Motown Direct Amplifier-inspired Preamp?

[untune]

I wish someone would do a nice steel 19 inch case with aluminium front and back panels - they are so much easier to woodwork with than steel.

Ian, I have used these from CPC in the past for an AML EZ1073 and an Oakley filter, case is 1.2mm steel and the front panel is 3mm aluminium... despite the rear panel being steel I have put XLRs, jacks and an IEC/switch in them and don't remember it being too much hassle.

CPC doesn't have any good technical documentation on them for some reason, but look at Penn Elcom's site here at the Q&As and they state that if you send them the necessary design, they will custom cut the rear panel for you, not sure on cost though. For some reason it's cheaper to buy them from CPC than PE directly but apprently they price match so, maybe worth enquiring

Only things I didn't like on these was: the venting in the top panel—not for any real reason other than aesthetic preference, but if you were doing anything with valves you might want them to breathe anyway! And minimum 220mm depth, they felt a bit oversized (for my needs at least) but if you like a lot of space then again, not really a concern.

Cheers

 

[MidnightArrakis]

Since I see that you fellows are discussing "rack-mount" chassis and enclosures, I thought I would share with you another potential supplier that sells their products in both the EU and the USA. They are called -- OKW Enclosures -- and they have a rather wide range of not only "rack-mount" enclosures, but also all manner of desktop, handheld, wall-mount and even wearable enclosures. Here are some links to their different "rack-mount" series of enclosures:

OKW Enclosures -- Combimet Series

OKW Enclosures -- Unimet Series

OKW Enclosures -- VersaMet Series

OKW Enclosures -- Mettec Series

And.....just for general enclosure browsing, you can visit their website at: okwenclosures.com

In addition, should you not be interested in drilling/filing/hacking your new enclosure for your project, OKW will also do the machining for you (at a cost).

I hope this helps everyone out!!!

/

 

[untune]

Quick question... in my haste to trim the fat from the schem, I scrubbed the 5v tap from the PSU thinking there would be no need for one. Only afterwards did I think about the possibility of a power indicator/pilot lamp. What is the best approach—use a power transformer with a dedicated additional tap, or is it safe to feed a bulb from the 6.3V tap in parallel with the heaters (as long as there is sufficient current)?

 

[abbey road d enfer]

Quick question... in my haste to trim the fat from the schem, I scrubbed the 5v tap from the PSU thinking there would be no need for one. Only afterwards did I think about the possibility of a power indicator/pilot lamp. What is the best approach—use a power transformer with a dedicated additional tap, or is it safe to feed a bulb from the 6.3V tap in parallel with the heaters (as long as there is sufficient current)?

That's how it's been done for years. However I would recommend the use of an LED-based pilot light. Less current draw, less heat, no maintenance.

 

[untune]

That's how it's been done for years. However I would recommend the use of an LED-based pilot light. Less current draw, less heat, no maintenance.

Cheers, I was 95% sure that was the case but you know how it is, better to bump up that extra 5%

 

[ruffrecords]

On a GZ thread someone wanted to build a pre using a 6SL7 and a 6SN7. I suggested the MB-1 schematic and then added that there is a version that uses a 6V6 as the output cathode follower. So how about making this 100% octal and using a 6SL7 and 6V6 plus a suitable octal rectifier tube?

Cheers

IAn

 

[untune]

I did consider that early on, inspired by some of the Ampeg B-15 preamp suggestions; in the meantime I've managed to acquire five USA-made 6V6s that all test really well so I settled on 12AX7 (since I have a handful) + 6V6 in place of the 6AQ5. It would be interesting to see a 6SL7 version though!

I decided to go octal for the output tube because I was intersted in trying the 6BX7 (as stated was in the original) but I've no idea of the best way to incorporate a double triode such as that in place of the 6V6. They also appear incredibly rare in the UK. I had previously got hold of a 6BL7 as the specs are very similar (small differences in mu and plate resistance IIRC) but it's gone in a drawer.

The only major piece I'm missing for this project at the moment is the power transformer, delaying ordering it for the moment until I've got time to work on it. I'd settled on solid state following the revised MB-1 design, and asked about details of the original power setup (tube vs ss) but didn't hear back. Never heard back from primarywindings either so I was going to go with a Hammond 250-0-250 such as the 369JX and work from there, reasoning that would allow for experimentation with tubes with hungrier heater current requirements down the line if need be

 

[kaltavilla]

So the control room and the musicians

So the guys in the control room and the musicians were listening two different mixes? May be... even if it sound strange to me.

