Dual Mila preamp.

[ruffrecords]

Sorry, what I meant to say is I connected yellow and purple to the power switch (which is also connected to the mains), and 2 brown ones to the rectifier.  ;D

Only yellow and white had a resistance. So the pairs are yellow/white and orange/lilac.

OK, that at least is safer but I still think it is not right. From your description it looks like you have 220V mains connected across a single 115V primary winding - this means the secondary voltages will be much too high which explains the higher than expected  secondary voltages.

So you really do need to have the two primaries connected in series to get the right secondary voltages. Check the continuity between the orange and lilac. If it is open circuit then you have a faulty transformer.

Cheers

Ian

 

[Anthon]

White-yellow and orange-lilac have continuity.

Originally, I soldered yellow and orange (didn't connect it to anything), and connected white and lilac to the mains (thru a switch).

The two pairs were connected in series, but I'm not sure which wires should go to the mains and which should be fused and insulated.

Or should 0V's be connected and 115V's go to the mains  ???

 

[rock soderstrom]

Hi Anthon, Ian is right. You have to connect the two 115V primary windings in series. I would do it this way:

First bridge the two windings in series, connect white with orange then connect  lilac and yellow to the mains through a fuse and a switch.

As always, no guarantee! Don't fry your transformer or yourself! 

 

[Anthon]

Hi Anthon, Ian is right. You have to connect the two 115V primary windings in series. I would do it this way:

First bridge the two windings in series, connect white with orange then connect  lilac and yellow to the mains through a fuse and a switch.

As always, no guarantee! Don't fry your transformer or yourself! 

I described it wrong again 

In a nutshell:
I did lilac/white to mains (orange/yellow connected in series) it gave almost 0 volts on the secondaries. Certainly not good (edit: it burned the fuse).
I also did lilac/yellow to mains (orange/white connected in series) gives 300V on the secondary. Too high 

I don't see any other way to connect them (aside from parallel, but that will be not good for sure).

I checked my mains - they have normal 230V.

 

[ruffrecords]

OK, orange/white tied together and mains across yellow/lilac sounds right to me. This should give 250V AC across the two brown secondary leads when the winding has a load that draws 100mA  At lower loads this voltage will be higher due to the finite output resistance of the secondary winding. You are probably only drawing 20mA on the HT rail so it is not surprising the HT rail is higher than expected. An extra 2K in the HT RC chain will drop 40V at 20mA. The resistor will dissipate 800mW so better make it a 5W type or just increase all the series resistors in the HT supply such that the total series resistance is 2K. That should sort out the HT.

The heaters are more puzzling. Your load is 1.8A and the transformer is rated at 2A . I would therefore expect the loaded heater voltage to be a little above 6.3V but not much - certainly not 7.3V. I get the impression that both secondariness measure much too high. This implies one of the primaries may be faulty. The sort of fault that would cause the symptoms you see would be shorted turns in one of the primaries. You can test this by measuring the dc resistance of each primary winding. They should read almost exactly the same. if they differ significantly then I think you have a faulty transformer.

Cheers

ian

 

[Anthon]

I measured the resistance on each pair and it is 35.0 ohm and 37.4 ohm. Would this indicate faulty transformer?

 

[ruffrecords]

I measured the resistance on each pair and it is 35.0 ohm and 37.4 ohm. Would this indicate faulty transformer?

Probably not. One primary winding will be wound on top of the other so it will be slightly longer. There is only 7% difference between the two and I can believe the outer winding could be 7% longer.

The only other test I can think of is to check the inductances of the primary and secondary winding, Are you able to do this?

Cheers

Ian

 

[Anthon]

I measured the resistance on each pair and it is 35.0 ohm and 37.4 ohm. Would this indicate faulty transformer?

Probably not. One primary winding will be wound on top of the other so it will be slightly longer. There is only 7% difference between the two and I can believe the outer winding could be 7% longer.

The only other test I can think of is to check the inductances of the primary and secondary winding, Are you able to do this?

Cheers

Ian

I do have an inductance meter.

White-yellow: 114.2 mH
Orange-lilac: 115.2 mH
Brown-Brown: 20.20 H
Green-Green: 19.23 mH

 

[ruffrecords]

I do have an inductance meter.

White-yellow: 114.2 mH
Orange-lilac: 115.2 mH
Brown-Brown: 20.20 H
Green-Green: 19.23 mH

Turns ratio is proportional to square of inductance ratio. 114.2/19.23 = 5.93 which squared is 35.26. 230V divided by 35.26 is  6.52. Unless my calculation is wrong, this implies each of the primaries is a 230V winding not a 115V winding.

