diceman

Valve Pre-amp & phono thoughts for future project

20 posts in this topic

Some excellent advice guys & guy ;-) Many thanks to all, even current & Ex manufacturer's chipping in - gotta luv the wam.

Interesting that the russian 6h30n tube is sounding quite nice, I have read a lot about this and has been lebelled the "super tube". As a single valve it looks quite interesting as has plenty of current compared to most.

The russian Reflektor & Voskhod tubes sound very nice in my book and are normally quite cheap (apart from the 6h30n it seems).

Gain is indeed a very important consideration and easy to overlook. I suspect what makes sense is to use very low/no gain as line stage and try and match phono stage output to typical CD of 2V rms.

This will all be thrown out the window if I use it with my Leak Stereo 20 though as opposed to any of my regular amps!

thanks again chaps.

A quick clarification on copper. Is there any benefit in using it for both PSU & Signal boxes or just one?

My metalworking skills are iffy / so-so. I do have access to a guillotene and folding machine for small jobs though although I am concerned that the copper would be badly marked from the process.

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Interesting that the russian 6h30n tube is sounding quite nice, I have read a lot about this and has been lebelled the "super tube". As a single valve it looks quite interesting as has plenty of current compared to most.

The russian Reflektor & Voskhod tubes sound very nice in my book and are normally quite cheap (apart from the 6h30n it seems).

The 6H30 has become rare, costly and hyped in the past 10 years, but the 6N6P is almost the same, sounds almost the same, and hasn't yet hit the big time. Either buy matched pairs from a good dealer, or a bunch of NOS ones from Eastern Europe and sort them yourself.

Other interesting alternatives could be to go for an octal tube, Thomas Meyer likes the 6AH4, and Gary Dews uses the low mu part of the 6FD7/6DE7 family.

Gain structure is important, I tend to aim for an overall gain of around 6 to 8 dB, as is output impedance which should be determined by what power amp load you wish to drive. Many valve amps may have a 100K to 220K input impedance, so a valve plate impedance of 2 - 3K may be acceptable, however transistor amps can go as low as 10K which sets a real challenge to a cap coupled common cathode stage, partly because of the low plate impedance required, and partly the size of the cap! This is where an OTX is useful, as it avoids using a cathode follower.

I have also been playing with battery grid bias on my line stage recently, which enables the electrolytic bypass cap to disappear, which is a very nice game.

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I recently found a 'new to me' indirectly heated triode which has low gain & which ought to make quite a fun, if current hungry, & grunty cap coupled line stage. I've just received a few but haven't got around to building anything yet. It's a 7233. Might make a nice 1-2W power amp too.

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I think that the 6N6P is a wonderfull valve! I use it myself in a heavyly modified DIYHiFiSupply TRAM line amp. Not in production anymore! But You can find "The recipe" on Troels Gravesen website: http://www.troelsgravesen.dk/Diy_Loudspeaker_Projects.htm The best linestage for the money, Silk transformers highly recommended! Use only "super caps" they make quite a difference ("the best caps are no caps").

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i've had a 6N6P preamp based around modernized "euridice" WOT schematics and it's a killer. very easy to build, you just need some good iron (chokes and output transformers) and voila. this may be a good reading point to start:

http://basaudio.net/pubs/jfl_euridice_2005.pdf

and from there on, there's a plenty of variations around on DIY fora, just google "euridice preamp". the design has a huge following among DIY-ers.

a step forward is this - a differential parafeed line stage:

http://www.raleighaudio.com/MI%20articles.htm

it's a well elaborated DIY design with very little chance to make something wrong as you will find a plenty of different components tested and chosen for sound merits. it's based around lundahl iron but obviously you can put anything there...

for me the output transformer is the must in a line preamp, otherwise you will end (in most of the cases) with either a difficult, high output impedance that will discriminate almost anything but a 100k tube power amps or a cathode follower that will just not sound as good.

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