My Pultec MB-1 clone

[drjet]

Hi there,
my name is Jojo and I just finished my first DIY tube preamp, a Pultec MB-1 clone.

Before I start let me introduce myself.  I run a small homestudio only for myself since 1985 but I didn´t do any diy but soldering cables and patchbays and tweakin my guitars. As my kids grew up I had more time so I started buiding my first stompboxes and a isolation-cab a few years ago. As a first tubeproject I did some heavy modifications of my Fender Champion 600. Next thing I did was a low wattage guitar tube amp kit. I learned a lot and got more and more addicted to diy (like a lot of you too, I guess). So a year ago after finishing my guitaramp I decided to build a tube MIC preamp as a next project. After reading a lot about electronic and tube theory (I didn´t have any electronic background before) and studying preamp-circuits  I decided to do a MB-1 Clone.

For the circuit I followed the Original as close as possible, but I used different input and  output transformers. For the powersupply I designed something for my needs. The additional phantom-powersupply is based on the Gyraf G9.
I wanted to do the layout myself  (for learning) and did a turret-board design. Only Phantompower was build on strip-board.
After a lot of testing  and troubleshooting I powered it up and it worked. And it sounds great, very low noise.
Here´s  a  first impression.

 

[drjet]

Here´s my schematic

 

[drjet]

some details: the preamp-board. Some parts are soldered to the tube sockets underneath the board

 

[drjet]

and the power supply board

 

[drjet]

and a look underneath the board

 

[ruffrecords]

Excellent. Nice build. Good to see a classic tube mic pre design brought back to life.

Cheers

Ian

 

[drjet]

Thank you, Ian!
I did learn so much from your posts!


To be honest, I was thinking about adding your poor man pultec eq to this, there´s a lot of space left in the housing and on the frontpanel.

Best wishes from Germany
Jojo

 

[ruffrecords]

Thank you, Ian!
I did learn so much from your posts!


To be honest, I was thinking about adding your poor man pultec eq to this, there´s a lot of space left in the housing and on the frontpanel.

Best wishes from Germany
Jojo

And I learnt a lot from the MB-1. It is one of the few tube mic pres to close the NFB loop at dc, which is a significant aid to stability. I use the same basic technique in the EZTubeMixer design.

Cheers

ian

 

[kambo]

whats max  gain ?

 

[drjet]

Thank you, Ian!
I did learn so much from your posts!


To be honest, I was thinking about adding your poor man pultec eq to this, there´s a lot of space left in the housing and on the frontpanel.

Best wishes from Germany
Jojo

And I learnt a lot from the MB-1. It is one of the few tube mic pres to close the NFB loop at dc, which is a significant aid to stability. I use the same basic technique in the EZTubeMixer design.

Cheers

ian

So this is why I measured 13 VDC on grid V1a?  I could not explain this. But I have 14 VDC on the kathode, so the grid is negativ with  respect to the kathode.

 

[drjet]

whats max  gain ?

Sorry, but I can´t answer you this question right now, as I just finished the project. I can tell you that I get a good level with a  Sm57 and a  TLM 103, more than enough to get a decent level in my daw.

Jojo

 

[ruffrecords]

whats max  gain ?

Sorry, but I can´t answer you this question right now, as I just finished the project. I can tell you that I get a good level with a  Sm57 and a  TLM 103, more than enough to get a decent level in my daw.

Jojo

The amp max gain should be around 40dB to which you have to add the gain of the input transformer. The amp max gain is approximately the 18K resistor divided by the 120R resistor which is 150X or about 43dB. In practice, because the open loop gain is not infinite the actual max gain is a little less than this - hence my 40dB guesstimate.

Cheers

Ian

 

[kambo]

40 db good enough for me
i will slap one together, when i get a min....

 

[drjet]

Hi,
I´m thinking about adding two features to my preamp

1. I´d really like to have a Hi-Z input for bass and guitar. I changed the schematic and added a switchable Hi-Z input with a capacitor blocking dc and a 1M resistor to ground. I have never seen an instrument input on any mb-1 schematic before. Is it possible the way I planned it?

2. As a future project I want to turn  my preamp into a channel-strip, but only preamp and eq, no compressor. I´d like to build a passive eq design with a make-up stage. How do I get this connected to the preamp? Do I need a interstage transformer or can I connect the unbalanced signal behind the 2u capacitor directly to the eq circuit? What kind of make-up amp do you suggest?

Jojo

 

[drjet]

I took some measurements of my mb-1 clone. I connected the output to a line input of my audio interface (Focusrite Scarlett 18i6). No phantom power activated, no pad. I recorded the preamp without  mic  connected and recorded it with Studio one. I used the onboard analyser for measuring.

1. zero gain : there´s a  inaudible -109 db peak at 50 herz

 

[drjet]

2.  gainpot  set half way: almost the same,  a -109 peak at 50Hz

 

[drjet]

3. Gainpot at maximum: I see a lot more noise with a -88db peak at 50 Hz and noise all over the frequency spectrum

This noise is audible at high levels. It appears only on the last few millimeters of the pot´s range. Is it caused by the gainpot, which bypasses the 1.5k resistor to ground at maximum setting? Could this get better if I insert a small resistor, maybe 10 ohm, in series with the pot?
What do you think?

Jojo

 

[buildafriend]

if you mount the toroid on the side of the chassis you might do a little better with noise. that's the first thing I would try if it bothered me, not sure how much you care though. I'm a bit of a noise reduction freak.

 

[ruffrecords]

First you need be sure your grounding scheme is right. I suggest you read this document I wrote on the subject and check it against your own implementation :

https://www.dropbox.com/s/25va7g7kpn0jkxv/grounding101v2.pdf?dl=0

If you are seeing predominantly 50Hz hum then there are several possibilities to consider. As others have mentioned, there is the possibility of induction from the the toroid. This can enter the circuit either by induction in the mic transformer or in the leads to and from the mic transformer. The fact your hum gets higher as the gain increases implies it gets into the input stage. So you need use good quality twisted screened mic cable from the input XLR to the transformer and keep the secondary leads as short as possible. The input circuit is very high impedance  so you can sometimes get hum induced directly in the tube. The screens you have should stop these but they need to be connected to 0V to be effective. Some tubes, especially NOS types, have poor heater/cathode insulation. With elevated heaters, as in the MB-1, poor heater/cathode insulation manifests itself as hum so it might be worth trying different 12AX7 tubes. For lowest noise and microphones I prefer the Sovtek 12AX7WA.

Cheers

Ian

 

[Jeremy.Starseed]

Nice build! May I ask which power transformer you used?

Thank you for sharing. Might want to give this one a go.

Point-to-point seems a sensible place to start building (after a bunch of
restorations)...having all the parts laid out like that makes it look deceptively easy