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12HL7-triode SE Headphone Amp - I finally built something!
12HL7-triode SE Headphone Amp - I finally built something!
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Old 5th July 2018, 08:50 PM   #11
H713 is offline H713  United States
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Good idea. I've never tried LED biasing... How does it compare to cathode bias?
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Old 5th July 2018, 08:55 PM   #12
Wavebourn is offline Wavebourn  United States
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12HL7-triode SE Headphone Amp - I finally built something!
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.
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Old 5th July 2018, 09:03 PM   #13
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
Quote:
Originally Posted by H713 View Post
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.

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Originally Posted by H713 View Post
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?
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.

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Originally Posted by H713 View Post
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.
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Old 5th July 2018, 09:07 PM   #14
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
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Originally Posted by Wavebourn View Post
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.
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Old 5th July 2018, 09:13 PM   #15
Wavebourn is offline Wavebourn  United States
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12HL7-triode SE Headphone Amp - I finally built something!
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.

Speaking of 12HL7, I use 6P15P, or 12HL7, or 6J52P depending on gm that I want.
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Old 5th July 2018, 09:32 PM   #16
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
Quote:
Originally Posted by H713 View Post
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.
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Old 5th July 2018, 09:39 PM   #17
H713 is offline H713  United States
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Quote:
Originally Posted by rongon View Post
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.
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Old 5th July 2018, 09:51 PM   #18
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
Quote:
Originally Posted by Wavebourn View Post
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.

Quote:
Originally Posted by Wavebourn View Post
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.
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Old 5th July 2018, 09:56 PM   #19
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
Quote:
Originally Posted by H713 View Post
it's always preferable to use tubes from the "tubes I don't know what to do with" bin than to add to it.
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. !!! !!!

Quote:
Originally Posted by H713 View Post
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 by rongon; 5th July 2018 at 10:05 PM.
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Old 5th July 2018, 10:12 PM   #20
H713 is offline H713  United States
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Quote:
Originally Posted by rongon View Post
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. !!! !!!

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.

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.
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