NewYorkDave Mila Dual Tube Preamp Help Thread

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Hmmm... You did use 12AV7 for V1 and V2, right? (No substitutions). Just for laughs, did you try a different 12AV7 for V1? Does V1 have a shield? Are the leads for V1A and V1B dressed away from one another? Is the - end of C9 connected to the B- buss at around the same physical point as R7, VR1, R9 and R11?

Check the lead dress all around, make sure grid and plate leads are not crossing one another. Also pay close attention to the leads connecting to V1A cathode; I still think there might be some unintended feedback happening there since the problem occurs when that cathode is unbypassed.
 
Dave, or anyone ....

Interested in building one of these MILA tube mic pre's

Is there a list of direct replacement transformer substitutions for

Input

Output

Power

Just in case prices or availability pose a challenge.

Also , does anyone have a BOM for this project in .pdf format. I hate having to install MSExcel just to open BOM's.

Thanks
 
Here is the BOM in Word (.doc) format if that helps:

http://www.stagefrightrecords.com/milabom.doc


I'm going to leave it the way it is for now and build the second one. If that is ok I will just redo all the wiring. It sure seems like lead dress. Cause initially I had white noise in all ranges. But moving components and wires got it down to low range only. I'll give it to my son in the meantime and exchange it for my new improved version 2 when completed. :) As it is now it is completely usable. I can't turn it into the problem range without going over 0 VU on my convertors anyway. So he will be fine with his 002.

Thanks
 
I finished wiring up my Mila last night and fired it up this morning to check voltages, etc. I have not passed any audio through it yet, so I don't have any comments on how it sounds.

Pilot LEDs and tube heaters work fine - I'm measuring 5.91 V.

I definitely have a problem with my circuit though... My B+ is high at about 312 V and the voltage drop through R21 is really low. I'm reading 285 V at C8, so the current through R21 is about half of what is shown in Dave's schematic.

I think the problem has to do with V2 - the plate voltage is waaaay too high (228 V) and the cathode voltage is way too low (1.31V). I'm going to re-check my wiring for V2 when I get home... seems like the current draw for this tube is much too low.

The cathode voltages on V1 are right on, and although the plate voltages are high, the voltage drops across R8 and R12 are proportional to the higher B+ that they are being fed (i.e. the current determined from the voltage drop I=V/R matches up with Dave's schemo).

So I'm reasonably certain that the problem lies somewhere in my wiring of V2. Like I said, I'm planning on checking out my wiring as soon as I get home, but if anyone has ideas to point me in the right direction, they would be much appreciated!

Can' wait to sort this out and hear what this baby sounds like.

Mason
 
hmm... I checked out my wiring of V2 last night and couldn't find anything that was obviously wrong.

Anybody have any ideas of what kind of problem I'm seeing based on the odd voltages on the plate and cathode of V2 that I reported above?

Mason
 
[quote author="pstamler"]Bum tube?

Peace,
Paul[/quote]

I was going to mention this as well... especially since it's a proven design & your wiring IS "correct"
 
Success!

I triple-checked everything and realized that I hadn't grounded the XLR output connector at pin 1. So I did that, switched out V2, and fired it up...

All the voltages look pretty good, the plates are running just a tad high (~2-4% higher than the numbers quoted on the schemo) and the cathode are running just a tad low (about the same deviation), but overall it appears that everything is within range.

Gotta run to work now, hopefully my new cables will come in today so I can hook it up to my audio interface and start passing audio through it. Can't wait to hear what it sounds like.

Right now I have a cardboard front panel on the unit because I wasn't sure of the final layout (I'm thinking of building something else into the box). I'll post picts as soon as it's "presentable". Sound clips will follow eventually.

Mason
 
New York Dave,

Sorry to be PITA, a lot of questions.

Hopefully I can get some help on this help thread.

can you recommend any other input iron for this project - I missed Kubi's group order for the Beyers on this project. I saw stancor mentioned earlier and read that any xfmr could be used as long as it meets the 200:20K ratio 1:10 or 150:15K ratio also 1:10. But this leaves a wide field of mind twisting choices, and I want to keep it simple and sounding good.

A list of direct replacement input tranformers would be helpful since the Beyers are not readily available now.

