seemed fine. Oh well. Brought it up again. All seemed to be going well so I connected RCA cables to the inputs and gave it the Bzzzzt Test (touch the central pin in each RCA plug in turn, slowly turn up the volume, listen for the bzzzzt from each channel in turn). Hmmm, no sound from the right channel. Oh no... Continuity checked out OK. All resistances OK, including both OPTs. The output jack? Brought it out, disconnected it. Clip-leaded in a new jack and there it was. Would you believe a defective jack? Man, I'm snake-bit!

Finally, all was looking good. I took the amp off the variac, powered it directly from house AC. All good. Time to listen.

I connected the amp to my PC's soundcard, plugged in a pair of beater headphones, gave it a listen.

Four hours later and I'm still listening. This thing sounds really, really good to me. Clean and clear. Not "warm." Lots of impact. Drums go "thwack." Isaac Stern's violin is really nice. Not 'electronic' sounding at all. Bass has real body. I think it goes low enough to be enjoyable. (I was worried about that.) Overall the amp is 'livelier' sounding than my Objective 2 amp. I don't want to go down the rabbit hole of audiophool jargon blather, so I'll just post some pictures of my fuggly build and the schematic of the basic circuit.

In the schematic there's a B+ supply common to both channels, then the DC splits off into an RC filter for each channel of 560R and 330uF (R2 and C4). Later I hope to replace that RC filter with a simple voltage regulator member Elvee basically designed for me (thanks Elvee!). The idea is to have a couple of pretty 0D3 glow tubes standing on top of the chassis. I rigged it up with the passive supply to get it going.

I was worried about hum from AC heaters. I very carefully followed the suggestions in the "Heater Wiring - The Good the Bad and the Ugly" thread. I'm getting absolutely zero hum of any kind. (I'm so happy about that.) I also followed the suggestions for grounding layout in Merlin Blencouwe's Valve Wizard Grounding article. That worked perfectly too. (I'm elated about that.) I also referred to Merlin's "Designing High Fidelity Tube Preamps" book a lot, which was especially helpful for power supply stuff.

So it's a success. Gain is about 2X, I think. I haven't measured it yet, as I don't have my signal generator and scope fully accessible right now. I need to get to that.

Finally, I need to figure out whether I need damping resistors across the OPT secondaries for using the amp with 300 ohm headphones. The amp sounds excellent to me into my Fostex T50RP headphones (60 ohm impedance). It also sounds clean and clear into my Sennheiser HD650 headphones (300 ohms), but these supposedly 'dark-sounding' cans sound bright and a little bass-shy from this amp. I'm wondering if that's an artifact of putting a higher than optimal 300 ohm load on the nominal 50 ohm secondary. This weekend I'll have some time to clip in some different values of parallel resistors to see what sounds good to me.

But for now, I'm just happy this thing is working well. Thanks again to the diyAudio community.
--
 

Attachments

  • rongon_12HL7-triode_SET-headphone-amp_4July2018.png
    rongon_12HL7-triode_SET-headphone-amp_4July2018.png
    26 KB · Views: 922
  • IMG_20180704_180829235.jpg
    IMG_20180704_180829235.jpg
    500.6 KB · Views: 913
  • IMG_20180704_180741879.jpg
    IMG_20180704_180741879.jpg
    574.2 KB · Views: 822
  • IMG_20180704_180240405.jpg
    IMG_20180704_180240405.jpg
    679.9 KB · Views: 803
  • IMG_20180704_180249480.jpg
    IMG_20180704_180249480.jpg
    765.1 KB · Views: 831
Last edited:
Looks pretty nice. Reusing an old chassis always makes things harder to get a clean look, but still not bad. My first build is way uglier on the inside. Definitely would be cool to do a regulated power supply.

Also, congrats on your (I assume) first project, and you did not make the same mistake I did with my first preamp project. You chose a large enough chassis, and you didn't try to hot-glue in your filter caps in (yeah, hot glue + heat from tubes = not glued anymore).

