Pultek MB-1 Mic Preamp

[davoux]

I am looking at building this preamp, Schematic at:
http://www.frontiernet.net/~jff/pult...onePreamp.html The system auto changes the link to his main page. For the schemo, go all the way to the bottom and click on the highlighted "here".

Perhaps I'm out of my league, but it would be fun to try and my son says he needs a tube preamp. I have some experience with fixing 3 guitar amps (with a lot of help from Enzo and other wonderful people on this site)

I'm having trouble finding the parts he used. The first part is the UTC A-11 multigrid transformer. Its specs are: Low Z to 1 or 2 grids/ Primary Z 50,200,500/ Second Z 50K CT/ Mag Shield Yes/ Resp. 20-20K/Pri DCR 52/ Max Level +3/ Max MW 30. Does anyone know of a readily available replacement? Other questions, what is the purpose of input impedance selection, aren't all mics roughly 600 ohm? Also, if I connect phantom power to the input connection, will that change the whole configuration of the input section?

Second part is the Triad R-4A Power Transformer. This is rated at 555VCT@ 40 ma. This yields about 325 VDC on the B+. I am looking at the Hammond 369 JX. It produces 250-0-250 @ 60ma. Is this a good replacement or is something better ( about same price range $60). Does it matter that the B+ would drop to something around 300 VDC?

The Hammond also has a 50V tap for the bias. Can this be used to provide an easy phantom power, or would you lose too much in filtering , etc? Are there any easy schematics for this option? If not any other easy phantom schematics nicely laid out, almost pictoral, like Joh Fleig did on this schem? I like the nearly "solder by numbers" schemos like this.

Last question. When I finish, how can I test it to be sure it's within safe parameters before I try it connecting to far more expensive equipment my son will use it with? Can I measure output pin 2 to 3 and what should the reading be?

Thanks for your help John

[Steve Conner]

Any 600 ohm to 50k impedance converter transformer should do the trick, since the centre tap of the 50k side isn't used. There are dozens of different models from $20 to several hundred bucks, and of course the more expensive ones sound better

Not all mics are exactly 600 ohms. Some perform a bit better into different impedances. You'll probably not be bothered by the lack of an impedance selector switch, though.

Phantom power should be easy enough: the only thing to watch when using it is that you don't send spikes into the primary of the impedance converter transformer when you plug and unplug things or turn the phantom on/off. It can step a 48V spike up to several hundred volts, possibly enough to arc the windings. The standard phantom power hookup with its DC block capacitors and feed resistors shouldn't cause this problem, but if you connected the phantom power to the centre tap of the 600 ohm winding, say, that would be a different story :-/

[Amp Kat]

That's going to be a very difficult transformer to find as they were made probably in the 40's. I've tested that exact transformer up to a jensen and they are very close. There is also an output transformer on that schemo and it's not vital that you use one as a capacitor coupled out works just as well. You may even go to the jensen website and build one off of there at http://www.jensen-transformers.com and check out the schematics. They are excellent circuits if you build them exactly to specks. Most Microphones impedance are 150 ohms and if you hook up phantom power you have to isolate the inputs with around 6.81k resistors. I'm currently building the JT 115 Cascode Mic pre off of that site which I've built before and it is a really nice pre but one thing is keep your tranny big because I used the Hammond 169 bbecause the voltage was right and it sounded very weak and thin. The Marshall tranny sounded awesome and I used Kevin O'Connors power scaling circuit to regulate it down to the desired voltage. Not trying to sway you from the circuit as no doubt it would most likely sound awesome but finding the part's just seem like a chore when you can build just as nice a box for way less hassles.

[davoux]

Confused

I'm getting a little confused. Steve, you suggest any 600ohm to 50k would work. Amp kat, you talk about 150 ohm. Is the solution that the balanced xlr is 150 ohm and the unbalanced is 600 ohm? Most input xfers I've found are one or the other, but none have a 50k secondary. If I changed to some other secondary impedance, would that screw up the whole circuit?

If I understand you correctly, you are suggesting it would be a bad idea to use the bias winding from the OT for phantom power, as it could send some huge spikes into the input xfer. I'll look for a second small circuit to do it then.

When I look at the pictures of the filtration board, it looks to me like he added 2 caps paralleling the 2 47uf caps. They look to be about 10uf each. What impact would that have on the circuit?

He uses a vintage WIMA MK-4 3.3uf to couple to the output xfer. Would a metalized polyester be a viable replacement?

TNX, John

[Amp Kat]

Ok lets try this one more time but I'll even put you directly on it. Everything you'll ever want or need to know about mic pres is right there or somewhere on that site.

http://www.jensen-transformers.com/a...rs Chapter.pdf

[Steve Conner]

When I said any 600 ohm to 50k transformer would work, I meant 150, 200, etc. as well as 600. I always just think of that whole mess of low mic-style impedances as "600". My fault for being sloppy, I suppose.

The 50k isn't critical either. In fact, ideally it would be 1M I guess, but 50k is about the highest impedance you can practically make without hurting the sound in other ways.

I saw a 600 ohm to 50k transformer for $20 in an electronic parts catalog yesterday, so how rare can they be?

[Amp Kat]

When I said any 600 ohm to 50k transformer would work, I meant 150, 200, etc. as well as 600. I always just think of that whole mess of low mic-style impedances as "600" ohms. My fault for being sloppy, I suppose.

Not sloppy at all Stephen, since everything in microphone,telephone, and anything that has to do with db or measuring systems were all referenced to "600". It's been the standard for years and will continue to be. As long as you somewhat match the impedance to the transformer theres a large window there as to when you'll actually hear a difference on the input and on the high impedance side also.