NewYorkDave Mila Dual Tube Preamp Help Thread

[Animatic]

My first post here.
This preamp is what got me to join here.

And PRR is also here, I know his expertise in general
from the Hoffman board. He informed me of this great BBS.

I have been wanting to build remote tube pre-amps
with 48v phantom, but also remote gain controls and remote LED metering.
NY Dave if you have any comments on adding this to your circuit,
I would be beholden.

I have a Trident in NYC, but that's half way round the world,
and anyway I want tube stages to counter balance digital recording etc.

I plan to breadboard up a Mila and a 1 tube out of what ever parts
can be scrounged in Bangkok's Ban Mor district,
and then see how I can add the other two elements.

I am thinking also a higher mili-amp PT could power 4-8 of these
easily like a console but split into smaller groups.
250 one side 250 the other and not much tube draw on either side.
If I can get 8 a side I can do much of the big room like this.

3-5v DC lines to the preamp from the control room etc. Once it's DC
it won't bug the mics and high gain guitar amps too much.

So no doubt I will be asking some other 'under informed' questions
in the future.

 

[MasonAtom]

[quote author="Animatic"]My first post here.
This preamp is what got me to join here.

And PRR is also here, I know his expertise in general
from the Hoffman board. He informed me of this great BBS.

I have been wanting to build remote tube pre-amps
with 48v phantom, but also remote gain controls and remote LED metering.
NY Dave if you have any comments on adding this to your circuit,
I would be beholden.

I have a Trident in NYC, but that's half way round the world,
and anyway I want tube stages to counter balance digital recording etc.

I plan to breadboard up a Mila and a 1 tube out of what ever parts
can be scrounged in Bangkok's Ban Mor district,
and then see how I can add the other two elements.

I am thinking also a higher mili-amp PT could power 4-8 of these
easily like a console but split into smaller groups.
250 one side 250 the other and not much tube draw on either side.
If I can get 8 a side I can do much of the big room like this.

3-5v DC lines to the preamp from the control room etc. Once it's DC
it won't bug the mics and high gain guitar amps too much.

So no doubt I will be asking some other 'under informed' questions
in the future.[/quote]

Phantom Power is easy enough to put in. I used a totally different PSU in mine, featuring a 15k dropping resistor and a couple of 24V zener diodes and it works very well. I'm not sure that I ever drew up the whole schematic anywhere, but I can take a look. I think I basically used the "Mike Vaughn" preamp PSU from boozhound labs and wired the switch as per the phantom power schematic that is floating around on the Jensen website.

A bigger tranny should work to power multiple units. IIRC, the quiescent current through the tubes is like 10 to 13 mA or thereabouts. Figure you need about 1A to power the two tubes and whatever LEDs that you are running. Hammond makes some burly big transformers that should do the trick. Also, Antek makes a 250V, 150VA transformer that supplies 3A on 6.3V and 2A on 12.6V... so you could probably run several units off that. As a bonus, it is small enough that it will fit in a 2U enclosure.

BTW it is, IMO, a very very good preamp. I finally finished the engraving of my frontpanel and will hopefully post some pictures once I get the engraving backfilled and assemble the unit.

Mason[/url]

 

[MasonAtom]

[quote author="Animatic"]My first post here.
This preamp is what got me to join here.

And PRR is also here, I know his expertise in general
from the Hoffman board. He informed me of this great BBS.

I have been wanting to build remote tube pre-amps
with 48v phantom, but also remote gain controls and remote LED metering.
NY Dave if you have any comments on adding this to your circuit,
I would be beholden.

I have a Trident in NYC, but that's half way round the world,
and anyway I want tube stages to counter balance digital recording etc.

I plan to breadboard up a Mila and a 1 tube out of what ever parts
can be scrounged in Bangkok's Ban Mor district,
and then see how I can add the other two elements.

I am thinking also a higher mili-amp PT could power 4-8 of these
easily like a console but split into smaller groups.
250 one side 250 the other and not much tube draw on either side.
If I can get 8 a side I can do much of the big room like this.

3-5v DC lines to the preamp from the control room etc. Once it's DC
it won't bug the mics and high gain guitar amps too much.

