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Tube Preamplifiers

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Hi everyone!

This is my first post in this board!

Anyway, I'm looking for a DIY tube preamp with balanced inputs and outputs. Know of any? Most of the projects and kits uses unbalanced RCA input & output.

The other thing I'm curious is does anyone one know of diy preamp design utilizing tubes normally used only in an output stage of a poweramp?! E.g A preamp that is driven by 300Bs! Check this link!

http://www.zenn.com.sg/Zenn prod.htm

Unfortunately this is only available as an assembled product. Any DIY tubed preamps like that eh? A 2A3 preamp? Or a 211 preamp? hehehe.
Off the cuff, I'd expect that it'd be a dicey proposition to put together a preamp with power tubes, as they generally have a pretty low mu (amplification factor). That's not to say it can't be done, particularly if you string enough of them together.
I'm short on time at the moment; will look at the link later.
How much gain do you need for your balanced preamp? If all you're doing is listening to CD or tape, then you can get by with, say 6 to 10dB gain (although most preamps have more). A simple differential front end, followed by cathode followers would do the trick. Use the tube of your choice: 12AX7, 6DJ8/6922, 6SN7, etc. If you're planning on phono, consider a little more gain, even though most of the work will be done in the phono stage.
There are zillions of DIY sites out there: hit the links and start fanning out looking for schematics for balanced stuff.

Although it is by no means a complete preamp, you could take a look at the 5687 LTP on my site. This circuit would form the basis for a balanced preamp. Reduce the degeneration resistors until the gain is where you want it, add a pot or stepped attenuator, etc.

There are also a number of preamp schematics for the 5687; I know because I referenced some of them when working on my page. Try a google search--some people like the sound of this tube.
that it'd be a dicey proposition to put together a preamp with power tubes

you have no idea ho dicey (see moniker) this proposition is :) ROTTFLMAO :) have exactly that in mind .
MC-input trannie deeding the grids of a long-tailed pair of choke loaded EC8010 (could also be triode-wired E180F), directly coupled to a long-tailed pair of choke loaded 12A4 or 6900, directly coupled to a long-tailed pair of PP-output-transformer-loaded 12B4A.
That's for phono; split RIAA tucked between the stages. For line, a complex switch circuit replaces the direct coupling between stage 2 and 3 by an input transformer, Lundahl LL 7901.
The output stage of my preamp is nothing but a straight lil' PP power amp as linestage, with enuff headroom. OPT: Lundahl LL1660. outputZ tuned to 600Ohm.

The 12B4A is as juicy as a 2A3, just indirectly heated and gain a tennyweeny bit higher. In fact, it is 2A3's grandchild and also its replacement in similar industrial and military applications

No idea if this will work or how it will sound, but my tube guru friend Manfred was unable to find the hair in the soup of my circuit design albeit he feels uncomformtanble about it -- burned child avoides fire, but in his case direct coupling (mis-)happened in single-ended topologies. WE both feel my intended circuit has huge sonic potential and i wil find out if it is worth the hassle :). But it will have hot-pluggable phono EQ. Yippie! just the right thing for my shellac collection.

second your recommendation for the 5687 as linestage. Makes also a worldclass SE line stage in combination with the Shilton pa102.

Vinylsavor has directly heated triodes for linestages since i know him. Always Xfrmr-coupled, mostly the 10/10Y/801 with gain 8. Raved elsewhere about his system. But he told me he was not happy with longtailed pairs of OPT-loaded 10s due to the stages virtual ground collecting hum and noise unbearable at this signal level.
Peaceseeker and all,

preamps with power tubes, especially DHTs have big sonic
advantages. Power tubes are usually run at rather highish
bias voltages (compared to the usual preamp tubes). Even
if run at reduced power dissipation compared to power amp
applications. This gives the preamp huge headroom. In my
opinion you can't have too much headroom. A typical CD
source has 2V RMS output, some more than 3. 2V RMS is
about 5.6V pk-pk. That means the line tube needs a minimum
bias of 2.8V. But that would be absolutely zero headroom.
Even with a moderate 6dB headroom a bias of 5.6V is needed.
Better yet 12dB or more!

Phono is an entirely different story. As Bernhard reported, some
records can have peaks of 17dB above nominal 0dB output!

Typical power triodes would run at bias levels of 20-40V.
Plenty of headroom, no danger to even come near the
region of overload.

This also means that the tube will never swing anywhere
near the non-linear reagions. At line levels power triodes
will operate with extremely low distortion.

There is a prejudice that power triodes will have less resolution
ate line levels. Not true! In cintrary due to the above reasons
they will have heaps of detail.

And last but not least they will bw able to provide some power,
which is good to drive long cables, or as in my case low
impedance line level cross overs in active systems.

Of course all this does not come for free. As somebody already
mentioned, they typically have low mu. With a step down
transformer at the output, even this low mu will be eaten up.
That's why I prefer tubes like the 10 or 801 for line level
application. It's mu of 8 is not so extremely low.

Other factors to consider are microphonics and filament
supply. DHTs with their flexible filaments tend to be womewhat
microphonic at such levels and definitely need some means
of shock mounting. The filament supply needs to be DC, no way
to get away with AC here. Especially thoriated tungsten
filaments tend to show significant sonic differences depending
on the filament supply. Best solutions are current rsources,
or passive filters with end with a choke.

Peacekeeper, I'd love to hear about your progress with the
211 line stage. I'd expect more microphonics due to it's size
and the filament supply will be quit hefty. Which kind of
transformer do you have in mind ?

Ciao ... Thomas
Re: Re: Re: Preamp Schematics

planet10 said:
No discussion of balanced pres would be complete without pointing out Allen Wright's RTP3s.

A more thorough discussion of this circuit in his Preamp Cookbook -- a hoot to read even if you don't build anything out of it.

david, my little brother is namned so - not GOLIAT
what you know of EF86 qualities
for the present

Joined 2001
Paid Member
Re: Re: Re: Re: Preamp Schematics

gromanswe said:
david, my little brother is namned so - not GOLIAT


what you know of EF86 qualities

a pentode. used in Quad IIs. burn out alot in my friends Leaks so he is going to mod it for ECC83 (why an ECC83 i don't know). I got an amazing amount on eBay for a pr of Mullards. I'll probably stick to triodes except in the output stage.

planet10 pendeler of ours

Originally posted by gromanswe

david, my little brother is namned so - not GOLIAT
what you know of EF86 qualities
for the present

So, what brand do I buy these days?
You did not answer my question right.
Joined 2001
Paid Member
Re: planet10 pendeler of ours

gromanswe said:
So, what brand do I buy these days?
You did not answer my question right.

I have no experience using this tube, so i am not qualified to make any more comments then i already have -- those all circle around the answer to your question.

Anyone like to comment on EF86s for our friend in Sweden...

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