Pultec MB-1 Jon Fleig design

[tjdavoux]

hello everyone,
Years ago my pops built me the Pultec dual mic pre based on the Fleig schematic.  What a guy...  He had used this forum to ask some questions and received quite a bit of helpful advice.. http://www.groupdiy.com/index.php?topic=20658.0;prev_next=prev#new ....Due to health issues he is no longer able to work on electronics.  Me being the recipient of this pre and being VERY much a novice, am looking for further advice.  How could i go about modifying this preamp to give me more harmonic distortion or saturation in its output????  I have 3 rough theories, and am hoping for a little guidance..

1.  Simply change the tubes.  Currently I have a 12AX7 and a 12AU7 powering 1 channel (same for the 2nd channel). Both made by Electro Harmonix...

2.  Change the resisters in the gain selector nob.  Modify them to achieve more gain to hit the current tubes...?

3.  Change the resistor between the A-11-8 and A-11-6 (the first parallel resistor after the input transformer).  My father used a 150k resistor and the original design called for a 82k resistor... 

Once again, I am very novice about electronic design. but any advice to achieve a more classic retro saturation out of this pre is what i am hoping to do..

Thanks!

 

[okgb]

Post a schematic to this page / thread , will make it easier for people to advise on the circuit
otherwise guessing to offer advice or general  , subtle distortion can be hard to control , do you know
anything that successfully does what you want your's to do ?

 

[BigDad]

Hi tjdavoux,

I am trying to build a Pultec MB-1, and i'm interested by your questions.

1. Change your tubes can completely change your sound on this preamp !
What I mean is : This préamp is really direct --> Input -> Input Transformer -> Tubes -> Output Transformer -> Output.
I know that the Electro harmonix tubes's are realy realy bad for a mic préamplifier, especially if the sound depends of the tubes. For a guitar préamp maybe but for a mic preamp... no no no ! I advise you rather NOS Tubes (especially military) or the exceptional Telefunken Tubes.

2. Change the resistors in the gain selector knob (to decrease the resistors), could allow you to send more gain into the tubes and thus to saturate, but maybe to change the 330K (or 240K ?) resitors (on this schematic http://www.gearslutz.com/board/attachments/geekslutz-forum/133771d1251343261-help-mb1-diy-mb1-schem- mod-2.jpg), could to allow an small "overload" of the 12AX7 in the 12AU7.

3. Change the 150K resistor by a 82K depend on the secondary of the input transformer ! What transformer is used ?

Just for information, how would you define the sound of the Pultec MB-1?

 

[tjdavoux]

MY pultec (being slightly modified from original schematic i.e. in and out transformers ( BigDad, Its In trany is a CineMag 3440A)) sounds clear, and has some depth.  I did tho however just changed the tubes, pulled the mullards out of my guitar head and swapped em'.  Definitely is an improvement.  Still. I want to be able to push this bad boy even more.  As far as what kind of pre sound i am chasing.. I am not exactly sure.  I have a modest/reasonable amount of gear, and have been favoring the older "cheaper" gear.  (i.e. my tascam m30 mixer, fostex mixer and its eq.)  The best pre i own IMHO is my Akai gxc 750d tape deck!  Its tape player is actually broken, but plug in a mic with a stereo 1/4 in' and this things sounds phat and warm and saturated.  Funnest pre i have ever used.  I might just buy 3 or 4 broken ones on the cheaps, to mic a drum set.  Getting off topic, I will be attaching the schematic my father referenced.  If anyone has any suggestions of how to modify this pre; to give me the OPTION to saturate and push its output, thats my goal!

 

[tjdavoux]

attachment 2

 

[tjdavoux]

and 3...

 

[EKADEK]

hi tjdavoux. ive been making MB1 for ages after finding one in melbourne early 90s. made them into singles - duals - portables - mixer amps -  control room monitor units -  multi multi channel units. and these days as output stages for equalizers and phono amps. they are pretty dam fine sounding things.  quiet self noise and distortionless flat flat frequency response. low low (no) hum. and have a fairley low component count.

ive refined and experimented with the design and can give these advices.

1.  the edcor 15k/600 ohms is no good for MB1.  they dont have the mojo of a proper made for the purpose trafo. the edcor lacks the bass response. they roll off significant below 50hz within the MB1 topology. a proper made trafo will perform right down to 10 hz. right up past 40 khz.

these MB1 have a terrific bass response with a good trafo as was designed by pultec way back then. the trafo design requires at least 3 layers (half prim- whole sec - half prim ) and wound around the best grain orientated jap steel.

2. use wirewound resitors in the cathode circuits. 1k5. 1k8. and 18K. this helps the noise to stay way down and makes for fat forever stability.

3. use hi stab 1 watt  for plate resistors. (non essential actually) . but for peace of mind only costing a couple of extra bucks.

4. use the best most expensive .047 coupler you can get. i use french solen ones. they are small of size and dont leak. use the biggest non leaky 2.2 uF trafo signal ground feeder cap - go bigger if u want but wont make any difference.