So no doubt I will be asking some other 'under informed' questions
in the future.[/quote]

Phantom Power is easy enough to put in. I used a totally different PSU in mine, featuring a 15k dropping resistor and a couple of 24V zener diodes and it works very well. I'm not sure that I ever drew up the whole schematic anywhere, but I can take a look. I think I basically used the "Mike Vaughn" preamp PSU from boozhound labs and wired the switch as per the phantom power schematic that is floating around on the Jensen website.

A bigger tranny should work to power multiple units. IIRC, the quiescent current through the tubes is like 10 to 13 mA or thereabouts. Figure you need about 1A to power the two tubes and whatever LEDs that you are running. Hammond makes some burly big transformers that should do the trick. Also, Antek makes a 250V, 150VA transformer that supplies 3A on 6.3V and 2A on 12.6V... so you could probably run several units off that. As a bonus, it is small enough that it will fit in a 2U enclosure.

BTW it is, IMO, a very very good preamp. I finally finished the engraving of my frontpanel and will hopefully post some pictures once I get the engraving backfilled and assemble the unit.

Mason[/url]

 

[Animatic]

Mason, Thanks for the reply.
I will look around for this stuff.
I have found a small Bangkok transformer company
that will custom wind one offs to my specs .
They seem to be supplying the small audiophile amp market here.
I got a 30 watt PP transformer , primary is multitap up to 10K and sub-divisions,
with 8Ω and 16Ω secondary for $14 US....
I don't imagine their others are much more expensive.
I say; here's a Hammond spec, and they build it.

I think they likely can also do 16 mic input transformers easily too.
We'll see.
Good to hear it is a VERY nice preamp.
Simple and clean works for me, and juggle the tones AFTERWARDS.

I started with just an Ampex 351 and 4 Norelco C60 mics into
a custom P to P mixer with 1/2 dual triode for each mic
and 1/2 of another for a gain/summing channel for each pair.
So I like the classic; use the acoustics and play it right the first time. methods.

But also the edit it digitally and use time lines for precision thing too.
Sure beats razorblades...
I haven't used a board in 15 years, just pre-amps.
But my new room is now MUCH bigger. And I want line level returns
from 15 meters off, not long mic level runs. So spread the pre-amps out
Groups if 2-4 from a common 'PS and metering web' and then just look at
the gain knobs and LED bridges for each space.

Big boards impress people, but you don't really need them.
And they cause mixing room issues like early, short reflexions etc.
I like to record in the control room also, so why deal with it;
give me space.

PS I looked at your 2watt amp. Very nice.
I grafted a AX84 pre-amp onto a Angela Super SE amp and like it very much.
It uses a 6SN7 as the 2nd tube rather than the 12AX7,
and I moved tone stack and gain around because it's SE and has no PI,
but it's cool.
it runs 1 or 2 6V6 or 6L6. into a BIG Weber WSE25 OT.

I like the idea of a 6SN7 powered amp. Good tone at low volumes works for sure.
I see an open octal socket is that for a second parallel 6SN7?
What was the OT impedance youi chose for the 6SN7?

 

[MasonAtom]

[quote author="Animatic"]Mason, Thanks for the reply.
I will look around for this stuff.
I have found a small Bangkok transformer company
that will custom wind one offs to my specs .
They seem to be supplying the small audiophile amp market here.
I got a 30 watt PP transformer , primary is multitap up to 10K and sub-divisions,
with 8Ω and 16Ω secondary for $14 US....
I don't imagine their others are much more expensive.
I say; here's a Hammond spec, and they build it.

I think they likely can also do 16 mic input transformers easily too.
We'll see.
Good to hear it is a VERY nice preamp.
Simple and clean works for me, and juggle the tones AFTERWARDS.

I started with just an Ampex 351 and 4 Norelco C60 mics into
a custom P to P mixer with 1/2 dual triode for each mic
and 1/2 of another for a gain/summing channel for each pair.
So I like the classic; use the acoustics and play it right the first time. methods.

