NewYorkDave Mila Dual Tube Preamp Help Thread

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bernatvm said:
R1 (on the psu board) burns out. This happends when i connect the main board with the psu, powering on the psu unconnected doesn't make any smoke :p

Without the main board your PSU is stabil and with reasonable voltage output? This could be a sign for excessive current flow in your amp, due to a short to ground, wrong components (wrong biasing of the tubes) or faulty tubes on the main board. Please show a good photo of your amp.
 
rock soderstrom said:
Without the main board your PSU is stabil and with reasonable voltage output? This could be a sign for excessive current flow in your amp, due to a short to ground, wrong components (wrong biasing of the tubes) or faulty tubes on the main board. Please show a good photo of your amp.

Yes, when amp unplugged everything seems ok. Voltage is 380V when its supposed to be lower, but without the tubes I read is normal.
 

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Input TX was backwards, corrected it and still smoke. R1 in psu is 4.7k instead of 2.2k (need to buy some 2.2k). Anyone seeing any kind of short? The problem is too much current flowing on b+ even with no tubes.
 

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Hi, your build is pretty messy, I do not know where to start. You must avoid in any case that your wires have unintentional contact with each other or against the case. The resistors at the sockets are very suspicious, your PSU PCB seems to have no save isolation against the case, this one screw in the second photo is not enough. Is this your layout?

- check all the components
- avoid any unintentional contact, find the short
- isolate suspicious contacts
- work safe and don´t kill yourself

Your layout will probably have problems with hum after you have established basic function, but first things first.
 
I would undo the mod. Start with the basic circuit, make it work first. Have you measured resistance between B+ and B- (without tubes and PSU)?
 
No more smoke! The rubber spacers I was using happend to be a bit conductive (they give a reading on the ohmeter). The one on the superior right corner of the amp was shorting b+ with b-. Changed them for plastic spacers, smoke is gone.
 
bernatvm said:
No more smoke! The rubber spacers I was using happend to be a bit conductive (they give a reading on the ohmeter). The one on the superior right corner of the amp was shorting b+ with b-. Changed them for plastic spacers, smoke is gone.

That´s good news! Check for further security related issues, make it safe. ;-) How is the sound, any hum problems?
 
Didn't wire the mic input yet, but instrument input sounds very good! I only have hum with very high gain and higher range positions, but it's soft, so I think  it's the normal behavior.
 
I have some problems with phantom power. With the same configuration in the preamp (gain and range). I get more volume from my sm57 than my condenser AT2050. I checked it with my interface preamps and in that case the condenser gives more volume as you'll expect.

Also tested the 57 with the phantom power on and, as expected, nothing changes with it.

What could be the problem here?

Also, do I need to block phantom power with capacitors? Or the Lundahl LL1935 in the input stage is already doing the job?

Thanks guys!
 
bernatvm said:
Wired the mic input and sounds ok but maybe hum is a bit over what would be desirable... Where do I start?

Rotation of the toroidal power transformer and keep AC wiring maximum away from high impendance inputs and DC wiring.
 
bernatvm said:
What could be the problem here?

No idea. Stange phenomenon, maybe impedance mismatch due to wrong wiring of the input transformer section.

No, you don´t need blocking caps as long you use thigtly matched 6k81 "phantom" resistors (R2/R3), There is no DC at the primary side of your Lundahl.

Check you wiring, take the Jensen standard input circuitry as reference.
 

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Still having issues with phantom powered mics... readings between pin 2 and 1 on the input XLR are 44.6V. Same between pin 3 and pin 1. Any ideas? Is it too low?
 

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