Also, I've found that Crestron devices have pretty nice chassis.
 
Thanks. It's actually not my first project at all. Just the first successful from-scratch build in a few years. The last thing I built, a push-pull power amp, didn't work out. It hummed. I was traumatized, so was very careful to do this one more properly. It seems OK.
 
Last edited:
With my preamp project, I chased lots of hum issues. At the time, I didn't know about grounding the center tap of a heater winding. I figured out that grounding the heaters through a .1uF capacitor killed much of the noise. And that was a DC heater, so it was equally confusing.

Also confusing was the chasing down a massive hum that was ultimately due to a bad pot.

Anyhow, 12HL7... not a tube we see every day. What made you choose it?
 
With my preamp project, I chased lots of hum issues. At the time, I didn't know about grounding the center tap of a heater winding. I figured out that grounding the heaters through a .1uF capacitor killed much of the noise. And that was a DC heater, so it was equally confusing.

Also confusing was the chasing down a massive hum that was ultimately due to a bad pot.

Anyhow, 12HL7... not a tube we see every day. What made you choose it?

Having lots of experience built up over many builds is a huge asset. Unfortunately, that takes a lot of time and effort. Since I'm not properly set up for metalwork, I find it hard to stay motivated to start projects. Layout is a challenge for me, as is grounding. I think I'm getting better at it. But I need to build more stuff, learn what doesn't work and how to make the right compromises.

I read about 12HL7 here on diyAudio and found a sleeve of them for cheap. They have high transconductance so low plate resistance when wired triode, with a fairly hi mu of ca. 30. That makes it a good candidate for a 'spud' headphone amp. I'd use something fancy like a D3a or 7788, but this was a kind of 'test build' using $15 Edcor OPTs, so I didn't want to throw expensive parts at it. Definitely a junkbox project.

--
 
See, I have the opposite problem. I start new projects before I actually finish the previous.:eek:

Yeah, I know how that is. :guilty: I went through a period of thinking I could make a large pair of efficient speakers with horns. Now I have several large speaker cabinets, waveguides and a bunch of boxed up speaker drivers cluttering up my closets. I bought a pair of JBL LSR-305 on a whim, and now I have less interest in speaker building. I feel a speaker parts sell-off coming on...

Also, I must say that I like your design quite a lot. Been thinking about a headphone amp for a while... this might be it.

Thanks! I think it's pretty good, and certainly is easy enough to put together.

So far I like it best matched up with my Fostex T50RP headphones. That makes sense, since those are 50 ohms impedance with flat impedance curve, being that they're planar magnetic.

Last night I listened a bit with my Sennheiser HD650 cans hooked up. I didn't like that as much. They were kind of 'thin' sounding driven by this amp. Very clean, but the bass is somewhat attenuated. That's probably because of the impedance mismatch between the OPT 50 ohm secondary and the high and variable impedance of the HD650s (varies from 310 to about 500 ohms). I'm going to try a suggestion from famousmockingbird to place a 100 ohm resistor across the OPT secondary to smooth the load seen by the output tube through the OPT. Perhaps that will allow the HD650s to play better with this amp.

Today's the first day I've had to listen to some 32 ohm headphones with the amp. They work just fine. Very nicely, in fact. Only one complaint. What I'm finding is that the tubes are a bit microphonic, which is likely to be a problem if using efficient 32 ohm cans (like Grado). With the volume set for normal listening, if I mute playback and tap the top deck of the chassis, I clearly hear a tinkling and ringing noise. It dies down quickly, unlike some other RF pentodes I've tried triode-strapped (6J52P anyone?). However, with less efficient cans that require turning up the volume control, the microphonic noises are now low enough in level that I can easily ignore them. I know this would drive some people crazy, though.

Another thing is that channel balance is dependent on tube characteristics, since there's no negative feedback employed. I'm finding that channel balance is shifting slowly back and forth between left and right as the amp plays over the course of several hours. I suppose the tubes will settle in at some point. Tube matching mandatory, so buy extras to sort through.

