Motown Direct Amplifier-inspired Preamp?

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The 12AD7 is basically a low microphonics 12AX7 so that will be fine. The 6AR5 looks like a regular audio output pentode so it should be fine for the output stage. You may need to adjust some resistor values to get it biased right.

Edit: 12AD7 seems to be quite rare. Only one pair for sale on eBay that I could find and the price was over £70 per tube.

Cheers

Ian
 
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 :unsure:

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
Another supplier in EU - try these guys - https://www.adamhall.com/shop/nr-en/19-inch-rack-accessories/19-inch-rack-housings/
 
Hello all,

First a quick disclaimer, there’s a lot of background info here but if you prefer to skip it and look at the design goals I’ve put them in the next post.

I’ve been getting the itch to tackle another valve project lately and given that the Motown Direct Amplifier was the single piece of equipment that originally got me thinking about valve electronics in the first place, I’m thinking about attempting to recreate it in some way. Sadly, I just recently found out that the designer of said direct amplifier, Mike McLean, passed away last November. His schematics were never made public as far as I am aware, and there is a company producing a modern version of his circuit (allegedly from the original schematic) whom I very much doubt will ever make it available. Therefore, to build a picture I thought it best to consolidate all the information that was put forth by Mike himself along with his esteemed colleagues. Given that the aforementioned recreation is/was made in limited numbers, info is very scarce and I’ve had to study a handful of pictures and brief discussions to work out (roughly) which components were used. While I doubt that it will have been made 100% faithful to the original, it might be enough to give some ideas on topology and make some educated guesses in order to design something more of an homage than a straight copy.

By way of brief history for those who aren’t aware, the Motown Direct Amplifier was an amplification device (pic from theproaudiofiles.com attached) which was designed and built in-house and used at the Motown Hitsville USA Studio; it was intended to remove the need for artists to lug multiple heavy amps in and out of the studio when time and space was at a premium. The unit sat at the back of the room and gave five musicians (usually the bassist & guitarists) the ability to plug in their instruments, set their recording level, and monitor themselves in the live room. Importantly, it also satisfied the need for “a high fidelity, high performance preamp between the guitar and the line input of the console mixer position.”

To quote Mike further:


In terms of a technical brief, to quote Mike again:


I expect that modern line input standards might modify the point about driving a 300 Ohm load, but the rest would stand. Any input on the above is appreciated.

Mike didn’t give any indication of topology, but various sources give suggestions. We know that they were vacuum-tube preamps, and Ed Wolfrum said:


Mike himself said, when referring to the Motown studio in general:


There was also speculation online that some of the designs may have been borrowed or expanded upon based on existing equipment at the time, but it’s impossible to say how true this is:


From Ken Sands, who joined Motown in 1967:


That’s also pretty much the extent of what is known via the original designer and those who worked with him. Again, it’s hard to say just how accurate these details are. The rest is speculation based on the appearance of the modern recreation; note that this also has a mic preamp included that was not part of the original unit (although apparently is based on Motown designs.) I’ve briefly broken down the components in notes that I have gathered:


The thing that stands out is the use of the 6V6, which (in my limited understanding) should probably be driving a speaker rather than a line level output? I have triode-strapped an EL84 for a line output in a previous project and that worked, but I was led to understand it is very uncommon. My first thought when I saw it was that it was a rectifier tube; I struggled to identify it, before later finding confirmation that it was a 6V6 (or a variant.) Perhaps it was common in guitar amplifiers at the time (I think the Fender Champ has a similar topology) and was incorporated for some reason? They do seem to crop up in certain Ampex preamps (or more specifically, speaker amplifiers e.g. 620, 622), but always in push-pull. Again, I’d be interested to know anyone’s thoughts on this.
Hello all,

First a quick disclaimer, there’s a lot of background info here but if you prefer to skip it and look at the design goals I’ve put them in the next post.

I’ve been getting the itch to tackle another valve project lately and given that the Motown Direct Amplifier was the single piece of equipment that originally got me thinking about valve electronics in the first place, I’m thinking about attempting to recreate it in some way. Sadly, I just recently found out that the designer of said direct amplifier, Mike McLean, passed away last November. His schematics were never made public as far as I am aware, and there is a company producing a modern version of his circuit (allegedly from the original schematic) whom I very much doubt will ever make it available. Therefore, to build a picture I thought it best to consolidate all the information that was put forth by Mike himself along with his esteemed colleagues. Given that the aforementioned recreation is/was made in limited numbers, info is very scarce and I’ve had to study a handful of pictures and brief discussions to work out (roughly) which components were used. While I doubt that it will have been made 100% faithful to the original, it might be enough to give some ideas on topology and make some educated guesses in order to design something more of an homage than a straight copy.