1. Should the 6.3V heaters off the power supply be regulated or does it matter ?

2. The resistors in the power supply - should these be power resistors rather than say metal film or does'nt matter ?

3. The bridge in the power supply d1-d4 IN4007 - can a regular bridge rectifier be used instead of 4 individual diodes , or does'nt matter ?

4. The rotary 2x3 switch , should this be shorting or non-shorting ?

5. Instead of 12AV7 tubes , would 5965's be a good sub ?

6. Can I take input xfmr with a primary of 20K and secondary of 200
and use it in reverse to get my input tranformer ?
 
A list of direct replacement input transformers would be helpful since the Beyers are not readily available now.

Any good-quality 1:5 to 1:15 input will do the job. There are so many possible choices out there--and if someone wants to make a list, that's fine, but I don't have the time to do it. The other guys seem to be happy with that Cinemag input they're using, so that seems an obvious alternative to the Beyer. I just used the Beyer because I had it on hand, not because of any special characteristics it had.

As for the other questions:

1. Answered by the schematic. I used an AC supply riding on a DC bias, but you can build a regulated supply if you like.

2. Power resistors: metal oxide, wirewound, whatever... Just make sure they meet or exceed the power rating given on the schematic.

3. You can use a packaged bridge if its ratings are adequate. If you don't have one that's rated at least 1A and 600V PIV, you're better off using the 1N4007s.

4. Either.

5. 5965A will work but might be noisier.

6. In theory, yes. But how well it will work depends on the particular transformer.

By the way, since this seems to be taking on a life of its own as a "project" on this board, now is a good time to post a disclaimer:

I have presented this schematic for reference only, to document something I built for myself. I offer no warranty, expressed or implied, nor accept any responsibility for the outcome should someone else choose to build the circuit for his own use. I do not recommend this circuit to anyone who is not experienced in the safe construction, testing and use of high-voltage tube equipment. My participation in "help threads" or the answering of technical questions from constructors does not constitute my endorsement of the MILA as a DIY project. No commercial use (e.g., manufacturing) is allowed without my expressed permission.

(Catching breath) Whew! OK, that should do it.
 
I built mine and sounds awesome! My son was extremely excited when I gave it to him for Christmas. Especially when he tried the DI input with headphones and A Jackson. I used the Cinemag CMMI-12C and pretty much sounds like a UA 610 with a mic. I tried both units back and forth and is REALLY close. I gave him an unlimited warranty on it. ;) And would like to thank NewYorkDave for such a great sounding device.

I have another to build for myself which I will finish this coming week. Before I get busy again...

Built as in the schematic it takes less than a minute to drain the capacitors to a safe voltage. Using capacitors from the BOM. So exercise caution and you won't have any problems. But use a meter to be sure!!

I am really sure a Jensen 115 would work equally well. But are like 75 bucks each. The Cinemag is probably a better choice.

Actually my son just called me a few minutes ago and is going crazy with it. LOL. He loves it. He said he had no idea what a difference it would make to his music. Now he wants a tube compressor...

Actually his biggest surprise was how clean the signal is at lower gain settings. He thought tubes always saturated. But as a DI he said he will never again use his tube plugin. (Samplitude 2) :green:
 
And the aluminum channel I used is less than 2 bucks on ebay ;)

Think was $1.91 each and was cut already to size.

PS... I didn't notice any differences lifting the output ground on the xlr. So I left it enabled.
 
CMMI-12C - no such transformer on Cinemag website ?

Whats a aluminum channel ?
 
Hello Dave, etc...

I asked this question earlier but may have been buried... If I am building a two channel unit, do I need to build a separate power supply for the plate voltage of each channel (sharing nothing but the power transformer)? Forgive my lack of experience with such topics. This question was addressed in another thread but no conclusion was reached.

Thanks everyone for your help!

Tim
 
Finally got a chance to test my MILA a bit today. Some good, some bad.

My initial impressions:

[Let me preface this by saying that I've never owned a true "high end" piece of equipment, so I don't really have a great reference point for comparisons.]

A)
I fired up the unit and hooked up my craptastical Dean Bass to the Hi-Z input. Let me first say that this bass has a replacement DiMarzio pickup. Before replacing the stock pickup it used to sound godawful terrible. After I replaced the pick-up it sounds run-of-the-mill bad...