But also the edit it digitally and use time lines for precision thing too.
Sure beats razorblades...
I haven't used a board in 15 years, just pre-amps.
But my new room is now MUCH bigger. And I want line level returns
from 15 meters off, not long mic level runs. So spread the pre-amps out
Groups if 2-4 from a common 'PS and metering web' and then just look at
the gain knobs and LED bridges for each space.

Big boards impress people, but you don't really need them.
And they cause mixing room issues like early, short reflexions etc.
I like to record in the control room also, so why deal with it;
give me space.

PS I looked at your 2watt amp. Very nice.
I grafted a AX84 pre-amp onto a Angela Super SE amp and like it very much.
It uses a 6SN7 as the 2nd tube rather than the 12AX7,
and I moved tone stack and gain around because it's SE and has no PI,
but it's cool.
it runs 1 or 2 6V6 or 6L6. into a BIG Weber WSE25 OT.

I like the idea of a 6SN7 powered amp. Good tone at low volumes works for sure.
I see an open octal socket is that for a second parallel 6SN7?
What was the OT impedance youi chose for the 6SN7?[/quote]

Thanks for the kind words on the 2W amp. I really like the 6SN7 output in a push-pull amp. Has a much "bigger" sound than you'd expect of a 2 Watt amp, which is cool for recording in a home studio. I can't really crank up a 100W Marshall living in a townhouse...

I believe the OT impedance was 22K. I have a few new preamp designs and I want to be able to test them quickly without having to build up completely new amps, so the I'm going for a modular approach. I'm building a 2W PP poweramp based in one 2U rack and will rack up the different preamps in 1U racks. The poweramp is based on Cliff Chappell's October Studio (AX84).

What did you think of the 6SN7 as a preamp tube? I've been dying to try it in either a guitar amp or mic pre.

Mason

 

[khstudio]

[quote author="timmygrimm"]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[/quote]

Bumping this because I'm building a "Stereo Unit" too.

Should the (Individual Plates) B+ to the Left & Right channels be decoupled :?:
& if so... Could someone explain how best to do it??? :green:

 

[dissonantstring]

hey kevin,
i just built a 2 channel MILA, but i haven't gotten around to any serious testing, but so far so good.

i used a Hammond 269JX (too much B+ when rectified - some 390V and i don't suggest using it as adding resistance doesn't help much with such low current draw), but it worked fine supplying B+ to both channels after some tweaking to get the B+ down.

Dave (or just about any of the more knowledgable member here) will know better than i if running two channels of B+ from the same transformer will have inherent problems, but i don't think so. as long as your current draw for both channels is within the transformers capabilities. it hasn't really caused any issues on my build, but as i said, i've not done any serious testing yet. it passes signal, PAD/POL/48V all work. just a little hum and it had some osc, but after some burn in time on the NOS 12AV7s the osc went away. i'm trying to add the switchable zobel network on the input, but i haven't found too much difference in them. i really should run proper tests on them with my friend's A2.
-grant

 

[dissonantstring]

deleted (double post)

 

[Animatic]

I just had this same discussion with a transformer builder this AM.
I found a small company that will build me anything I want,
OT's and PT's, chokes and line drivers,
and maybe a few interstage trans to try out.
.
And they will take a shot at small sheilded input Trafo,
more or less Cinemag CMM 7... something, style and/or Jenson,
like the Prodigy Pro group buy NYDave used for Mila 1.

He's worried that the quality of small lamination metals isn't good enough here.
Any of y'all know of a source for good transformer lamination metals???

I got him the spec sheets, and he will see what he comes up with for input.
Seems like a nice intelligent guy, family operation.
Grandma talking soaps next to me while I talk mic preamps,
her great grandchildren web surfing next to her.

We discussed running 4 mila on one PT and he suggested 100ma
but four 125v taps on the same iron, each with a diode bridge.
Should be little or no interaction.
This isn't like over-driving EL34'as in any case.

I have gotten almost all the parts for 2 Mila's to be built in the same frame.
And then likely duplicate that in several 2 mic pairs,
but with reduced transformer count.

Should be interesting.
I have a cheezewiz LED bridge for monitoring levels.
But haven't made up my mind on a DC remote level scheme yet.
Gotta reup my 'sand' knowlege base...

