Dual channel Pultec MB-1 project finished

[quote author="vibrolax"]I prefer using Keystone double level hollow turret pins,
mouser part # 534-1509-4. I'm pretty sure they ship worldwide.
[/quote]

I think Kent may have told me the same thing ... I should take my own advice and listen more.

BUT
when I looked into it something fell through ??
could have been the OS shipping ... or payment

I would group buy if I could guarantee delivery

thanks Jon ! :thumb:

I PM'ed you Kev.

Joel

[quote author="rafafredd"]On the opamplabs site they also sell a 422 and 422b that looks right for this...[/quote]

The 422 and 422b are electronic (opamp)-type impedance matchers/drivers.
I want iron.

I checked out EDCOR. The 0.5W 10K:150 ohm matching transformer is spec'ed for 5V RMS maximum input. This is for matching the high impedance line output of a mixer to a low imp. input.

The cathode follower on the Pultec puts out at least 90V peak to peak - too much for the Edcor.

-Jon

[quote author="b3groover"]Just ordered one of those rack chassis from Sescom![/quote]

Bad call, IMO... The Sescom cases are a pain because they're held together by 4 screws going through each side panel... If you want to remove the top panel to work on the insides, you basically need to dismantle the whole box!

Peace,
Al.

That's a drag. Maybe it can be modified.

I don't see anything on the page for the WSM10K/150 that specifies 5VRMS maximum input. I only see a 0.5W maximum power spec. God knows I've run these transformers WAY above 5VRMS input! In fact, I've used the WSM series at levels of +27dBM (1/2 watt) at the output of a tube line amplifier and had no problems at all. In the case of the 10K:600 model, this equates to 17.35VRMS x 4 x 1.414 X 2 = 196V peak-peak!

The limiting factor is the insulation breakdown rating, which is usually going to be good up to a few hundred volts.

For the output of the MB-1, the WSM10K/600 would be more suitable anyway. It's closer to the stock ratio. And a ~10K load should not present a serious challenge to the cathode follower. There's also a WSM 15K/600 available even though it's not listed on their webpage. Call them and ask about it.

[quote author="NewYorkDave"]I don't see anything on the page for the WSM10K/150 that specifies 5VRMS maximum input. I only see a 0.5W maximum power spec. God knows I've run these transformers WAY above 5VRMS input! In fact, I've used the WSM series at levels of +27dBM (1/2 watt) at the output of a tube line amplifier and had no problems at all. In the case of the 10K:600 model, this equates to 17.35VRMS x 4 x 1.414 X 2 = 196V peak-peak!.[/quote]

My mistake. I incorrectly thought the WSM transformers were used in the Matchmaster boxes, which have a 5V RMS max input level spec. WSM's look like a great value.

Thanks for setting this straight.

-Jon

[quote author="Kev"]
I look forward to seeing the schematic and the layout
[/quote]

Full-size turret board layouts, component values as built, and wiring details are now available athttp://www.frontiernet.net/~jff/pultec_mb1/images/DualPultecMB1Layout.pdf

-Jon

cool


looking at the pdf now

you keep calling it a Dual but that just your current assembly - right ?
this looks like one channel on one board ... yes I can see that in the pictures at the web site

how about a schematic ?

I checked the html link with a cut and paste from the pdf page
http://www.frontiernet.net/~jff/pultec_mb1/DualTubeMicrophonePreamp.html

and it all works

very good stuff

:sad:
should have been at the DIY Factory

congrats to a good job

Thanks NewYorkDave for your suggestion to try some Edcor WSM 15K/600 transformers as outputs in my Pultec MB-1 clone. The guy at Edcor asked how I knew they would wind non-stock items.

They arrived freshly varnished a week after I ordered them. The DC resistance in the primaries and secondaries for all four samples matched to 0.1 ohm.



I'm very happy with how they sound in the circuit - it's hard to imagine a better value. I've updated my web site and schematics accordingly.

-Jon

Glad I could be of assistance. Your preamp looks great and the web page and documents are well put-together.

Other DIYers should note that 15K:600 was a common ratio for preamp output transformers. The popular Altec 1566A comes to mind. The Edcor WSM15K/600 should be able to substitute in most if not all such output applications where you might find, say, an Altec 15095A. Just don't run DC through it.

I was about to start building this preamp for myself as a first bigger project than guitar pedals and amps which works with batteries...

I noticed that the schematics says "120VAC" but in Finland we have 230V/50hZ. What kind of modifications I'd have to do in the circuit to use it in Finland? Different transformer?

Yes Jussi. Only different transformer. Hammond 369JX might be the one? Look at http://www.edgaraudio.se/tubes/trafo.htm
A nice guy in Sweden who sells them. My neighbour 

The specs of the transformer in the schematics are:
555 VCT @ 40 mA
6.3 VAC @ 2A

I checked this table about those transformers your neighbour is selling, but none isn't exactly the same. I don't know how close each value should be...
http://www.mouser.com/ds/2/177/5c0028-41664.pdf

370BX

Very nice build. It is always good to see people building tube gear. I have long been a fan of the MB1 and it is the inspiration for my own EZTubeMixer design. It is one of the very few tube preamps to solve the NFB stability problem  by using NFB down to dc. As you have discovered, the problem with this is the clicks as you change gain. Using a pot with a series capacitor is only a partial solution. This is  because the dc switch method not only changes the NFB but it also alters the gain of the first stage. This means that the amount of feedback remains nearly constant which helps ensure stability. The guys at Pultec were smart cookies.

Cheers

Ian

In the schematics there is UTC A-11 input transformer which is almost impossible to find. Any equivalents?

EDIT: Found this but they are $90 each:

CMQEE-3440A from Cinemag should work for the input tx (and as a matter of fact, it was used in this MB-1 build: davoux's MB-1 .

CMMI-10 series should work too and has been used on at least one other MB-1 clone that I know of.

Cinemag's site

Nothing from Edcor will be suitable for the mic input (they don't have electrostatic or magnetic shielding) . Their 10K:600K output tx's will work for the output. They can custom wind a 15K:600K output tx for you too so it more closely mathces the original tx.

Click to expand...

Never saw this one, looks good. 

I have to say I'll never use a Sescom rack case again, they are really lightweight, not meant for any sort of weight load, would not stand up to being moved much, and don't hold up with repeated assembly/disassembly.  The few things I built using them left me holding my breath in dread as the builds progressed.  Ground contact between planes is the worst I've experienced as well. 

JussiR said:

In the schematics there is UTC A-11 input transformer which is almost impossible to find. Any equivalents?

EDIT: Found this but they are $90 each:

CMQEE-3440A from Cinemag should work for the input tx (and as a matter of fact, it was used in this MB-1 build: davoux's MB-1 .

CMMI-10 series should work too and has been used on at least one other MB-1 clone that I know of.

Cinemag's site

Nothing from Edcor will be suitable for the mic input (they don't have electrostatic or magnetic shielding) . Their 10K:600K output tx's will work for the output. They can custom wind a 15K:600K output tx for you too so it more closely mathces the original tx.

Click to expand...

Click to expand...

How can you wire  CMQEE-3440A so you can select other impendances than 600 with impendance selector?

you can use UTC A-10 or A-12 for the input if you wish,