Hi all,
Early last year I happened to buy a box of 100 6DX8 tubes off eBay on the cheap, and have been playing around with the idea of using them for a small amp. I had a hard time finding a design (was going to go for a Push Pull design, but couldn't find a schem) and I'm too new to this to design my own, but I happened to stumble upon this one (the 6DX8 amp is two thirds of the way down). I've got SE 5K to various taps transformers on the way, so it will be an SE design, but I was looking for any critique of this design anyone might have. The main issue is that it can run large headphones to a decent loudness. Would it help if I threw a 12AT7 stage in front? Any help is appreciated.
Nic.
Early last year I happened to buy a box of 100 6DX8 tubes off eBay on the cheap, and have been playing around with the idea of using them for a small amp. I had a hard time finding a design (was going to go for a Push Pull design, but couldn't find a schem) and I'm too new to this to design my own, but I happened to stumble upon this one (the 6DX8 amp is two thirds of the way down). I've got SE 5K to various taps transformers on the way, so it will be an SE design, but I was looking for any critique of this design anyone might have. The main issue is that it can run large headphones to a decent loudness. Would it help if I threw a 12AT7 stage in front? Any help is appreciated.
Nic.
What are you going to drive it with? CD Plalyer?
Given the high Mu of the triode, I would expect pretty good gain to drive the second stage Pentode. By the looks of the data sheet you should be able to get around 3-4 W out of it, so I don't think you will have any problems driving headphones. In fact, it should be able to drive high efficency speakers.
High plate impedance is going to be your limiting factor in finding a output transformer to suitably match it to your load if you drive a low impedance (speaker) load. You could always try an 8 Ohm speaker on the 4 Ohm tap of the transformer if you have multiple taps. This would set your load impedance at 10K with the 5K transformer
What is the impedance of your headphones?
Given the high Mu of the triode, I would expect pretty good gain to drive the second stage Pentode. By the looks of the data sheet you should be able to get around 3-4 W out of it, so I don't think you will have any problems driving headphones. In fact, it should be able to drive high efficency speakers.
High plate impedance is going to be your limiting factor in finding a output transformer to suitably match it to your load if you drive a low impedance (speaker) load. You could always try an 8 Ohm speaker on the 4 Ohm tap of the transformer if you have multiple taps. This would set your load impedance at 10K with the 5K transformer
What is the impedance of your headphones?
Most likely my MacBook, iPod, CD Player, in that order.
Do you think it would be better to run at 10k than at 5k?
My current set has an impedance of around 150 ohms if memory serves, but I want to but a good set of Sennheisers sometime soon, and the ones I am looking at are around 600 ohms.
IIRC the output's specs are 5W SE 5K:8/32/64/150/600 (w/ centre tap on primary)
Other than that, do you think the design should be okay?
Do you think it would be better to run at 10k than at 5k?
My current set has an impedance of around 150 ohms if memory serves, but I want to but a good set of Sennheisers sometime soon, and the ones I am looking at are around 600 ohms.
IIRC the output's specs are 5W SE 5K:8/32/64/150/600 (w/ centre tap on primary)
Other than that, do you think the design should be okay?
Not only that, but this tube wasn't designed with audio-quality linearity in mind. Given that the intent of all this is to build a low power amplifier to use with headphones, I would never consider a tube like this. For less than $100, one can buy headphones whose sonic quality rivals speakers costing $5,000 or more. Low power tubes capable of producing commensurate quality are available at very reasonable cost, and it's false economy to use a $2 television valve instead. There are dozens of better options for this amplifier, none of which require a higher OPT winding than 5K: EF86 to 6AQ5/6V6/6BQ5, EF86 to 1626, 12AX7 to 1626 (all-triode with no NFB), etc. If you want something smaller, there are also a number of sub-mini triodes and pentodes more suited to audio. Sorry, I don't mean to rain on anyone's parade, I just think the time required to build a piece of tube gear can't be justified on the basis of saving a couple bucks on odd tubes. That's particularly true when those tubes won't serve the intended purpose as well as others that are commonly available.
You really should listen to a pair of Grados first. Even the bottom end of their line is superb. Unlike Sennheisers, the Grados exhibit a synergy with tube amplifiers unlike anything else I've heard.
Very interesting headphones. Had a quick look around Grado's website and they have some very nice looking products. Which of theirs have you heard? Thoughts? Are they comfortable to wear for long periods?
I auditioned several models up the line (sorry, don't remember the numbers now), but ended up purchasing the least expensive. I think they were about $80. The thing is, even the cheapest is so far above everything else I've heard (including highly touted and *much* more expensive Senns), paying more for upper-end Grados seemed to be diminishing returns. As for comfort, that's one of the problems. These phones are so comfortable and so astounding to listen to, I never want to take them off. Incidentally, I'm a diehard DIYer and cynic, and there are very few commercial products that I recommend. Other than tubes, that is. 🙂
MellowTone started out stating he had a box of 100 6DX8s and wanted to build an amp from them.
