DIY Tube Preamp

[FBLEAGH]

Hi guys,

I want o have a go at making a preamp as my first foray into tube DIY.

I found the SJS line preamp (http://www.users.globalnet.co.uk/~valveamp/ under read it, line preamp , and second design + powersupply ) and the design looks nice and simple and fairly easy.

i jsut wanted a bit of help with the power supply section.
a) would a toroidal transformer be a good choice ?
b) what specs should i be looking for in the transformer. i guess it would be 220V primaries and 2 6.5v secondarys. but what about VA etc ??
c) where cana i get tubes locally ? ie australia

I was gonna chat to http://ourworld.compuserve.com/homepages/harbuch/ and see what thier price would be on the xformer.


any other things i should be aaware of with this design ??

[timc]

This link has a list of valve suppliers in Australia - http://home.mira.net/~gnb/elec/valveaus.html
Even Jaycar is selling a limited range of valves.
I find it just as easy and cheaper to order them across the net from places like http://www.thetubestore.com
You can also try guitar/music shops.

For the transformer, I think toroidals are fine, but many people have different opinions/preferences. They are one of the easiest to mount.
I would be interested to see the costs of the harbuch transformers in small/custom runs...

Looking at the design, if your are going for easy, I would use a solid state power supply instead of the valve rectifier shown.

Regards,
Tim

[halojoy]

But I found this page with a good tutorial.
(searched Google, keywords: tube line amplifier beginner)

My First Tube Amplifier

Here is another "TUBE DIY Page"
http://tubesall.hihome.com/

This is a real good one. Well described projects:
http://www.glass-ware.com/tubecircui...t_Archive.html

let's tube it

[Brett]

Howdy,

I'll make this really brief as I'm madly trying to finish a project for tomorrow. I live in northern NSW and build tube gear (and have for years). I'll try to add more later or feel free to email.

a) Transformer: Torroids are too wide bandwidth and let in far too much junk that's a hassle to filter out later. An EI is a much, <b>much</b> better solution. An electrostatic shield between the primary and secondary is a good investment too, esp if you're having them custom made. Try www.tortech.com.au in Sydney (strathfield?) All custom transformers are expensive, esp as you will need a few different winding for B+ and heaters.

b) I took a look at the schematic and I like it.
Transformer specs;
Primary: 230V
Secondary 1: 330V - 0 - 330V at 100 mA for the B+
Secondary 2: 6.3V at 2A for the GZ80
Secondary 3: 8V at 1.5 A for the signal tubes

The first two secondaries are over-specified by a large margin. Most of the cost of a custom trans is on labour, so it's better to over spec it, because it sounds better, and because most people who play with tubes will build something else later and it's good to have the extra rating in case you want something with a lot more grunt, always handy in a line stage. Besides, over-speccing means parts run cooler and are more reliable.
Actual continuous current draw on the secondaries will be a max of about 15 - 30mA, 600mA and 600mA.

I also wouldn't bother with DC on the heaters as they're indirectly heated, and so would get a 3.15V - 0 - 3.15V heater for the signal tubes, and use a resistive divider from the B+ to bias them via the centre tap to 40V or so.

c) Best place in Oz for tubes is David Crittle at www.retrovox.com.au.
However, he may not have what you want.
I'll give you a pair of new 6N1Ps if you send me your address details. Nice tubes.
I hardly buy any tubes locally. Most of mine come from the US, HK, Italy, Lithuania or the UK and it is a zero hassle fast reliable deal. I can recommend reliable sellers I've dealt. Airmail is fast, reasonably cheap, and unless you buy expensive tubes, you'll seldom cop import duties or Govt Suck Tax (keep under $A499 landed incl postage). I will provide more details of parts and tube sources later (I don't have all the URLs from where I'm posting. Trust me, importing is a yawn.

The PSU chokes are a minor hassle, but they're small so postage should be cheap. These will probably have to be impoerted. I always use Lundahls, but they cost around $A160ea and take about 4 weeks to order through the Australian importer.

More manyana

Cheers
Brett

[fdegrove]

Hi,

Keep in mind Mr.Shiltons' warnings against either topology 2&3.

To my mind #2 is the better of the two,but you can't drive long interconnect cables with that due to high output impedance.

The PSU as proposed is pretty much universal so no quibbles there.

Depending on the tube of your choice you may have to burn off some excess voltage.

If that's the case,I would regulate it ,if you want to do that with tubes you will then have to provide the extra heater current unless you want to just put in some glowdischarge tubes or a string of zeners that are decoupled locally.
To my ears it well worth the extra outlay.

Happy building,

[fdegrove]

Hi,

Here's a nice little circuit that is up to the task.
It will be a lot better at driving cable capacitance and should mate well with the PSU.



Cheers,

[G]

Hi Fred,

What is the approximate output impedance of the circuit you posted? Would it drive a Zen V4 with an input impedance of 47K or so?

G

[fdegrove]

Hi Gavin,

Since I don't see anyone named Fred here I assume you mean the circuit with the 6SN7s' a bit higher up.

Zout should be around 2K and it should be able to drive a 47K Zin.

Cheers,

[dhaen]

Thanks for the clarification Fred.

[G]

Sorry Frank. I haven't had my gallon of coffee yet this morning.

G