Stagefright13

Mila Dual Tube Preamp Help Thread
« Reply #40 on: December 04, 2007, 07:51:40 PM »
I guess I could add some caps on the output if need.
Amateur experimenter


NewYorkDave

Mila Dual Tube Preamp Help Thread
« Reply #41 on: December 04, 2007, 08:02:11 PM »
Yuck... Don't stick that switcher inside your preamp.

If you must have phantom built-in, use a linear supply. For my part, I use an external phantom supply.

Stagefright13

Mila Dual Tube Preamp Help Thread
« Reply #42 on: December 04, 2007, 08:16:35 PM »
Is a switching supply noisy? I didn't order them yet cause I wasn't sure. Should I build a Bo Hansen one instead? I have to make another mouser order anyway cause I am missing a couple loose parts. And the entire phantom supply. I accidentally ordered the wrong cases so they are kinda big and have enough room to park a truck inside. LOL So room is NOT a problem or heat for that matter.

Never mind I checked the Meta again :oops:
Amateur experimenter

MasonAtom

Mila Dual Tube Preamp Help Thread
« Reply #43 on: December 10, 2007, 05:24:14 PM »
Newbie Question -

If I'm using a mic input transformer with a 50K secondary (Cinemag CMQEE-3440A, 150:50K), do I have to adjust R6 to give the proper load? If so, how do I determine the proper value?

Also, the VP from Cinemag suggested that I put in a coupling cap between the load resistor and the grid of V1A so I don't blow out the winding. Does this sound reasonable?

Mason

MasonAtom

Mila Dual Tube Preamp Help Thread
« Reply #44 on: December 14, 2007, 08:28:52 AM »
I just picked up a bulb for the pilot light and noticed that the current rating was much higher than I expected (.15 A). I'm using an Allied traffo that is rated to 1 A on the 6.3V tap, and I'm a little worried I'll be going over spec (two 12AV7 = 2 X .45A = .9A+.15A for the bulb = 1.05A). I was also planning on running an LED off this, but it's only about 20 mA, so in total I should be around 1.1 A, 10% higher than the Allied is rated. Should I be concerned?

In previous DIY guitar amps I've built, I've used Hammond PTs and I've found that they can easily handle about 20% higher current draw than they are rated for.

Mason

Stagefright13

Mila Dual Tube Preamp Help Thread
« Reply #45 on: December 14, 2007, 09:50:32 AM »
Well if you did the math it is probably too much. You have already answered  your question. I personally wouldn't use A transformer above it's rating. Use an LED for the power light if ya need to. They come in many pretty colors.  :green:

I am using voltage from the line to power the lamp.
Amateur experimenter

raysolinski

Mila Dual Tube Preamp Help Thread
« Reply #46 on: December 15, 2007, 12:23:39 PM »
I built a MILA last night...double checked everything..fired her up...voltages are a tad high ..330 B+ and a couple of volts higher at the measuring points...except at pin 6 of V1...Should be around 70 volts after the voltage drop across the 220k..but mine is 170 (afd the resistor is a 220k).. All the other voltages are ballpark..Means that plate isn't drawing the right amount of current, right? (ohm's law! :) )..and consequently is not passing audio..
1 thing I did differently than the schematic: I did not have a switch laying around so I wired the feedback with a 100k between the output and pins 8 and the cap on pin 3  of V1, I also have a 22k from pin 3 to ground...no doubt I did something wrong there  :oops: ..any thoughts tube gurus?

Cheers,
Ray
"orange whip?..orange whip?...3 orange whips please"

babyhead

Mila Dual Tube Preamp Help Thread
« Reply #47 on: December 15, 2007, 03:24:28 PM »
Start with a signal trace...

raysolinski

Mila Dual Tube Preamp Help Thread
« Reply #48 on: December 15, 2007, 04:07:40 PM »
OKAY...no laughing...this is a good one... :oops:
I fired up the MILA today...testing voltages, considering the feedback loop, changing tubes...looking at the schematic and the turret board layout....ready?
It wasn't passing audio because...I didn't have the output (pin 1, 6 V2) hooked up to the output cap!....wire to the top of the nice, fat solen 24uf and voila...BEAUTIFUL sound..gotta say I much prefer the MILA to the one or two bottle...gonna have a few  :guinness:  :guinness:  tonight, cut some tracks and watch the snow fall..

Cheers,
Ray
"orange whip?..orange whip?...3 orange whips please"

NewYorkDave

Mila Dual Tube Preamp Help Thread
« Reply #49 on: December 17, 2007, 11:01:03 AM »


pstamler

Mila Dual Tube Preamp Help Thread
« Reply #50 on: December 17, 2007, 11:57:25 AM »
Quote from: "MasonAtom"
Newbie Question -

If I'm using a mic input transformer with a 50K secondary (Cinemag CMQEE-3440A, 150:50K), do I have to adjust R6 to give the proper load? If so, how do I determine the proper value?


Typically the proper value is 10 times the secondary impedance; in this case that would be 500k. You may also need a Zobel network; the specs for that should come with the transformer.

Quote
Also, the VP from Cinemag suggested that I put in a coupling cap between the load resistor and the grid of V1A so I don't blow out the winding. Does this sound reasonable?


