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Old 13th January 2010, 03:42 PM   #1
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Default 6DX8 Amp for Headphones and Low Volume Needs

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.
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Old 13th January 2010, 04:00 PM   #2
TheGimp is offline TheGimp  United States
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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?
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Old 13th January 2010, 04:31 PM   #3
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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?
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Old 13th January 2010, 04:57 PM   #4
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Originally Posted by TheGimp View Post
High plate impedance is going to be your limiting factor...
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.
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Old 13th January 2010, 05:06 PM   #5
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I want to but a good set of Sennheisers sometime soon
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.
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Old 13th January 2010, 05:25 PM   #6
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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?
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Old 13th January 2010, 05:37 PM   #7
Cookj is offline Cookj  Belgium
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Wavebourn Kaidan headphone amp

http://wavebourn.com/forum/viewtopic.php?f=7&t=998&start=0[/URL]
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Old 13th January 2010, 06:01 PM   #8
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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.
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Old 13th January 2010, 06:07 PM   #9
Stixx is offline Stixx  Germany
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the Grados exhibit a synergy with tube amplifiers unlike anything else I've heard.
I'd call that very subjective...
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Old 13th January 2010, 07:23 PM   #10
TheGimp is offline TheGimp  United States
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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.
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