How are those Edcor transformers?

Surprisingly good, I think! However, the primary doesn't have high enough inductance to get great bass response. It's good enough, but I can tell right away that my Objective 2 amp goes deeper in the low end, and with better control (probably due to much lower output impedance). However, I do like that tube 'liveliness' to the midrange reproduction, and the extra-smooth highs. Vocals, saxophones, and orchestral strings and percussion sound great through triodes and transformers.

Quick switch back and forth, Ugly Beastie vs. Objective 2 -- the tube amp sounds 'warmer' for sure, the solid-state amp goes deeper in the bass, for sure. It's really hard for me to choose which I prefer overall.

An interesting experiment might be to use a higher gain pentode like 6J52P (or the pricey D3a) and employ it as a plate follower (apply plate-to-grid negative feedback). Only about -6dB of NFB would be available, but that might be enough to tame the 6J52P's microphonic tendencies and lower its plate resistance enough to get better bass out of the OPT. The result could be a more versatile design, able to successfully drive a wider range of headphones.

Anyway, thanks for the encouragement.
--
 
Last edited:
All my headphones are 32 ohm, and I've been thinking of playing around with a 6J7 design, maybe triode-strapped. Might be considering the 12HL7 now too though. I've got some thinking to do, but I better do it quick because the roommate might not like my rather large speakers and big tube amp. I mean, who doesn't want a 50 pound heater in their room that makes a whopping 30WPC? :D

Also, I'm really not an expert, but wouldn't 600 ohm headphones work better with an OTL amp?
 
Hi, Good show, I will pass along a suggestion I learned the hard way some years back. Using LEDs is a fine way to get bias....but to get more current capability using them in parallel can be an issue. The voltage drop across each diode will not be exactly the same. The result is that the one with the lowest drop will pass the most current. In some cases nearly all of it. This can result in failure do to excess dissipation. The solution is to put a small value resistor in series with each diode. They will then share the current. The larger the value of the resistor the more equal will be the sharing. If you make it too large though they will will alter the bias setting that you wanted.
 
All my headphones are 32 ohm, and I've been thinking of playing around with a 6J7 design, maybe triode-strapped. Might be considering the 12HL7 now too though.

I think 32 ohm headphones are a good match for this Edcor transformer.

Looking at the 6J7 data sheet (http://www.r-type.org/pdfs/6j7-1.pdf), I don't think that's going to work as a headphone amp tube. Its mu is almost high enough, but more importantly, its triode plate resistance is much too high for this use. 6J7 is more of a preamp tube than a power tube. It's a very different beast from a 12HL7, which is closer to something like an EL84. In other words, it's a small power tube, which is what you want for a headphone amp.

If you're looking for a single tube per channel, you need high gain AND high transconductance. That usually means high transconductance RF pentodes. It's much easier to make a two-stage amp. There are a few dual triodes that have a medium-mu 'preamp' triode and a low-mu 'power' triode in one bottle that would work well for this. Some are really cheap.

There's another thread happening where a parallel-pair 5687 (or 6N6P) is proposed for use with an output transformer for headphones. That would work with this Edcor transformer too.

I've got some thinking to do, but I better do it quick because the roommate might not like my rather large speakers and big tube amp. I mean, who doesn't want a 50 pound heater in their room that makes a whopping 30WPC? :D

LOL. I was using a PP 2A3 amp for years. Probably 40 lbs to make a whopping 6 watts per channel, and lots of heat. My headphone journey began when I moved into a condo apartment with persnickety neighbors downstairs. They objected to my bedside stereo (class D amp and small single-driver speakers). It wasn't even loud, but they can hear anything I do. So it's headphones or no bedtime music listening at all. Hence the headphones.

Also, I'm really not an expert, but wouldn't 600 ohm headphones work better with an OTL amp?