5. get the HT up to that 340 - 350v  level. the amp will work and sound good with much lower HT (250 - 300) .but it really fires and sounds real HOT with the higher potential.

6. follow circuit detail and apply the dc (potential only - no current of course)  to the heater circuit. should measure around 50 v or so. this not only makes right the cathode - heater spec for the 12ax7 - but also significantly cancels/negates any heater -cathode leakage current. it will completely cancel that type of induced hum.

7. use a 6AQ5 or 6BQ5 pentode as V2. in place of the parallel coupled 12AU7. change the feeder plate resistor to 220K (frig with that value to play with the pentode's bias) . that tube will make a nice difference to the sound of the amp.  be careful tho. those tubes require twice as much heater current. the amp will sound HOT.

this pentode output style IS a pultec design that was used in the SP3 panning mixer . here's the circuit of that particular item.

ive made this mixer and its the fattest cleanest fantastic sounding mix master beast.

 

[okgb]

Try increasing the 330k to ground in between the first and second stages , if that does anything for you
you can add a pot , 330 is already a fair bit though , try  1 MEG .  Add attenuator after the output transformer
try a higher ratio input transformer to drive it harder
if you really want to start digging , the output 12au7 is paralleled so you could wire up the first half as a gain stage
but more gain more noise and some kinds of distortions are just plain nasty
Hopefully someone who is more of a designer can offer better tips

 

[emrr]

Critique from the cheap seats; this design really doesn't interest me.  It's not what I want tubes to do, which is also not what it sounds like you want tubes to do either.  Wrong circuit to get good harmonic distortion IMO, I wouldn't even bother. 

It's based on negative feedback gain control, so you are fighting it's very essence.  Keep it for your clean channel, and build an RCA BA-2 or Collins 6Q, basically anything with little to no negative feedback.  If you want to hack it and start over with the transformers, sockets, and chassis, you could find something suitable. 

You want to hear it without NFB?  It might be terrible since it was designed with it; I don't know what range the mod plan covers.  Disconnect the 17K5, be ready to turn it right back off.  Gain will probably be a lot higher, and you won't have any gain control anymore. 

Changing the 330K won't do much. 

The original 82K on the input transformer secondary makes it a MATCHING circuit, with true 600 ohm loading on the mic when connected as shown on the original schematic.  It hardly affects gain (less than 6 dB from combo of iron loading and mic loading), and neither does the 150K.

Changing the tubes hardly does a thing for gain.  That's a guitar amp thing to do; they generally have no negative feedback in the relevant places and tube type does have an effect there.  NFB is a bit self correcting, change the tube type and the NFB will mirror what the tube does in relative ratio. 

The gain resistors do what they do, it's not an unlimited freeway of possibility. 

 

[EKADEK]

emrr's points are real good ones .

if u wanting a valve amp to give a potential maximum state of harmonic distortion - then choose some other non - nfb design. the rca and collins designs are cool . the MB1 is a super clean thing.  it was designed that way original to match with the pultec EQs.

when u take out the nfb by grounding the first cathode with a large electro in series with a pot. wired around that 1k5 cathode resistor  - 220uF -  2k2.  the MB1 pretty much behaves like any other non nfb amp tho. the gain roars up , but not so much as to be destructive. depends on the level u putting in.

so u can have the best of both worlds. 

 

[ruffrecords]

The MB-1 is a very good design of clean mic pre. It does all the right things; low noise high gain early stage, low gain high power output stage; NFB that works down to dc to make it unconditionally stable, and a simple means of altering gain. It was this design that inspired my EZ Tube Mixer design; all I did really was move some gain from the first stage to the output stage, change the output stage to push pull and extend the gain range. Other than that they are topologically identical.

Since significant negative feedback is used it has very low distortion even at high output levels into 600 ohm loads and is largely immune to the variabilities of tube characteristics. So if you change tubes and can hear differences you either are cursed with incredibly sensitive hearing or are using some particularly bad tubes.

As others have said, it you are looking for a design with significant tube distortion this is not it.

Cheers

Ian

 

[EugeneKei]

hi tjdavoux. ive been making MB1 for ages after finding one in melbourne early 90s. made them into singles - duals - portables - mixer amps - control room monitor units - multi multi channel units. and these days as output stages for equalizers and phono amps. they are pretty dam fine sounding things. quiet self noise and distortionless flat flat frequency response. low low (no) hum. and have a fairley low component count.

ive refined and experimented with the design and can give these advices.

1. the edcor 15k/600 ohms is no good for MB1. they dont have the mojo of a proper made for the purpose trafo. the edcor lacks the bass response. they roll off significant below 50hz within the MB1 topology. a proper made trafo will perform right down to 10 hz. right up past 40 khz.

these MB1 have a terrific bass response with a good trafo as was designed by pultec way back then. the trafo design requires at least 3 layers (half prim- whole sec - half prim ) and wound around the best grain orientated jap steel.

2. use wirewound resitors in the cathode circuits. 1k5. 1k8. and 18K. this helps the noise to stay way down and makes for fat forever stability.