Yup, two different mixes. The same way that in a live situation the musicians on stage are listening to a monitor mix, and the front of house guys make their own mix for the audience. When I worked a live recording truck, all the sources were first fed into a transformer isolated splitter. The direct out goes to the recording truck, ISO 1 goes to the front of house, ISO 2 goes to the monitor engineer. Everyone can do whatever they need to with each source. The Motown system did pretty much the same thing, except the musicians set their own balance for the room. They were, in essence, their own monitor engineers, and the control room engineers did whatever they needed to, to get it all down on tape.

 

[ruffrecords]

I did consider that early on, inspired by some of the Ampeg B-15 preamp suggestions; in the meantime I've managed to acquire five USA-made 6V6s that all test really well so I settled on 12AX7 (since I have a handful) + 6V6 in place of the 6AQ5. It would be interesting to see a 6SL7 version though!

Yes a 6SL7 plus a 6V6 would give an all octal version. Both tubes are readily availble. Interesting you decided on a 12AX7 for the first tube. You could also marry that with an EL84 output stage wired as a triode for a 'European' version.

I decided to go octal for the output tube because I was intersted in trying the 6BX7 (as stated was in the original) but I've no idea of the best way to incorporate a double triode such as that in place of the 6V6. They also appear incredibly rare in the UK. I had previously got hold of a 6BL7 as the specs are very similar (small differences in mu and plate resistance IIRC) but it's gone in a drawer.

6BX6 is a very interesting tube a kind of souped up 12AU7. 6BX6 seem to be quite cheap on eBay at the moments. Best way to incorporate it is to wire the two triodes in parallel (as in the non-6V^ version of the MB-1) and set up the bias accordingly.

The only major piece I'm missing for this project at the moment is the power transformer, delaying ordering it for the moment until I've got time to work on it. I'd settled on solid state following the revised MB-1 design, and asked about details of the original power setup (tube vs ss) but didn't hear back. Never heard back from primarywindings either so I was going to go with a Hammond 250-0-250 such as the 369JX and work from there, reasoning that would allow for experimentation with tubes with hungrier heater current requirements down the line if need be

The 369JX is a good general purpose tube transformer. I have used one before for a project.

Pertsonally I would like to try a 6SL7/6V6 version if only because the very first tube amp I built back in 1963 used a 6V6.

Cheers

Ian

 

[pucho812]

At the brain trust last night, John Windt shared this this bit of information.
While it is not a complete schematic, it is a flow chart of the 5 channel amp.
Basically the direct box was a 12ax7 and a 12au7. there was a trim pot between tubes which was used to set level, it's why it is described as a fixed gain preamp. The rest is self explanatory. I didn't get to ask, but when I showed this to a friend he wondered if there was a buffer for the VU meter. I suspect that because of when this was built combined with using a true VU meter, that a 3k9 resistor was just attached at the output and would feed the meter and that it wouldn't cause any increase in distortions, but that is just a guess.

 

[ruffrecords]

At the brain trust last night, John Windt shared this this bit of information.
While it is not a complete schematic, it is a flow chart of the 5 channel amp.
Basically the direct box was a 12ax7 and a 12au7. there was a trim pot between tubes which was used to set level, it's why it is described as a fixed gain preamp. The rest is self explanatory. I didn't get to ask, but when I showed this to a friend he wondered if there was a buffer for the VU meter. I suspect that because of when this was built combined with using a true VU meter, that a 3k9 resistor was just attached at the output and would feed the meter and that it wouldn't cause any increase in distortions, but that is just a guess.

Interesting because that is completely different to the other info that has been provided that said the DI contained a 6V6. Interesting. Who do we believe? Maybe someone can pop over an have a look inside the real thing?

Cheers

Ian

 

[pucho812]

Well John windt worked at Motown. he was friends with mike McLean their chief tech and even went for the same job position at one point. John is chock full of good info. Perhaps the 6v6 was in the McIntosh amp?

