Dual Mila preamp.

[Anthon]

Hi there,

I was looking for parts for a dual Mila preamp by New York Dave.
After building a Madamp G3, I decided to make it P2P as well (in a 2U 19'' case)

I figured out how to get most of the parts for a reasonable price, but still can't find transformers.

Input: Can't find Beyer transformers.
I wanted to use OEP - A262A43E. Would it be a good substitute? Any other options?

Output: Edcor would be good, but they charge to much for shipment to Europe.
Any alternatives in the same price range?


Power: this one?
http://www.ebay.co.uk/itm/220V-115v-high-quality-35W-R-Core-Transformer-for-tube-preamp-220V-2-6-3V-2-/321734412250?hash=item4ae8dd87da:g:NWQAAOSwPhdVGRjr
Would it be enough power for 4 tubes? (concerned about 2x 0.8 amp on the 6.3v rail)
I'll add a small generic transformer for phantom power.


Finding parts is the hardest part  :-[

I'll make updates as I progress.

 

[ruffrecords]

Don Audio do a good range of toroid mains transformers:

https://www.don-audio.com/toroid-toroidal-audio-transformers-power-trafos

For output try Carnhill from audiomaintenance in the UK:

http://www.audiomaintenance.com/acatalog/Output_Transformers.html

For input to a tube mic pre I would suggest you need to look for a higher ratio than 1:2.

It would help if you included your location in your profile.

Cheers

Ian

 

[rock soderstrom]

Hi, the heater windings are too small for the 12AV7. Plz check http://www.r-type.org/pdfs/12av7.pdf

 

[ruffrecords]

Hi, the heater windings are too small for the 12AV7. Plz check http://www.r-type.org/pdfs/12av7.pdf

Not quite sure what you mean. 12AV7 heaters can be wired for either 12.6 or 6.3V operation.

Cheers

Ian

 

[Anthon]

Hi, the heater windings are too small for the 12AV7. Plz check http://www.r-type.org/pdfs/12av7.pdf

Not quite sure what you mean. 12AV7 heaters can be wired for either 12.6 or 6.3V operation.

Cheers

Ian

Maybe he means the power rating: the document states 12av7 needs 0.45A in 6.3v operation, while the transformer I suggested gives only 2x 0.8A.

I think I'll go with Don Audio transformer then.
I know Don Audio sells Edcor transformers - maybe I can contact them for XSM10k/600 (currently not in their supply).

Can you please specify which Carnhill transformer should I use? (VTB2291 perhaps?)
For input transformers, I've found Sowter, Lundahl version - but they to expensive  :-\
Any cheaper alternatives? (20-40 EUR)

I updated my location.

Anthon

 

[rock soderstrom]

Hi, the heater windings are too small for the 12AV7. Plz check http://www.r-type.org/pdfs/12av7.pdf

Not quite sure what you mean. 12AV7 heaters can be wired for either 12.6 or 6.3V operation.

Cheers

Ian

Edit: Anthon was faster 

Hi Ian, I am talking about the heater current. The ebay transformer from the first post can deliever only 2x 0.8A at 6.3V. Four 12AV7 for a stereo Mila will draw 4x 0.45A=1.8A.

Cheers

thorsten

 

[ruffrecords]

Ah, I had not realised the AV7 has a higher than usual heater current. Good catch. Finding the exact mains transformer you need is often a PITA.

The VTB2291 would be ideal for the output transformer. I use them all the time. They are exceptional.

Good input transformers are always expensive. It is the first thing your precious signal passes through. Any distortions it adds cannot be later undone. The only reasonably cheap ones which are still capable of acceptable quality are the OEP ones but you need to be very careful which one you select. I think NYD specificed a 1:10 transformer. The nearest OEP one I can find to this is this one, the A187A10C.:

http://www.oep.co.uk/page-content/datasheets/1438270402A187A10C.pdf

but I do not know where you can buy it.

Cheers

Ian

 

[Anthon]

Okay, I'll consider the VTB2291.

I know that quality is expensive, but I don't have much experience with studio grade projects (assembled guitar pedals, amps and 1 PIC controlled analog synthesizer). So I would like try out different options - and then see what works best for me. From what I've read, OEP seems like a decent alternative for the price.
Unfortunately, I can't find the A187A10C either  :-\

If I can't find it, I'll probably go for a Lundahl.

I see Don Audio has Lundahl LL1935 for 60 EUR. (1:10 200:20k)
Does AudioMaintenance has a suitable Carnhill option? (couldn't find it in their list).

I'm new to audio transformers, so just to make sure: when looking for a substitute should I look at turns ratio and impedance ratio (e.g. 1:10 200:20k) or is there something else?

Anthon

 

[Anthon]

Actually, it turns out Don Audio does have Edcor transformer 10k/600 (suggested by NYD).
https://www.don-audio.com/Edcor-XSM10K-600
Somehow I missed it...

Now I'm leaning towards buying Edcor as output, Lundahl LL1935 as input and also mains toroid form Don Audio. (will save shipment costs).

 

[ruffrecords]

Actually, it turns out Don Audio does have Edcor transformer 10k/600 (suggested by NYD).
https://www.don-audio.com/Edcor-XSM10K-600
Somehow I missed it...

Now I'm leaning towards buying Edcor as output, Lundahl LL1935 as input and also mains toroid form Don Audio. (will save shipment costs).

Sounds like a plan! A 1:10 transformer has a 1:100 impedance ration so 200:20K is 1:10. Transformers are poorly specified as a rule by manufacturers so it is best to check the data sheet (if there is one!!) The Lundahl LL1935 is a well specified transformer and should be fine for the Mila.