Basically no. If there's a cap between the load resistor and the grid then, barring the use of an additional grid resistor, the grid will have no DC path to ground other than leakage. This will mis-bias the tube. I grant that using a cap could prevent the winding getting fried from a plate-to-grid short in the tube, but in 35 years of messing with tubes I have yet to see a small-signal triode do this. If you really want to use a cap, connect it between the transformer and the load resistor.

Peace,
Paul

Stagefright13

Mila Dual Tube Preamp Help Thread
« Reply #51 on: December 17, 2007, 01:27:01 PM »
I am building my Milas right now however I appear to not have ordered the 100 ohm resistors for the heater circuit. Could I use quarter watt there? Those I have. I'm only using the 6.3 to power the tubes only. No lamp. How much power do the tubes draw?

Thanks!
Amateur experimenter

Stagefright13

Mila Dual Tube Preamp Help Thread
« Reply #52 on: December 18, 2007, 06:23:47 AM »
I will have the correct wattage resistors here tomorrow but I was really trying to calculate this out myself and learn something. It looks like R5 is there to drain the high voltage caps on power down? But still a little stumped what R3 and 4 do. And how to calculate how much current goes thru them... And why they are connected to the 329 volt supply. I do believe it would be preferred to have a matched pair there? And another matched pair for R1 and 2.  Please bear with me as I learn. :roll:  I have been wearing this site out lol!

http://ourworld.compuserve.com/homepages/Bill_Bowden/ohmslaw.htm

One thing I can say about my build so far is I am glad I accidentally ordered larger cases than I at first planned. I have ALOT of stuff getting crammed in there. :)

(EDIT) Nevermind I think I kinda figured that out after actually powering the supply...
Amateur experimenter

Stagefright13

Mila Dual Tube Preamp Help Thread
« Reply #53 on: December 21, 2007, 08:59:39 AM »
WOOHOO! Mila #1 is up and running! Sounds awesome. Big and Fat. I'll test it more after I fix this problem I have:

When I turn the gain up, when I hit about 3/4 and above I get a loud burst of white noise. It happens a little sooner with the range switch on low. Anyone have any ideas?? The only thing I changed in the circuit was put a 178k for r4 and didn't use C1...

Here's my voltages:
B+ in 314
heaters: 6.86 (seems high)
V1: pin1: 106
pin3: 2.42
pin6: 93.6  (Seems High)
pin8: 1.71

V2: Pin1: 158
pin2: 2.43

I have 344 volts coming out of the power supply rectifier. Not 329 like the schematic.

Thanks
Amateur experimenter

MasonAtom

Mila Dual Tube Preamp Help Thread
« Reply #54 on: December 21, 2007, 09:21:24 AM »
Just tested out my power supply last night and everything seems to be up and running smoothly. My PP measured out at 48V with no problems and the switch works as planned. Just have to finish wiring the preamp board to the tube sockets and wire up the Pad and Range switches, then I should be able to fire it up.

One quick question on wiring up a Lorlin 2 X 6. I put the collar in the correct place to limit it to 2 X 3, but I'm a little confused about the lugs, which are numbered 1 to 12. I'm assuming that I want to use lugs 1,2,3 and 10,11,12, is that correct? Wouldn't it be easier if they numbered each side 1 to 6?

Mason

Stagefright13

Mila Dual Tube Preamp Help Thread
« Reply #55 on: December 21, 2007, 09:30:24 AM »
I think on a 2X6 it would be A corresponds to 1,2,3,4,5,6.  And B corresponds to 7,8,9,10,11,12. Just check with an ohm meter to be sure.

Yeah my phantom power works great. This thing really sounds good too so far. Just mine bursts into LOUD white noise after about 3/4 gain. But 3/4 is actually way more gain than I need. But this is a Christmas present for my son so I gotta tame that out...
Amateur experimenter

MasonAtom

Mila Dual Tube Preamp Help Thread
« Reply #56 on: December 21, 2007, 09:42:49 AM »
have you switched out the pot to check if it's faulty?

Stagefright13

Mila Dual Tube Preamp Help Thread
« Reply #57 on: December 21, 2007, 10:27:14 AM »
Yeah I tried another pot and same thing. Just seems I have too much gain. I am still checking it. My converters get buried using an ldc at less than 1/4 on the gain knob with range on high.
Amateur experimenter

NewYorkDave

Mila Dual Tube Preamp Help Thread
« Reply #58 on: December 21, 2007, 10:49:38 AM »
There's no point in having R4 if you're not using C1.

Did you install the grid stoppers R6, R10, R14 and R15 directly on the tube sockets?

That voltage reading you gave for pin2 of V2 is a typo, right? You meant pins 3 and 8? Because you shouldn't be seeing any DC on 2 and 7.

MasonAtom

Mila Dual Tube Preamp Help Thread
« Reply #59 on: December 21, 2007, 10:59:02 AM »
Dave,

Regarding the R6 grid stopper - I'm using a mic input transformer with a 50K secondary - do I have to adjust this grid stopper to take into account the change from a 20K to 50K primary? pstamler said that you usually use 10 X the secondary (500K), but that doesn't make sense to me because in your schematic you use a 270 ohm resistor there with your 20K primary.

Thanks for the help.

Mason


 

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