What to look for in a tube preamp?

[KenCalgary]

I have never used tube audio equipment and was wondering what would you be looking for when purchasing or kit building, say, a decent tube preamp? Would you be looking for a certain type of tube, a particular size/type of transformer, a circuit design? What makes one tube preamp better than another, parts-wise (except for better quality of parts)?

[Gordy]

Suitability for purpose!

Such as...

how many inputs?
how many outputs?
how much volume ('gain')?
stepped or non-stepped volume control?
do you need a low output impedance?
do you need a phono / RIAA equalisation stage?
does it have a regulated power supply (if not how likely is voltage variation to be a problem)?
is it DC heated (if not how likely is AC heating to be a problem)?
is the tube available in the market at a reasonable price?
(I am sure other members can add more...)

...and the main one for me: at the operating point of the tube what does the load line look like?

[tinitus]

Out of Gordys suggestion, I would first look at gain and impedance matching

Tube availability, consitent quality, and being affordable would also be on my "personal" list

But as a newbie it might also be a good thing to look at who designed it, his credentials
Thats more important than components

I think its safe to suggest looking at John Broskie

[KenCalgary]

I don't have all the answers to Gordys' questions but here are a few:
1. I most likely only have the need for two inputs (CD and turntable)
2. Only one output? going to power amp (solid-state at this point of time)
3. My current solid state preamp has 12dB of gain and < 60 ohms output impedance.
4. Currently I have a very accurate (RIAA), but not too expensive, separate solid-state phono stage however I have no idea if a good tube phono stage would "sound" better or not.

That's about it for what I know or as answers to the questions.

I think my goal in trying a tube preamp would be to get closer to a transparent rendering of the music with a sufficiently low noise floor that the nuances of that music can be heard.

One of my problems is not knowing what features of a tube preamp allow these to come through: Is it high cost components, good design, certain tube types, other parts/pieces that I failed to mention.

[Gordy]

Originally Posted by KenCalgary

One of my problems is not knowing what features of a tube preamp allow these to come through: Is it high cost components, good design, certain tube types, other parts/pieces that I failed to mention.

The key is the topology of the circuit and it's implementation, or 'good design' as you put it. The power supply is also a prime consideration, and your unrecognised friend is 'good wiring practice, especially ground'.

The 12 dB that you have sounds a sensible max for most systems that include a standard front end (1 - 2 Volts output) and a power amp of 'normal' sensitivity (typically 1-ish Volts input for full output). You might be able to use a lower line stage gain (possibly 6 dB), depending on how 'hot' your music is recorded and how loud you like it (...but only you can tell).

One widely recognised kit is the Aikido (from John Broskie, as noted by tinitus). The site is here: John Broskie's Guide to Tube Circuit Analysis & Design .

If you decide to roll-your-own, a simple one stage common cathode design should work fine as long as you choose a tube with low gain and low internal (plate) resistance. I suggest you read up on common cathode stages and load lines as a good place to start.

[KenCalgary]

Gordy, thank you for the good information. I shall read up on common cathode stages and load lines as you suggest. Actually, I'll start by finding out what those terms mean which shows just how much I need to learn

[Eli Duttman]

Ken,

When wired common cathode, most triodes are not capable of directly driving the IHF 10 KOhm "standard" I/P impedance. An exception is the 12B4. There are more 12B4 based designs than you can shake a stick at. FWIW, I've uploaded my version of a 12B4 based line stage.

[Gordy]

Originally Posted by KenCalgary

Gordy, thank you for the good information. I shall read up on common cathode stages and load lines as you suggest. Actually, I'll start by finding out what those terms mean which shows just how much I need to learn

I hope I did not sound patronising when I gave advice, if so my apologies.

To help you along the path you might like to take a look / click here: Technical books online

There are a great many classic and useful tube books available there courtesy of the good Mr. Millett. One of the most comprehensive is a 25 Mb file of the book: Radiotron Designer's Handbook, Fourth Edition. I think most contributors here have downloaded a copy or sought out a hard copy.

There is also a very good book by Morgan Jones simply called Valve Amplifiers (available from Amazon, etc.)

[KenCalgary]

Originally Posted by Gordy

I hope I did not sound patronising when I gave advice, if so my apologies.

Gordy I did not take your comments as being patronising. I appreciate all the advice and information I receive. Even though I grew up with tubed radios and TVs, I had little or no knowledge of how they worked. I've started to learn but have a long way to go. Ken

[Brit01]

Just be careful with the gain on your tube preamp matched with a SS power amp.

I made the same error when I entered the hobby. First kit was a Aikido with 5687/6N1P.

Wayyyy too much gain for my power amp. Luckily I have a level input control on my power amp to tame it.

I used lower gain tubes and an Allen Bradley or Holco resistor in series before the volume pot to reduce gain somewhat.

Also tried a Cathode follower (buffer, no gain) but preferred the dynamics of a common cathode preamp.

One day will get round to building my tube power amp.