That is until I heard it through the MILA. I was immediately shocked at the difference in gain from other DI's I've used, and also at the huge difference in clarity and articulation. The bass sounds lifeless and dull through everything I've ever used to record it, but through the MILA it finally had some life! There is that great springy/snappy warmth that you only get from tubes.

I pulled up a song that I've been struggling to get an articulated bass sound on, hit record, and suddenly the song came to life. Previously I had all kinds of effects Comp/EQ/bass amp models on it to get close to the sound I wanted, this time around it was plug-and-play and it sounded 10 times more energetic and vibrant.

HOWEVER, when I started fiddling with the controls, I noted two things wrong:

Problem 1:
the gain pot is responding in reverse - what I turn it all the way clockwise, the sound goes away, and when I turn it all the way counterclockwise it reaches max gain.

I am assuming I just have to reverse the connections on solder lugs 1 and 3 and that should be sorted out, right?

Problem 2:
The pad isn't working. There is so much gain in this baby - the range switch works as expected, but at this point it is overkill for me to use anything but the highest setting. So then I tested out the pad and.... no action. I was getting the same input level regardless of where the pad was set.

Edit: then I realized that I'm a dumba$$ and of course it shouldn't work since it comes before the Hi-Z input in the circuit. DUH!!!!!!!!!!

B) After fiddling with my bass for a bit, I went DI with my Gibson SG. Once again, there was great articulation - the clarity of notes is super and the snap and attack on the strings comes through wonderfully. I found that the preamp had a little bit too much top end for the SG, but this guitar is very heavy on the treble side, so I'm not surprised. I haven't played my Les Paul through it yet, but I expect it to give a more even sound. Regardless, there are a lot of very useable tones in there for DI guitar as well.

C) SHOCKED, absolutely SHOCKED at how quiet this thing is.

I am but a mere hobbyist when it comes to recording, so as I said I've never used high end gear, but the DI is soooo much quieter than any of the DI's I've used before, which include a Digidesign MBox (focusrite pres), a Presonus TubePre, a JoeMeek SixQ channel strip, and my Mackie Onyx 400F. The Mackie has the nicest DI that I've used before, but the MILA blows it away.

D) So I was super, super excited to throw a mic in the XLR in and hear what the pre sounded like with an acoustic guitar. Got out my JoeMeek JM37 condenser and my Sennheiser e609 dynamic, hooked up the JM37, and:

NOTHING!

No sound through the XLR in. Not a single peep out of either mic. There is a bit of noise when I plug the cable in or take it out, but other than that - nothing. I tried changing cables and that didn't make a difference. I tested the phantom power before firing up the pre and it was reading 48.7 V when I threw the switch. After I turned everything off I tested both mics with my Onyx pres and they worked fine.

So, in essence, I have a wonderful DI, a fully active range switch, a gain pot that works in reverse, and an XLR in that gives me nothing.

Debugging time! (if anyone has any suggestions on the first place I should check, I would be most appreciative). I'm not sure if I'll have the time tonight to get at this thing... obviously I made a mistake somewhere in the wiring of the XLR in to pad to input transformer section of the circuit since the DI passes signal through the preamp with no problem.

Mason
 
Came home and triple checked everything and reflowed a few joints that might have been iffy. Fired it back up and still nothing from the XLR. DI works great and sounds awesome. Put it through its paces with the Les Paul and was impressed.

I'm using a Cinemag CMQEE-3440A, wired for 150:50K as the input transformer. As far as I can tell everything is wired correctly. I'm pretty sure the mistake has to be somewhere between the XLR in and the Hi-Z in, as it seems like the circuit works fine from the Hi-Z to the output.

I removed the zobel network (R4/C1) because the Cinemag VP said you don't need one with this transformer. I replaced it with a jumper. The violet wire from the input traffo is tied to where R4 would have been, the gray wire is tied to the circuit ground. White goes to the chassis ground and black was connected to pin1 of the XLR in, but I tested it connected and disconnected.

If I had an oscilloscope, I'd start sending a signal in and tracing where it goes, but I don't have one. How do I check to see if the traffo is working correctly?

It's probably some dumb mistake like I have two wires connected backwards.... I'm perplexed because I'm really not seeing where the mistake is after checking and double checking and triple checking...

Mason
 
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