Pultec MB-1 as makeup amp for PEQ-1

[galapagoose]

Hi folks,

Haven't posted here before, but have been reading for many months which helped me the whole way through my GSSL build which takes pride of place in small home studio. Thanks to everyone for asking so many great questions (and answering them) so that I didn't even need to register until now! Particular thanks to Jakob for the great PCB design for the GSSL and all of your responses to the questions here!

I finally feel ready (and have the time/money) to build something bigger and using tubes + iron. I've built a couple guitar tube amps, so i'm well-versed with the tube sound, and working with HV. I've taken a lot of inspiration from ekadek (http://ekadek.com) across in new zealand. His affinity for the Pultec MB-1 circuit as a line-amp / output stage has convinced me I would like to build a stereo module to add some tube sound to my digital studio. I understand the MB-1 is a mic-preamp, though I would like to use it in position after a modified PEQ-1 as the gain stage. The unit will mostly (only?) sit on the 2-bus coming out of my DAC (RME fireface400) and not need to handle microphone inputs.

First and foremost, I would like to build an MB-1 for line-level use. The idea to use it as makeup for the PEQ-1 is that I would very much like a good quality EQ for mixdown (i'd like to move away from plugins) and the MB-1 has plenty of gain to deal with the attenuation of the filter circuit.


:::Resources:::
- I've been using this schematic + build history for info on the MB-1: http://www.frontiernet.net/~jff/pultec_mb1/DualTubeMicrophonePreamp.html
- Here is a post including the original schematic and some discussion: http://recforums.prosoundweb.com/index.php/mv/msg/26907/0/0/0/


I've drawn up a schematic, though I would like to run it past the huge knowledge base here before I dive in and start dropping dollars on the transformers and parts.

My primary concern is with the transformers. Per NYD's suggestions (http://www.groupdiy.com/index.php?topic=4107.0) I thought to use an Edcor WSM10K:600 (http://www.edcorusa.com/Products/ShowProduct.aspx?ID=286) input transformer to the PEQ-1 (I was a little late to the party to catch the NYD-EQ). This adds an additional 12dB loss, but the MB-1 should give 28-48dB of gain which should easily recover the loss (Vaguely remember 21dB loss at flat for the PEQ-1 >> therefore a total 33dB loss). As a result I was planning on starting my gain settings from the 33dB point.

Secondly, I cannot get my head around how the transformer works at the beginning of the MB-1. Something about the way the negative feedback returns to the input transformer continues to confuse me. To my understanding, the circuit is not balanced (unlike the traditional PEQ-1's makeup section), so I would like to omit the interstage transformer and wire it directly like in the Gyraf SRPP makeup stage. At this point I am stuck and don't understand my problem well enough to know exactly what to search for in the forum. If I were to use a transformer for the connection it seems a 600:2.4K or 600:10K would be correct - the MB-1 seems to expect about 10K load I think.

Finally I was planning on using another WSM10K:600 on the output. In the MB-1 document above it notes he had Edcor custom wind a 15K:600 which is now available stock, but I have since seen Rafafredd mention that the 10K:600 is a better match for the circuit. I figured it would be easier for me to simply order a bunch of 10K:600s than different models. Any recommendations here are welcome.

**POWER**
As the MB-1 requires around 320V I was planning to use the back to back transformer method as in the gyraf build. If the ~290V from rectified 240V (Australia) was insufficient I thought to use a 240:15V and then a 12:240V to boost the voltage up to around 330 (roughly). Guidance here is most welcome and I was planning on using 50VA toroidals. Will these be enough, or should I go up to the 80VA (or would the smaller 30VAs be enough?).

**FILTER**
I'm planning on starting out using the simplified version that lunytune used for his PEQ+GSSL build (http://www.groupdiy.com/index.php?topic=17527)
At a later point I will consider adding the inductors when time permits.

***
I think that covers everything and I will draw the schematic into the computer now and post it shortly. Any advice is welcomed and I look forward to hearing your responses whether good, bad or indifferent! If you think the idea is rubbish to combine the two (or needlessly complicated) I would prefer to go with a straight MB-1 build as an insert for my digital studio, so please try to help me with that!

Thanks again, and I look forward to having a finished design + pictures to return to this great community!
galapagoose

 

[emrr]

http://www.groupdiy.com/index.php?topic=35108.0

some good dissection.

 

[pH]

Hey Galapagoose, welcome to the forum!

I'm a fan of the MB-1 (and Ekadek's mad studio!), and I'm also planning to try it as the gain stage of the PEQ-1 filter I'm building.

About a year ago I went on a tube pre bender, and made an MB-1, a Hamptone, a MILA and finally a Redd47. It was really interesting to make all these around the same time and be able to compare builds and the way they sound. In the end, my most used are the Redd and the MB-1. I love the Redd for its clear top end (using Cinemag repros) and slightly aggressive character. The MB-1 I find a little woolier and softer, but that could also be the transformers I used (vintage Triad in, Edcor out). I find it a little more neutral than the Redd, so I think it might work well for the PEQ-1 or a summing bus.

I used a 250-0-250 transformer that was already in the tape preamp enclosure that I built mine into. An SS rectifier brought the plate voltage up to where the schematic wants it to be, but I like those 6X4 rectifiers for going easy on cold tubes. I'd recommend a 300-0-300 tx, I'd be that Edcor has one in their lineup.

Keep us posted.
-Paul

 

[galapagoose]

Such quick responses!! Thanks a lot.

It seems that the floating transformer before the MB-1 is a design requirement of the circuit — without it, the gain selection won't work properly. as a result, a PEQ+MB would require 3 transformers, like the original PEQ1.

How does this look for a power tranny? http://www.edcorusa.com/Products/ShowProduct.aspx?ID=591

Perhaps I would be best off building the unit as a straight MB-1 at first and leave room + panel space for the PEQ to be built in at a later stage.
In light of that idea, what kind of transformer would be the ideal input transformer to send line level (+4) to the MB? All of the builds I've seen have used input transformers for microphone inputs rather than line level.

On that note, what kind of impedance should the secondary of the input transformer have, and if you were wiring a PEQ in before it, how do you maintain the floating transformer (as mentioned in the above link from emrr)? Seems that you couldn't ground the primary of the interstage transformer, so I'm not quite sure how it could be hooked up.

....... many questions -- i love the feeling that i'm stretching my brain and (slowly) beginning to understand more about these lovely devices.

I look forward to your responses!

 

[pH]

Interesting issue. How do you build an MB-1 as a line amp without an input transformer? Seems like you need the input tx there for the gain/feedback network even if you bypass it with a line input. So you end up with a pultec eq with four transformers?