Given that, I still contend a reasonable amp could be built from it. If he is interested in specifically driving headphones, then that might dictate a topology other than SE with an output transformer.
Something like the Wavebourn Kaidan topology might serve him better.
Given that, I still contend a reasonable amp could be built from it. If he is interested in specifically driving headphones, then that might dictate a topology other than SE with an output transformer.
Something like the Wavebourn Kaidan topology might serve him better.
I have a handful of PCL84 here (series heated version of that same tube). When one of these is used as a cascaded two stage amplifier, driven by an MP3 gadget and working into ripped-from-an-old-radio 16R speaker through 100V line transformer, this thing is LOUD.
This being said, one could either apply a huge amount of NFB to bring the level down and linearize its response or try a different topology.
MellowTone: forget about extra voltage amplification, you will want to bring the level down, not up !
I was using 39K anode resistor for the triode, I'd have to check the pentode load though because I cannot remember it off the top of my head. I could draw the schematics if you want me to.
Remember, while some tubes may have not been designed with audio in mind, that doesn't mean they sound bad (just think of old transmitter triodes that are now considered to be "audiophile" tubes). Since you already have a boxful of those, make some use of them 🙂
This being said, one could either apply a huge amount of NFB to bring the level down and linearize its response or try a different topology.
MellowTone: forget about extra voltage amplification, you will want to bring the level down, not up !
I was using 39K anode resistor for the triode, I'd have to check the pentode load though because I cannot remember it off the top of my head. I could draw the schematics if you want me to.
Remember, while some tubes may have not been designed with audio in mind, that doesn't mean they sound bad (just think of old transmitter triodes that are now considered to be "audiophile" tubes). Since you already have a boxful of those, make some use of them 🙂
Hi MellowTone,
Well, you might consider something along my own headphone amp which actually is a full fledged push pull pseudo triode flea power design, even able to drive small speakers at about a single Watt of pure class A. It uses the series heater brother of 6DX8, named 15DQ8/PCL84 and most easily can be adapted to your box of 6DX8.
This amp intentionally was built as a showcase along an ETF lecture what to do with "cheap TV tubes". In headphone amp mode, it is able to drive any headphones from, say, 16 Ohms upward to Hi-Z types. I am using it myself with a 55 Ohms Sennheiser HD595 headphone or a AKG K600 professional monitoring unit.
Have a look at the attached schematic; plugging in a headphone disconnects the speaker outputs and internally connects a dummy load w/ the headphone in parallel by means of an internal switch in the headphone jack.
Just for fun I entered some local headphone / headphone amp shootouts with this amp and it performed very well compared to some highly regarded "audiophile" tube headphone designs. Its strenghts are especially with low-Z headphones of any type. And not to forget, it also can be used to drive even small bookshelf speakers to quite reasonable "background music" levels.
A plot for the ECL84/6DQ8/6DX8/6Phi4Pi, PCL84/15DQ8 pentode section strapped as a pseudo-triode like in this amp is available here.
Regards,
Tom Schlangen
Well, you might consider something along my own headphone amp which actually is a full fledged push pull pseudo triode flea power design, even able to drive small speakers at about a single Watt of pure class A. It uses the series heater brother of 6DX8, named 15DQ8/PCL84 and most easily can be adapted to your box of 6DX8.
This amp intentionally was built as a showcase along an ETF lecture what to do with "cheap TV tubes". In headphone amp mode, it is able to drive any headphones from, say, 16 Ohms upward to Hi-Z types. I am using it myself with a 55 Ohms Sennheiser HD595 headphone or a AKG K600 professional monitoring unit.
Have a look at the attached schematic; plugging in a headphone disconnects the speaker outputs and internally connects a dummy load w/ the headphone in parallel by means of an internal switch in the headphone jack.
Just for fun I entered some local headphone / headphone amp shootouts with this amp and it performed very well compared to some highly regarded "audiophile" tube headphone designs. Its strenghts are especially with low-Z headphones of any type. And not to forget, it also can be used to drive even small bookshelf speakers to quite reasonable "background music" levels.
A plot for the ECL84/6DQ8/6DX8/6Phi4Pi, PCL84/15DQ8 pentode section strapped as a pseudo-triode like in this amp is available here.
Regards,
Tom Schlangen
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The SR80i set look good, and I even found a pair of the SR225i's for quite a good price. Has anyone else had experience with these headphones?
The only limiting factor is that I have a pair of SE transformers on the way for a single ended set (I tend to buy before completely thinking things through) which limits me a bit, although could probably find a place for them.
One thing I definitely want is a proper B+ (don't like starved plates) but I want to be sure that I'm not going to have 170 to 250 volts through something that goes on my head 🙁 As per isolation, is that what the 1000uF cap is for?