 

[ruffrecords]

Well John windt worked at Motown. he was friends with mike McLean their chief tech and even went for the same job position at one point. John is chock full of good info. Perhaps the 6v6 was in the McIntosh amp?

The evidence for the 6V6 being in the DI is that there is a commercial product available that claims to be based on the original schematics and it uses a 6V6; Plus the schematic of the McIntosh 30W amp does not have 6V6 tubes in it.

Still confused. Perhaps you could clarify this directly with John Windt?

Cheers

Ian

 

[pucho812]

The evidence for the 6V6 being in the DI is that there is a commercial product available that claims to be based on the original schematics and it uses a 6V6; Plus the schematic of the McIntosh 30W amp does not have 6V6 tubes in it.

Still confused. Perhaps you could clarify this directly with John Windt?

Cheers

Ian

I believe the Acme product is based on ed wolfram designs. Ed was also a tech at motown. As to how much or how little he contributed I do not know.
I will definitely talk with john windt about it. I may even be able to procure a full schematic down the road.

 

[pucho812]

I also wanted to note that in flow chart above, you see how the output of the preamp shows up on the patchbay. they showed up as individual patchpoints and could be patched in any configuration needed. Usually summed to one track on the tape machine

 

[untune]

Hey folks, nice to see people still contributing here! I still have all the parts sat here waiting to start building but unfortunately, time hasn't been on my side this year. Thanks to Pucho for sharing that flow diagram, seems it was even simpler than we thought eh?

Just to comment on a couple of points above from the notes I made:

• We were told by a reliable source (i.e. from the original schematic) that the VU was unbuffered, strapped across the output as shown in John's diagram.
• Ian mentions the 6V6—this is the output tube chosen for the modern Acme design, and was not in the original. Acme kindly confirmed that the original was the 6BX7 driving the output tx. I think I mentioned I was going to go with the 6V6 as I had a stash of them, so that might have been why wires had gotten crossed there. As far as we can tell from the information provided, that makes the original tube compliment a single 12AX7 and a single 6BX7 (per channel).
  I assumed that the 6BX7 might have been paralleled such as with the 12AU7 in the MB-1, but John's diagram suggests otherwise. Actually the 12AU7 being there at all seems unusual but not unexpected, in my notes Mike stated in an interview he built "about a dozen preamps with a 12AX7 and a 12AU7 tube in each one" although I had assumed he was referring to mic pres there.

Might well be that, just as how the Pultec design became the prime candidate, some of the Altec mixers aren't far off the mark too (as was suggested way back at the start!)

Hope everyone is keeping well

 

[Jarno]

Yup, two different mixes. The same way that in a live situation the musicians on stage are listening to a monitor mix, and the front of house guys make their own mix for the audience. When I worked a live recording truck, all the sources were first fed into a transformer isolated splitter. The direct out goes to the recording truck, ISO 1 goes to the front of house, ISO 2 goes to the monitor engineer. Everyone can do whatever they need to with each source. The Motown system did pretty much the same thing, except the musicians set their own balance for the room. They were, in essence, their own monitor engineers, and the control room engineers did whatever they needed to, to get it all down on tape.

Replying to old message, my apologies

I believe Nashville studio guys used the "Music City Audio Inc. Mor Me Box":
https://reverb.com/item/17798574-music-city-audio-mor-me-box-personal-headphone-amp-di-system
which basically did the same thing, a personal mix, and DI out to the board. And, uh, yeah, interesting wiring, and that wallwart

 

[Fdieck56]

There is something worse than a wine snob….. a transformer snob.

P.S. I am a fiend for Motown recordings and 60s recordings in general.

 

[ruffrecords]

The very first tube amplifier I made back in 1962 used a 6J7 followed by a 6V6 with NFB from the secondary of the output transformer. Simply for nostalgia I would love to try a version of the Motown based on the MB-1 schematic but replacing the 12AX7 with a 6J7 as the first stage and a 6V6 CF for the output.

Cheers

Ian

 

[moonTV]

Hi everyone,

Thanks for all the awesome sleuthing! I am looking at repurposing a few akai monoblocks and thought maybe I could use the 12AD7 and the 6AR5 to whip up something vaguely inspired by what you have here. I could also go 12AD7 to EL84 but that is my second choice.

Should I stop dead in my tracks, or is this idea worth pursuing?