Well, theoretically, 600 ohm headphones allow you to choose either an OTL amp (White Cathode Follower or whatever) or a transformer output amp. I don't know which would be "better." I suppose the OTL is theoretically better, since you wouldn't have to deal with transformer issues like leakage inductance, ringing, insertion loss, etc. It's when you get down to 32 ohm headphones that you have to go for either an OTL tube amp using a large power tube and negative feedback to lower the output resistance (not an attractive option for most), a reasonable little tube amp with enough gain to drive a suitable OPT with fairly high step-down ratio, or go solid-state.

I wanted this one to work with either my 50 ohm or 300 ohm headphones. It's a challenge, yes. It sounds good with both, just better with the 50 ohm cans.
--
 
I just put all paralleled LEDs together so they touch each other equalizing temperature, unlike in Christmas tree lights where they are all in parallel and separated.

Is it enough to get the round-barrel LEDs to touch each other, or would it be better to choose square LEDs and use a spot of epoxy so that they're touching over a larger surface area?

Also, if you use LEDs that are all from the same production run, won't they have similar-enough current draw to survive being paralleled up?

I'll let you all know if these burn up. I had the amp running all day at work today, and not a hint of trouble. I'll measure the tubes' plate currents when I take it home.
 
Good idea. I've never tried LED biasing... How does it compare to cathode bias?

LEDs will act like a voltage reference if reverse-biased (in the case of a cathode load for a tube, used with the LED cathode connected to ground, and its anode connected to the tube's cathode).

Different color LEDs yield different forward voltages. Red LEDs usually yield about 1.8V to 2.0V, green about 2.0V to 2.2V (the ones I used seem to be 2.1V). White and blue LEDs are up around 3 to 4V. Infrared LEDs are down around 1.2V.

Let's say we take a green LED with forward voltage of 2V at 4mA. Its internal resistance will be around 10 ohms, more or less (I can't remember exactly).

If you use a resistor to drop that 2V at 4mA, that resistor would need to be 500 ohms. That's enough resistance to introduce current feedback to the tube (increases in current drawn by the tube will cause a proportional increase in voltage across the cathode resistor, causing the tube to conduct less = negative current feedback). This decreases the gain of the tube and increases its plate resistance, which means it also decreases the tube's transconductance (and also decreases distortion generated by the tube). This is not something you want in all cases, so to bypass that resistance for AC signal (audio signal) while leaving the 2V DC cathode voltage you'll place a capacitor in parallel with the resistor from the tube's cathode to signal ground.

The end result is that you can use a 500 ohm resistor to get that 2V DC at the cathode, but it will need to be bypassed by a capacitor of about 220uF to bypass AC signal down to below 2Hz.

Some say these large value electrolytic capacitors introduce bad-sounding effects. Others say they prefer the sound of a cathode resistor bypassed with a suitable capacitor over an LED. The LED is supposed to be technically superior, but I think the effect is pretty subtle, especially if you can find good electrolytic capacitors with very low ESR. (Don't use cheap no-name caps.)

LEDs are typically rated for forward current up to 20mA. In my amp each 12HL7 is drawing a pretty massive 55mA. So I paralleled four green LEDs per side (80mA max capacity). I'm going to glue mine together so they're touching each other, per Wavebourne's advice. Maybe that's all that's needed.
--
 
I think 32 ohm headphones are a good match for this Edcor transformer.

Looking at the 6J7 data sheet (http://www.r-type.org/pdfs/6j7-1.pdf), I don't think that's going to work as a headphone amp tube. Its mu is almost high enough, but more importantly, its triode plate resistance is much too high for this use. 6J7 is more of a preamp tube than a power tube. It's a very different beast from a 12HL7, which is closer to something like an EL84. In other words, it's a small power tube, which is what you want for a headphone amp.

If you're looking for a single tube per channel, you need high gain AND high transconductance. That usually means high transconductance RF pentodes. It's much easier to make a two-stage amp. There are a few dual triodes that have a medium-mu 'preamp' triode and a low-mu 'power' triode in one bottle that would work well for this. Some are really cheap.