3. use hi stab 1 watt for plate resistors. (non essential actually) . but for peace of mind only costing a couple of extra bucks.

4. use the best most expensive .047 coupler you can get. i use french solen ones. they are small of size and dont leak. use the biggest non leaky 2.2 uF trafo signal ground feeder cap - go bigger if u want but wont make any difference.

5. get the HT up to that 340 - 350v level. the amp will work and sound good with much lower HT (250 - 300) .but it really fires and sounds real HOT with the higher potential.

6. follow circuit detail and apply the dc (potential only - no current of course) to the heater circuit. should measure around 50 v or so. this not only makes right the cathode - heater spec for the 12ax7 - but also significantly cancels/negates any heater -cathode leakage current. it will completely cancel that type of induced hum.

7. use a 6AQ5 or 6BQ5 pentode as V2. in place of the parallel coupled 12AU7. change the feeder plate resistor to 220K (frig with that value to play with the pentode's bias) . that tube will make a nice difference to the sound of the amp. be careful tho. those tubes require twice as much heater current. the amp will sound HOT.

this pentode output style IS a pultec design that was used in the SP3 panning mixer . here's the circuit of that particular item.

ive made this mixer and its the fattest cleanest fantastic sounding mix master beast.

Hello. Do you have scheme for SP3 panning mixer or maybe turret layout?
Thanks in advance

 

[EKADEK]

sp3 panning mixer? not heard of that one sorry. and ive not used a turret layout with any MB1 builds.

 

[beigebaby]

sp3 panning mixer? not heard of that one sorry. and ive not used a turret layout with any MB1 builds.

Ten year old thread n all but do you sell your builds? Interested in a dual mono mic amp design.

 

[MidnightArrakis]

Ten year old thread n all but do you sell your builds? Interested in a dual mono mic amp design.

[Interested in a dual mono mic amp design] -- I don't know anything of your technical background or capabilities, but should it ever be needed.....here's my "2-cents" worth towards a "Dual-Mono Mic-Preamp" design.....should you need it:

https://app.box.com/s/exihmfu3hx6s4zf9icxisobrko96ow4f
"Standing By....."

P.S. -- You're not related to "Charles Manson", are you???

/

 

[beigebaby]

As a matter of fact, we are related. Except our objective has changed a bit now, we primarily target dimwitted forum members who answer questions directed at others.

Thanks, but you can keep your change.

 

[gyraf]

Midnight: Why this?

Beige: Have you looked at the G9 project, there's onboard local support for that?

 

[MidnightArrakis]

As a matter of fact, we are related. Except our objective has changed a bit now, we primarily target dimwitted forum members who answer questions directed at others.

Thanks, but you can keep your change.

[we primarily target dimwitted forum members] -- sigh.....yet another forum member who apparently doesn't have even a small shred of -- humor -- within themselves!!! And.....my -- dimwittedness -- even got me granted a U.S. Patent for the "novel and unique mechanical design of an electronic rack-mount chassis".

[who answer questions directed at others] -- I was only just simply offering you any mechanical and/or PCB-design services in your quest for a "Dual-Mono Mic-Preamp", that's all. I don't see any reason of why you would have to take offense to that!!! But, in today's "Cancel Culture Climate", people are being offended for even the most innocuous of inane and simple things!!! ENJOY YOUR BORING LIFE!!!

[you can keep your change] -- And, so.....I certainly will!!! There's no use with me trying to offer assistance to someone who is so touchy and has no sense of humor!!! Earlier today, I have received two PM's from other forum members who have requested that I assist them with their chassis designs, PCB-layouts and rack-mount "Electronics Packaging" projects. GOOD LUCK!!!

/

 

[MidnightArrakis]

Midnight: Why this?

Beige: Have you looked at the G9 project, there's onboard local support for that?

[Midnight: Why this?] -- Meaning.....???

/

 

[ruffrecords]

[we primarily target dimwitted forum members] -- sigh.....yet another forum member who apparently doesn't have even a small shred of -- humor -- within themselves!!! And.....my -- dimwittedness -- even got me granted a U.S. Patent for the "novel and unique mechanical design of an electronic rack-mount chassis".

[who answer questions directed at others] -- I was only just simply offering you any mechanical and/or PCB-design services in your quest for a "Dual-Mono Mic-Preamp", that's all. I don't see any reason of why you would have to take offense to that!!! But, in today's "Cancel Culture Climate", people are being offended for even the most innocuous of inane and simple things!!! ENJOY YOUR BORING LIFE!!!

[you can keep your change] -- And, so.....I certainly will!!! There's no use with me trying to offer assistance to someone who is so touchy and has no sense of humor!!! Earlier today, I have received two PM's from other forum members who have requested that I assist them with their chassis designs, PCB-layouts and rack-mount "Electronics Packaging" projects. GOOD LUCK!!!

/

Seems to me you both have an unusual sense of humour that the other doesn't get. I would suggest finishing any humorous comment with a smiley just to make sure everyone knows it is in jest.

Cheers

Ian