NewYorkDave Mila Dual Tube Preamp Help Thread

[Stagefright13]

Yeah I am sorry pin 3/8 is 2.43 volts. And actually R6 is pretty far away let me change it. I will report back in a bit after I play some more. BTW what does R4 do? I am using Cinemag CMMI12C for input. Do I need it?

Thanks!

 

[NewYorkDave]

Mason, you're confusing grid stopper with grid load. Short answer is, you don't need to adjust it. As for grid load, 1M (R5) is still plenty high even with a 50K primary.

John, R4 is part of a Zobel network. I don't know if you need the network with that particular transformer. This is where the ability to test with a square wave generator and scope comes in really handy.

 

[louder]

Hi
which resistors benefit from being soldered on the tube sockets?.
i have been thinking about this,while making the layout.
best regards
pedro

 

[MasonAtom]

[quote author="Stagefright13"]Yeah I am sorry pin 3/8 is 2.43 volts. And actually R6 is pretty far away let me change it. I will report back in a bit after I play some more. BTW what does R4 do? I am using Cinemag CMMI12C for input. Do I need it?

Thanks![/quote]

R4 and C1 are for the Zobel network to flatten the frequency response of the input transformer, as some transformers apparently overemphasize the high end (apparently like the Beyer that Dave used in the prototype). I'm also using a Cinemag and I talked to their VP and he said that you don't need a Zobel with the model I'm using, but you should check with them on yours (since it is a different model than the 150/50K that's in my MILA).

But I think that if you are getting a sound that you like and can live with, you don't really need to adjust the Zobel network.

Mason

 

[Stagefright13]

Yeah I took out R1 and C1 and sounds great. More clear highs. It works perfect unless I hit 3/4 or above on the gain. But at full gain it works fine again. At 3/4 I have buried my protools VU some time before. I may have a missed wire somewhere. The DI sounds GREAT with a Jackson plugged straight too btw... I would try a bass too but gotta get this ok for Wrapping paper! But I have some time left. Lemme post a pic maybe you can see something stupid.

Man dead quiet too. Except the white noise wich I am sure is my fault somewhere. Still looking.

 

[Stagefright13]

Pics:

 

[louder]

Hi John
very nice work you got there.
can you tell me(us) how it sounds?.
do you have a g9,that`s also a tube pre?.
best regards
pedro

 

[MasonAtom]

hey John, looks cool! Congrats on getting it working. Glad to hear that the DI sounds good, I'm looking forward to what it sounds like with a bass.

couple quick questions - 1) do you not have the -20 dB pad switch? I'm only seeing the one dpdt switch and you only have two connections to it (phantom power?). 2) why two rotary switches? one is for gain range, but what is the other? Just a little bit confused. 3) i think I see the chassis ground tied to your transformer - looks like you're running a wire from pin1 of both your XLRs to this, right?

Mason

 

[Stagefright13]

The first lorin is the pad I used a 4 pole 2/3 pos. The switch is phantom. It's got a huge bottom end should be great for bass DI. The guitar DI is awesome. And my son plays guitar and sings so this will REALLY rock his world.

Just gotta get rid of the white noise thingy at 3/4. I am getting close tho. And thanks Mason should look cool after I finish the lettering on front.

I won't probably be able to compare it to anything 'till after Christmas. And I wouldn't even try 'till I get it working as New York Dave designed it. This is my son's gift. Altho I have another to build also. But it does sound VERY high end. It pretty much sounds exactly like my UA 610's.

 

[MasonAtom]

[quote author="Stagefright13"]The first lorin is the pad I used a 4 pole 2/3 pos. The switch is phantom. It's got a huge bottom end should be great for bass DI. The guitar DI is awesome. And my son plays guitar and sings so this will REALLY rock his world.

Just gotta get rid of the white noise thingy at 3/4. I am getting close tho. And thanks Mason should look cool after I finish the lettering on front.

I won't probably be able to compare it to anything 'till after Christmas. And I wouldn't even try 'till I get it working as New York Dave designed it. This is my son's gift. Altho I have another to build also. But it does sound VERY high end.[/quote]

that was my suspicion, but I wasn't sure. Just out of curiosity, why go with a lorlin instead of a dpdt for the pad? Also, can you answer my grounding question?

Wish I could help you with the burst of white noise at 3/4 gain.

Mason

 

[Stagefright13]

Yes I am running it as the schematic. I didn't deviate from Dave's schematic. I am just an amateur experimenter! Try not to look to me for advice. (Ozzy Song "I Don't Know") Newyorkdave is the culprit who designed this awesome pre. BUT I might try a ground lift switch on the out xlr. Sometimes is nice if you don't plug all your gear into one socket...

I used the Lorin cause I have 4 extras here and no switches lol!

 

[Stagefright13]

Also hums bad when A mic is not plugged in. Any clues?

 

[timmygrimm]

This seems like a cool project. I'm tired of pc boards and am looking forward to my first from-scratch p2p project. I'm going to use the Hammond transformer (Hammond 270cax) to power two channels. It has 70ma of current for the plate voltage.

In another thread, people were talking about using a separate power supply for each channel's plates (shared transformer, separate rectifiers, caps, etc.) Is this necessary or significantly better sounding? I don't wanna skimp.

I'm also wondering about output transformers (I'm going to use the Beyer that NYD used for i/p (thanks kubarth!)). Do people feel there would be a significant improvement by using something pricier than the Edcors for o/p?

Also wondering about cabling/connectors for power, since I plan to do an external PS. I like the idea of a separate box for noise reasons and also like that I can detach the PS and worry less about getting zapped when I'm working on it!

Thanks for the project, Dave!

Tim

 

[Stagefright13]

Still having trouble with this. I got it so it is perfect in medium and high range. But in low range I still have bad white noise after 3/4 throttle. I may just remove the low range...

Could I add a capacitor or something? Just looking for suggestions.

On the schematic where R20 and sw2 meet there is a connection to B-. However there is no connection dot. Should I have connected this?

Thanks

EDIT: I tried it with out that wire and it works better with it.

 

[NewYorkDave]

Yes, the junction of R20 and SW2 is connected to B-.
Orcad (my drafting program) did not want to let me place a dot there, for some reason.

You say the noise problem only happens on the low gain range. Does it happen with both mic and high-Z inputs? What happens if you temporarily ground the grid of V1A--does it go away?

Double-check all the wiring associated with SW2, R18, R19, etc. Make sure you haven't accidentally cross-connected the two sections of the switch; there should be no electrical connection between them.

 

[Stagefright13]

yes makes the same white noise even when using DI. Wich pin is the grid?

Thanks

 

[NewYorkDave]

Pin 2. Make sure you get the right one--and keep one hand away from the workbench.

 

[Stagefright13]

Yeah still noise when I grounded it...

 

[NewYorkDave]

Did you...

Double-check all the wiring associated with SW2, R18, R19, etc. Make sure you haven't accidentally cross-connected the two sections of the switch; there should be no electrical connection between them.

?

I have a strong suspicion your problem could lie there. The only difference between low and medium gain mode is that in medium gain, R20 is shorted out, causing C2 to bypass the cathode resistor of V1A (thus increasing the gain of V1A). I'm wondering if somehow you have a cross connection between the sections of the switch, which would result in positive feedback when V1A cathode is unbypassed.

Note that the little dashed line between the switch section on the schematic is there to indicate that they are ganged together mechanically, it does NOT represent an electrical connection.

 

[Stagefright13]

Yep I have the switch wired correctly. I even checked it with a meter just to triple check. I have them on 2 seperate poles on a Lorin. It all works perfectly until I turn it up past 8 on the gain.