Tube Pre project

Hey guys,

I've been watching this forum for a few months but this is my first post.
For quite some time I have been building electronics for audio. Mainly microphones but also custom cables and repair.
Recently I got into the preamp world and my first build was a 312 clone.

Yesterday I was at a house that used to be a cinema and it had all kinds of nice equipment. I found an old box with spare parts and the guy that owns the house now gave it to me.
Some stuff that caught my attention in particular were some tubes. 2x ECC 83 and 1x EF 86. All Philips tubes, still in their yellow boxes and probably from the 1960's. Also I found a couple of input transformers, Sennheiser TM 003.

I'd really like to use those parts for something nice and I was thinking about building a pair of mic pre's, each with 1 ECC83 and an input transformer. It doesn't have to be perfect, I'd like it to sound like an old tube pre so simple is perfect.
I already found and bought this elektor article: http://www.elektor.com/magazines/2003/february/ecc83-(12ax7)-microphone-preamplifier.55031.lynkx
However I'd like your opinion. Has anyone built this schematic? And what power supply should I use?
Does anyone have better ideas? What would you do with those parts?

-Bergur from Iceland

Schematic?

Here it is, might be protected with copyright, please inform me if it is and I'll remove it.

In the article it states that it needs 350V and 12.6 V, 48V for phantom power would be nice to have too.

Output unbalanced and very high output impedance.
Don't count on it driving long cables or low-impedance inputs.
No easy way of changing this in a simple setup like the one shown.

Jakob E.

Ok, what about a UA 610 clone?
I have the schematic, could I use the sennheiser transformers for input? And possibly some Lundahl's or even just Edcor's for output?
And does anyone know what kind of a power supply the 610 needs?

-Bergur

do you know the ratio of the sennheiser, do you know anything about that transformer?

Mine are 1:15
Primary 200 ohms

Here is the datasheet: http://www.surplussales.com/Transformers/Pdfs/tm-tm003.pdf

Are you willing to add some tubes? How sophisticated do you want this to be? That transformer has a huge step up!

Yes more tubes are fine but I don't want it to get too complicated though.

Then just add an ecc88 srpp output stage into a 2:1 transformer and you're golden. run it at about 250v.

Do you want the transformer set up for 1:5 or 1:2.5?

1:15 was a pretty normal step up for tube mic amps...

Add it onto the Elektor pre schematic?

gemini86 said:

Then just add an ecc88 srpp output stage into a 2:1 transformer and you're golden. run it at about 250v.

Click to expand...

Yeah, probably a good place to put a 50k pot (or close to) for volume control. Take a look at Gyraf's g9 schematic for an example.

How much maximum gain do you want? I have a push pull tube amp that has 60dB of gain (without a transformer) and 0.03% distortion (but into a 10k/10k load), according to the SPICE sim. I have not built it so I don't know if Tina (my version of SPICE) is telling me the truth. Some folks told me it won't work, so I don't know. Bandwidth is not great, with -3dB at about 30kHz.

Well, I threw this one together. It's pretty good, but I don't have good spice models for transformers so I don't know what it's going to do with them. You'll need both input and output transformers. This one is much better than my other one but it doesn't have as much gain. Gain is controlled by changing the degeneration resistors. The diodes are the constant current type. You can make the -8 volt supply by rectifying and filtering the heater supply for the tubes. Gain is 35 or 45dB to 100kHz. This is just a rough sketch, subject to more refinements. Distortion is showing as 0.0001% for the low gain setting and 9x10^-5% for the high gain setting. Go figure. (1kHz, 1mVp-p input)

Great !
Thanks for the help guys, I will be looking into output transformers and other tubes now and decide what I want to do

-Bergur

Dirk,

Take a look at this:

http://www.gyraf.dk/schematics/Disa_91_b21_micamp.gif - not too different from your approach, and very good.

Jakob E.

gyraf said:

Dirk,

Take a look at this:

http://www.gyraf.dk/schematics/Disa_91_b21_micamp.gif - not too different from your approach, and very good.

Jakob E.

Click to expand...

Hey thanks! That makes me feel good, knowing I'm not crazy or stupid. I was pretty sure things like this had been around for over 80 years now. I tried other arrangements but this one seems to have the most gain and lowest distortion, at least in "simland". I've never built one of these though so reality may be different.

This is the one with over 60dB of gain. Bandwidth is not great because of the cascaded 12AX7's and their Miller capacitance. Distortion is low @ 0.002% (1kHz). This one has common mode feedback, but I've been told on another forum that it won't work because any imbalance in the tubes or components will make it oscillate. I have not built it so do not know from experience. Gain again is controlled by changing degeneration resistors. CMRR is well over 100dB, but of course this is in simland where everything is perfect.