Cheers

ian

 

[Anthon]

Sounds like a plan! A 1:10 transformer has a 1:100 impedance ration so 200:20K is 1:10. Transformers are poorly specified as a rule by manufacturers so it is best to check the data sheet (if there is one!!) The Lundahl LL1935 is a well specified transformer and should be fine for the Mila.

Cheers

ian

Okay, I'll order these transformers from Don Audio. Thank you for the advice.

About meters, I wanted to use couple of these https://www.don-audio.com/Vintage-Meters-VU-Meter-VM2103
Are they just connected in parallel with the output, as if it was a regular voltage meter? Like in this circuit http://www.hicksorganservice.net/misc/preamp.gif

I've seen some meters on Ebay, but they need some kind of a driver board... I would like to avoid that.

 

[ding]

http://www.jlmaudio.com/AT51%20DIY%20VU.pdf

 

[Anthon]

http://www.jlmaudio.com/AT51%20DIY%20VU.pdf

So basically, an expensive AC meter can be used without a driver, but it will distort the signal.
The driver circuit in the manual will accept both AC and and cheap DC meter, without distortion.

This article helped a lot, thanks.

 

[ding]

Or you can just get a dual secondary transformer and just bang a meter to the alternate secondary. No signal gets injected into your output.

 

[Anthon]

Or you can just get a dual secondary transformer and just bang a meter to the alternate secondary. No signal gets injected into your output.

It was already hard to find a suitable output transformer for a right price and reasonable shipment costs...
Can you maybe suggest such a transformer? (10k: 600/600)

 

[ding]

Best thing to do is have it custom made. The hard part about these vu meter things is that when you are building tube gear where are you going to get 12v for it. You can always do a doubler from the 6.3v heater but that's one more step in the mix. My feeling is that if you can't get an appropriate transformer just go without the meter. There is a reason many tube preamps of yesteryear don't have meters. Almost all meters are just a rough estimate of output anyways. They are never too accurate. Unless you spend some serious money. And you can always meter your output thru you DAW or whatnot which is what you should be doing anyways. So the options I see are as follows.

1. Strap a vu meter on the output with its resistor and deal with the slight distortion and impedance issues.
2. Build a separate power supply for the vu meter driver and build a meter driver to go with the vu meter.
3. Go without a meter.
5. Get an appropriate output transformer with dual secondaries and strap a vu on the 2nd secondary.
5. Someone smarter than me might have a much more elegant solution.

I am also working thru this right now. This is why I started a group buy for a custom Edcor transformer. If that doesn't pan out I will just go without or strap the vu meter right on the output and see what happens. You never know.

Edit: #5 seems to imply than I may know something about electronics. This can not be any further from the truth. 

 

[Anthon]

Best thing to do is have it custom made. The hard part about these vu meter things is that when you are building tube gear where are you going to get 12v for it. You can always do a doubler from the 6.3v heater but that's one more step in the mix. My feeling is that if you can't get an appropriate transformer just go without the meter. There is a reason many tube preamps of yesteryear don't have meters. Almost all meters are just a rough estimate of output anyways. They are never too accurate. Unless you spend some serious money. And you can always meter your output thru you DAW or whatnot which is what you should be doing anyways. So the options I see are as follows.

1. Strap a vu meter on the output with its resistor and deal with the slight distortion and impedance issues.
2. Build a separate power supply for the vu meter driver and build a meter driver to go with the vu meter.
3. Go without a meter.
5. Get an appropriate output transformer with dual secondaries and strap a vu on the 2nd secondary.
5. Someone smarter than me might have a much more elegant solution.

I am also working thru this right now. This is why I started a group buy for a custom Edcor transformer. If that doesn't pan out I will just go without or strap the vu meter right on the output and see what happens. You never know.

Edit: #5 seems to imply than I may know something about electronics. This can not be any further from the truth. 

No meter is the most rational solution (and the easiest one too), indeed.
I think they are more of a marketing trick. But people just love them - and so do I  ;D who cares if it's completely useless, if it looks cool?

It won't be hard to design a small PSU just for the meters. You can get lower voltage by adding another transformer - just a small generic one.
But I'll think about it - this project will be hard as it is.

 

[Anthon]

Update:

I ordered all transfo's from Don Audio (2x LL1935, 2x Edcor xsm10k/600, 1x toroid 'Mila') and 2x VU meters (couldn't resist  ).
Total cost inc shipment: 241 EUR (I have a VAT number)

Also bought 4x Amperex 12AV7 NOS tubes from eBay, 35 EUR incl shipment.

Costs so far: 277 EUR.
Past the point of no return now!  :

I'll order 2U 19'' enclosure (45 EUR) + other hardware (connectors, knobs, mounting materials) form Musikding.

Caps, resistors (CMF55) and other electronic components: from RS-components probably.

Btw: can someone recommend good quality caps? Axial if possible (going P2P here!)

 

[rock soderstrom]

Hi, I like f&t axial caps for the psu.

http://www.tube-town.net/ttstore/Kondensatoren/F-T-Kondensatoren:::31_37.html

You will find all the other brands( JJ, Solen Fast, Mallory etc.) in this shop, too.

cheers

 

[Anthon]

Thanks, seems like a great shop! A lot of other great stuff there, I see.

Question: the PSU has 3 caps (2x 22uF and 47uF) - should I just double all the values? (the schematic is for 1 channel)

Also - originally I wanted to put everything inside 19 inch 2U 250mm deep enclosure, but maybe a 3U 220mm  is a better idea?
2U: toroid will have to go inside the enclosure. Tubes will be mounted outside.
3U: Tubes and toroid can be mounted outside. More room for point-to-point wiring.

Which one would you use? I guess even 2 Meters, 4 knobs, 7 switches and a lamp can make 3U front panel look pretty crowded.