 

[rafafredd]

I think nowadays you should be able to get good permalloy or amorphous core from china for good input transformers.

 

[Animatic]

I think they usually want a shipping containers worth...

And specs on this metal?

 

[Animatic]

Now that I can have any transformer wound for powering my preamps,
it's possible to add a NPN or some such to replace VM1 after V1a.

I can ask for a winding set for a transistor voltage diode bridge,
in addition to the B+ and filament voltages.

Common Base stage? Emiter Follower?
AC coupled amp with RB2 as a remote low voltage/current pot?

Or just something that shunts R9 to ground
controled from a low voltage source?

MUCH too tired after the recent trip.
Forgot too much transistor info over the years.
Not finding what I want, so far, from searches.

Basically I want a 5Vdc level to control gain to V1b
from any distance I want. So 24v + / - levels for the chip
and divide some of that for a control voltage of the chip.

Basically I want a remote pot with low dc control voltage
to replace Mila VM1 pot.

I have little experience with hybrid tube/sand parts matching.
I know what I want to do, I know it can be done,
but have no competent english speaker to bounce ideas off of
face to face.

Ideally....
I don't want the signal passing through the transistor at all,
just having it control the gain of the signal.

 

[MasonAtom]

Finally assembled my MILA. I finished the circuit in January but was waiting to put the front panel together... it was my first experience in using a rotary engraver so it took me awhile to figure out how to write the G-Code and do the backfilling.

Here is the result:

I back-filled the engraved art with Testor's enamel paint. I used a syringe to apply the paint. This is tricky because you have to press done the plunger to get a flow, but then pull back on it ever so slightly to get a bit of back pressure so the flow isn't too fast. If it's too fast it results in bubbles. Then you have to use isopropanol to dissolve away any overflow.

Here's another image of the front, this time a close-up where you can see the two 12AV7 tubes hanging through a window. The window is back-lit by blue LEDs and looks pretty badass when the unit is on. Plus you can see the tube filaments glow... (I should have taken a picture of the unit powered up)

Here are some pictures of the front panel shot at an angle - you can see that the "Range" knob protrudes... I need to trim the post of the Lorlin rotary switch, just haven't gotten around to it yet.

This is what the guts look like... this is the first thing I had ever built from a schematic that didn't already have a well-established layout, so it's clearly not a great layout. I learned a lot from this, mostly what NOT to do. As you can see it is a rat's nest. A lot of that is due to the fact that in order to be able to change out the tubes, I needed some slack on the wires.

Oh, one side of the unit has a line level to instrument level converting circuit it in - rrrrrrrrrrr*amp.

I recorded some clips yesterday but my DAW ate them. Not sure when I'll get another chance to record, but I'll post results when I do.

Mason

 

[MasonAtom]

So the good news is that the DI sounds pretty fabulous. Bass guitar sounds great through it... it has that gritty slightly dirty tube character that just gives bass that extra 'umphh'. The DI sounds soooo much different than any of the solid state units that I've used in the past.

Now the bad news... the XLR input has some issues. There is a lot of noise, mostly in the 60 Hz and 120 Hz range. I chop-sticked the wires to the tube sockets, but that didn't help much, plus I'm pretty sure that the problem isn't there as then the noise would also show up when I'm using the DI, right?

So I figure that the problem must be between the input XLR and the mic input transformer. Any ideas of what would be causing some background hum in this part of the circuit? My mics sound really good through this but the takes are unusable because of the background noise. I'm planning on re-flowing all of the solder joints in the early part of the circuit to see if that is where the problem is. I'm also planning on putting in a metal partition between the preamp and reamp circuits to isolate the two - worried about the transformers picking up fields from each other.

Big thanks for NewYorkDave for a really awesome project. This unit sounds great and I can't wait to get it quiet enough to use on a regular basis.

Mason

 

[Animatic]

On the noise my guess is wire dress between heaters and grids.
On an SE amp I lifted the heater center tap to a cathode on a power tube
got a lot quieter, but should help on a 12 axxx too.

Maybe an alternative to the upside down tubes is called for...
I couldn't even clearly see what's heaters and whats P to P I/O for the tubes.

Otherwise looks cool, nice front etc.