Good to know, sounds like it should drive what I need. How is the sound quality?
I'd love to see a schematic if it isn't too much trouble.
Hi Tom, any chance of a shot of the guts? Looks like a nice amplifier you have there. The only problem I have is that I don't want to have to get another set of Outputs made up, as I'm aiming for a cost effective (well, cheap) amp.
Although the primary use of the 6DX8 was as a video amplifier, here in Australia it did find use as an audio valve in domestic 'radiograms' during the 1960's. The local Philips subsidiary published typical operating conditions - a copy of this data is attached.
I am currently building low-powered guitar amplifiers which use push-pull 6DX8's. The details of these amplifiers are here: The Impact Mk II
I am currently building low-powered guitar amplifiers which use push-pull 6DX8's. The details of these amplifiers are here: The Impact Mk II
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How's the sound quality compared to what ? I have already mentioned I used sub-par material to puit it together, what I didn't mention before was that it was put together on a protoboard. These are notoriously susceptible to NOISE so sound quality was, well, not good. On top of everything I only had one 47 uF capacitor behind the rectifier with no additional filtering (= even more hum). However that doesn't say anything about how it would have performed in a circuit built in a normal fashion.
I realized I still have that thing sitting in that protoboard almost intact so I made a photo just to illustrate the point above. The kind of ratsnest you see there is no reference for actual circuits, it is merely used to figure out static component values.
It was very loud though, as I mentied before (with regards to your question whether an additional stage might be required for headphone use).
OPT was 100V audio line transformer (scroll 3/4 down the page to section titled "100-V-Übertrager"). It sucks at low frequencies, but so does the speaker I was using so they were a perfect match
If that tube wasn't a tired B&W era TV pull, if I had a better output transformer on hand at that time, if I needed a lower power amp (plenty of ifs ...), I would have built a decent circuit.
Don't worry about whether this tube was made for audio output or not - this particular tube can be used for audio and it will sound just as fine as those other (more commonly used) tubes, provided you construct your amplifier with just as good material as you would with, say, ECL82. 6DX8/ECL84/PCL84 is far from being horrible, it's just somewhat underpowered compared to its brethren, but this not an issue with headphone amplifiers.
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If this is a major decision for you (it was for me), you need to listen at a local dealer. This is really important; you can't take anyone else's word for the quality or fit of headphones, not even mine. It's a very personal decision, and lots of folks online give advice when they don't know what they're talking about or have very little real experience. I suggest that you build the amp first and take it with you for the audition. When I listened to the Grados for the first time, I also took my old phones so I could do a quick A-B comparison. If this is completely impossible or there's no local dealer, the SR-60 is inexpensive enough to buy and try.
Incidentally, my 6V6 SE amp has a switch on the front panel for speakers/phones. When the switch is set to phones, it adds a 10 ohm resistor from each output to ground. That preserves the internal impedance matching of the amp. Then there's a 150 ohm resistor in series with each channel of the phones. This is just about right for an amplifier in this power class.
You can see the amp on the first page of this thread:
http://www.diyaudio.com/forums/tubes-valves/158330-akai-m8-rebuild-restore-will-work.html
Hi MellowTone,
Sure. The OPTs are mounted inside the chassis (3rd pic), the grounding is a classical star ground using 2,5 square millimeters solid wire (lower right corner of 1st pic).
Regards,
Tom
Sure. The OPTs are mounted inside the chassis (3rd pic), the grounding is a classical star ground using 2,5 square millimeters solid wire (lower right corner of 1st pic).
Regards,
Tom
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Hi -
Slightly OT & don't want to hijack, but this thread got me interested in making my own micropower tube amp. However, I was considering using a 6U10 with the med mu sections as a self phase split PP output & the hi mu section as a grid leak input. AFA phase splitting, I would consider an interstage xfmr if enough performance/power could be gained. Maybe also a 6BH11 instead of a 6U10. Anyone got any suggestions for suitable medium to high quality 2-5 watt output iron for an amp like this? The goal is a 'pocket size' stereo speaker/headphone amp. Edcor has a 16 ohm output tap option that doesn't look too oversized or down on bandwidth for this app, but I'm sure there's more out there.
Thanks much in advance.
Slightly OT & don't want to hijack, but this thread got me interested in making my own micropower tube amp. However, I was considering using a 6U10 with the med mu sections as a self phase split PP output & the hi mu section as a grid leak input. AFA phase splitting, I would consider an interstage xfmr if enough performance/power could be gained. Maybe also a 6BH11 instead of a 6U10. Anyone got any suggestions for suitable medium to high quality 2-5 watt output iron for an amp like this? The goal is a 'pocket size' stereo speaker/headphone amp. Edcor has a 16 ohm output tap option that doesn't look too oversized or down on bandwidth for this app, but I'm sure there's more out there.
Thanks much in advance.
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