The 20H for the HT winding is way out.  It should be similar to the primaries. Did you make sure you did the measurement at 50 or 60Hz.

Cheers

Ian

 

[Anthon]

I'm using Amprobe 37XR-A. I guess the meter generates some frequency to measure the inductance, but there is no way to choose it yourself. Specs don't mention which frequency it uses.
I just disconnected the transformer, and connected the wires to the meter, picked inductance metering and got the results.
HT winding starts at 23H, then falls to 20H.

 

[ruffrecords]

I'm using Amprobe 37XR-A. I guess the meter generates some frequency to measure the inductance, but there is no way to choose it yourself. Specs don't mention which frequency it uses.
I just disconnected the transformer, and connected the wires to the meter, picked inductance metering and got the results.
HT winding starts at 23H, then falls to 20H.

Looking at the user manual, the test frequency for the 40mH and 400mH ranges is 1KHz and for the 4H and 40H ranges it is 200 Hz. One problem is the inductance of mains transformers increases as the frequency reduces.  Ideally you should measure the inductance at 50Hz but clearly this is not possible. I see the meter has a range button which can be used to turn off auto ranging. So, I suggest you repeat one of the primary readings and press the range button to freeze the range then go and measure the HT winding. At least then the measurements will be at the same frequency.

Cheers

Ian

 

[Anthon]

Looking at the user manual, the test frequency for the 40mH and 400mH ranges is 1KHz and for the 4H and 40H ranges it is 200 Hz. One problem is the inductance of mains transformers increases as the frequency reduces.  Ideally you should measure the inductance at 50Hz but clearly this is not possible. I see the meter has a range button which can be used to turn off auto ranging. So, I suggest you repeat one of the primary readings and press the range button to freeze the range then go and measure the HT winding. At least then the measurements will be at the same frequency.

Cheers

Ian

I fixed the range while measuring the primaries:
Primaries gave 2.86H each
HT: bounces around 19.23H
Heaters: 0.01H

 

[ruffrecords]

I have a similar transformer from Don Audio. Same HT and phantom windings. The heaters are 12V at 2A. I will measure the inductances and report back.

Cheers

Ian

 

[ruffrecords]

Here are the measurements made with my Agilent 1731C  LCR meter at 100Hz and 1KHz

Winding                100Hz            1KHz              Voltage Primary                12.8H            3.4H                240 250VAC              17.6H            5H                      290 48VAC                  1.04H            320mH              54 12VAC                  88.5mH        29.4mH            14

The primary voltage is the actual measured mains voltage here in the UK. The secondary voltages are measured with no load. The no load voltages are in the ratio of the square root of the ratio of the inductances at 100Hz as expected.  Clearly the inductance at 1KHz is not a reliable indicator of the actual inductance eat the operating frequency.

The no load voltages are, as expected, higher than the rated voltage. I think it would be worthwhile you measuring the mno load voltages of the transformer you have as that should give a clearer indication of what is going on.

Cheers

Ian

 

[Anthon]

I've found the problem: the heaters were wired in series, instead of parallel. I just looked at my previous project for heater wiring, and I forgot to account for the fact that it uses 6xxx tubes. They were glowing and the preamp worked fine, so I never bothered to check it twice.
Murphy's law strikes again.  :

Anyway, now I get 6.67V on heaters and 315V on B+, which is a little bit too high, but acceptable I guess.

I plan to do some upgrades, like adding extra RC stage/choke to PSU, regulated DC heater PSU (I will probably need an extra transformer), low-cut filter, phantom power - but first I would like to measure how it performs with RMAA and do some recordings with it.

 

[ruffrecords]

That's more like it! Heater volts is well within the allowed 10%. HT is high because the HT secondary is lightly loaded. You could tweak it down a bit by increasing the RC filter resistor values and gain a little extra smoothing in the process. Probably not necessary because the tubes are amazingly tolerant of variations in HT voltage.  Pleased you got it sorted.

Cheers

ian

 

[Anthon]

Yes, I will add an extra RC and choke - so it will bring down the voltage.

Anyway - I had some time to try it out, so I plugged my Gretsch G5420 in the DI and played a bit. I also compared it side by side with Focusrite ISA ONE.
I didn't expect this, but it puts Focusrite to shame.
Mila has punchier lows, better clarity, gives more gain and has noticeably lower noise. It beats it hands down, no competition. 
And there are still some improvement that can be made.

I didn't compare mic inputs, but I surely will. Maybe Focusrite will redeem itself.  ;D