There's another thread happening where a parallel-pair 5687 (or 6N6P) is proposed for use with an output transformer for headphones. That would work with this Edcor transformer too.



--


Yeah, I have this habit of coming up with "great" ideas, and then I get a bit of a reality slap when I look at the datasheet. Time for another one: Pretty sure I have a couple sleeves of 12CA5s laying around somewhere.

Although, 12HL7s aren't expensive, but the microphonics could be a problem. That said, it's always preferable to use tubes from the "tubes I don't know what to do with" bin than to add to it.:D
 
I never had such problems. For 30 mA I used 2 LEDs rated for 20 mA each, they still work fine. Yes, they were from the same batch.

I have the 12BH7s drawing a massive 55mA each. Maybe I should back off from that. I could reduce the B+ a bit, from 165V down to maybe 150V. I chose the higher current operation to try to keep rp as low as possible, to counteract the low primary inductance of these cheap Edcor OPTs. The resulting bass response sounds good enough that I think the tactic is working.

Speaking of 12HL7, I use 6P15P, or 12HL7, or 6J52P depending on gm that I want.

I was thinking of trying 6J52P in this amp in place of the 12HL7s, but it will be tricky to get it to work. 6J52P has a pretty small grid base.
 
it's always preferable to use tubes from the "tubes I don't know what to do with" bin than to add to it.:D

6J7 as a triode looks a lot like 6SN7 (or 6J5, or 6FQ7, or 7N7). That would make a fine triode as the driver for a 2-stage headphone amp using a triode-strapped 6V6 or 6K6GT as the output tube. Just use an appropriate OPT. You might even consider using some (gasp!) negative feedback from the OPT secondary to the cathode of the 6J7-triode input tube.

Better yet, use the 6J7 as a pentode, and use plate-grid feedback with a 6V6 in pentode. !!! :D !!!

Time for another one: Pretty sure I have a couple sleeves of 12CA5s laying around somewhere.

Ah, now there's a worthy challenge. That looks similar to 12C5, 50C5, etc. I like that idea.
Challenges using that tube would be max plate voltage of only 130V, max combined plate+screen dissipation of 6.4W. As a triode it should have low enough rp so that you could use that Edcor OPT. As a triode:

Vp = 125V
Vg = -5V
Ip = 40mA
P+g2 diss = 5W
gm = 15mA/V (pretty good)

I should see if I can dredge up a couple of 50L6 or 12C5 from the bottom of the barrel somewhere. Those things used to be everywhere.
 
Last edited:
6J7 as a triode looks a lot like 6SN7 (or 6J5, or 6FQ7, or 7N7). That would make a fine triode as the driver for a 2-stage headphone amp using a triode-strapped 6V6 or 6K6GT as the output tube. Just use an appropriate OPT. You might even consider using some (gasp!) negative feedback from the OPT secondary to the cathode of the 6J7-triode input tube.

Better yet, use the 6J7 as a pentode, and use plate-grid feedback with a 6V6 in pentode. !!! :D !!!


I'm not too opposed to negative feedback. I mean, heck, I use a Heathkit AA-121 right now. It's a push-pull that's got feedback, pentodes, and a solid-state rectifier. I don't think I have too big an issue with a little negative feedback.:D

Yeah, my 807/1625 monster design might utilize a 6J7 for the phase inverter or preamp. I do like the look of a top cap. But that design might have to go on hold for the reasons stated before. I'm planning a 26" wide chassis made from 1/4" aluminum, and it's going to have an old Tektronix power transformer. Also will have two chokes, some massive soda-can sized filter caps, and god knows what else. I don't want to carry that up any stairs, that's for sure.

Back to headphone amps, I could do a triode-connected 12CA5, and perhaps use a 6J7 preamp... That wouldn't look the least bit goofy. Octal preamp pentode with a 7-pin miniature pentode running in triode mode for the power tube.

I should probably sit down and do a little reality check on that one.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.