Ok took another look.
For my tastes the green and black wires which I assume is heaters,
while wound, are pretty much weaving in and around your grids
too close.
Fenders like them up and over, Marshalls like them at the case lip down
flat on the far side from any signal to and from tubes.
I lean towards the later as being easier to work on.

And I would not have put the power transformer in such proximity
to the Cinimag input T, nor the heater wiresnear it too.

Not trying to be too critical, but I have been building high gain guitar
amps lately, and this stuff is pretty critical in them,
so I assume so in tube mic pre's too.

If you inverted the tube stand and lowered it, wire under it,
you could change tubes easier and still see them. A thought anyway.
I'm not a serious tube amp diagnostics expert like PRR.

3rd look, you have your AC surrounding your mic input transformer
2 sides at least. it's right next to the PT, has AC passing from switches and light...
I got taught power at the power end,
and sensitive low signal as far from that as possible. Yours are co mingled.

Nothing you can't fix and it will all be cool. Good luck.

 

[khstudio]

MasonAtom,

Dude... the faceplate looks GREAT!!! :thumb:

BUT, the inside has some problems.

#1 - Your input Tranz (Even if it's shielded) should be as FAR from the Power TX as you can get it :!: This could cause = HUM 60-120Hz :!: Especially with a Non Toroidal TX.

#2 - ALL your 6.3v filament wires are all over the place... crossing too many things. This = HUMMMMMMMMMMMMMMMMMM 60-120Hz :!: :!: :!:

You have to be VERY careful laying out TUBE gear. It's very sensitive especially to HUM. Also, Try not to CROSS the "Plate" & "Grid" wires = these are the "Input & Outputs" of the tube.
The filament 6.3v wires SHOULD be wound VERY Tight & run close to the chassis away from other wires. (Unless they're DC)

Good Luck.

 

[MasonAtom]

thanks for the comments/suggestions Animatic and khStudio.

yeah... the layout is a freakin' disaster. This was the first project where I went from a schematic and did the circuit turret board layout myself. I had one evening to drill the chassis and unfortunately I hadn't figured out my overall layout by then, so I went into the shop and just drilled holes to stuff everything in the chassis, 'cuz I didn't know when the next time I'd have access to the shop would be. Obviously not the best strategy, but you live and learn I guess. I knew PT and OT needed to be separated as best as possible but was unsure about the mic transformer.

I finished wiring the circuit in January and did some quick tests then, so I knew there were some noise issues that I'd need to resolve back then. In between the time I finished this circuit and finished the front panel (2 days ago), I built 2 more guitar amps, this time taking the time to do a thorough plan of the layout in advance, keeping the heaters from the grids, etc... These amps are pretty quiet and sound very good IMO.

So I figured that the heaters/grid dress would be an issue with the MILA.

But here's what I don't understand - why is the DI quiet if the heater/grid dress is the problem? I mean, it certainly LOOKs like this would be the first thing to fix... but since the DI signal goes through the same tubes, wouldn't it pick up the same noise as the XLR signal does? Am I missing something here?

I'm going to re-wire the tube sockets and reroute the heaters first. If that doesn't work then I'll think about moving the PT. BTW - the heaters are DC.

Should I used a shielded wire for the signal to the first grid too? I do that in all of my guitar amps.

Mason

 

[khstudio]

DC on heaters, good... then don't focus on them... try moving the Input tranny & input wires away from the Power TX or ANYTHING THAT'S AC!!!

 

[khstudio]

O... your DI is probably quieter because there's no "Step-up" in gain & it doesn't appear to be close to the PT or any AC lines... that's what your getting, 60Hz Hum bleeding into your audio somewhere... I'd bet the input tranny & or wires going TO it. :wink:

 

[MasonAtom]

[quote author="khstudio"]O... your DI is probably quieter because there's no "Step-up" in gain & it doesn't appear to be close to the PT or any AC lines... that's what your getting, 60Hz Hum bleeding into your audio somewhere... I'd bet the input tranny & or wires going TO it. :wink:[/quote]

Excellent. Thanks for the help. Not sure when I'll get the chance to work on it again but this is where I'll start.

Mason