By way of brief history for those who aren’t aware, the Motown Direct Amplifier was an amplification device (pic from theproaudiofiles.com attached) which was designed and built in-house and used at the Motown Hitsville USA Studio; it was intended to remove the need for artists to lug multiple heavy amps in and out of the studio when time and space was at a premium. The unit sat at the back of the room and gave five musicians (usually the bassist & guitarists) the ability to plug in their instruments, set their recording level, and monitor themselves in the live room. Importantly, it also satisfied the need for “a high fidelity, high performance preamp between the guitar and the line input of the console mixer position.”

To quote Mike further:


In terms of a technical brief, to quote Mike again:


I expect that modern line input standards might modify the point about driving a 300 Ohm load, but the rest would stand. Any input on the above is appreciated.

Mike didn’t give any indication of topology, but various sources give suggestions. We know that they were vacuum-tube preamps, and Ed Wolfrum said:


Mike himself said, when referring to the Motown studio in general:


There was also speculation online that some of the designs may have been borrowed or expanded upon based on existing equipment at the time, but it’s impossible to say how true this is:


From Ken Sands, who joined Motown in 1967:


That’s also pretty much the extent of what is known via the original designer and those who worked with him. Again, it’s hard to say just how accurate these details are. The rest is speculation based on the appearance of the modern recreation; note that this also has a mic preamp included that was not part of the original unit (although apparently is based on Motown designs.) I’ve briefly broken down the components in notes that I have gathered:


The thing that stands out is the use of the 6V6, which (in my limited understanding) should probably be driving a speaker rather than a line level output? I have triode-strapped an EL84 for a line output in a previous project and that worked, but I was led to understand it is very uncommon. My first thought when I saw it was that it was a rectifier tube; I struggled to identify it, before later finding confirmation that it was a 6V6 (or a variant.) Perhaps it was common in guitar amplifiers at the time (I think the Fender Champ has a similar topology) and was incorporated for some reason? They do seem to crop up in certain Ampex preamps (or more specifically, speaker amplifiers e.g. 620, 622), but always in push-pull. Again, I’d be interested to know anyone’s thoughts on this.
GREAT MAN! THAK YOU FOR SHARING THIS!! I read from Bob Olson interview that for mic preamps they mainly used the custom pre as you said 12ax7 ...probably push pull design... and the Langevin tube mic pres....seeing your great preparation about it and investigative method, do you think we can have more informations about the langevin mic pre ? Do you think '50 or '60 Langevin tube design ?? more penthode or more 6072 push pull king a low gain 12a type....THANKS again !!!
 
Just wanna let everyone in this thread know what a blast it was to read through all 18 pages of it!!!

As a fellow who was a recording studio engineer (tracking, mixing, and tech) since back in the 1970's, the info here was priceless....Especially Bob O's contributions. As a Motown fan, I always wondered what that box was with the 5 big knobs/meters at Hitsville, and now I know. I also personally own a Scully 1 inch 8-track, which is mostly functional. That's what I cut my teeth on doing studio engineering way back when.

One question though (if anyone is still listening)....Did anyone actually ever build one of these Preamp/DI/Monitoring mix boxes? I'm interested in building an octal 6SL7/6SN7/6V6(cf) version. Seems like there's plenty of info here for me to pull from. I have tons of NOS versions of those.

I've built a lot of DI's based on harvested UTC mic transformers (hooked up backwards) harvested from old broadcast consoles. RCA and Gates used some great ones in their consoles back in the 50's. The Bass channel will use one of those at it's input. That channel won't need that much more gain. Bob O. and Bob B. were right...that's where the great bass sound comes from! ("UTC iron magic" is what we call it)

Thanks again, even if no one replies.

Dave O.
 
Just wanna let everyone in this thread know what a blast it was to read through all 18 pages of it!!!

As a fellow who was a recording studio engineer (tracking, mixing, and tech) since back in the 1970's, the info here was priceless....Especially Bob O's contributions. As a Motown fan, I always wondered what that box was with the 5 big knobs/meters at Hitsville, and now I know. I also personally own a Scully 1 inch 8-track, which is mostly functional. That's what I cut my teeth on doing studio engineering way back when.

One question though (if anyone is still listening)....Did anyone actually ever build one of these Preamp/DI/Monitoring mix boxes? I'm interested in building an octal 6SL7/6SN7/6V6(cf) version. Seems like there's plenty of info here for me to pull from. I have tons of NOS versions of those.

I've built a lot of DI's based on harvested UTC mic transformers (hooked up backwards) harvested from old broadcast consoles. RCA and Gates used some great ones in their consoles back in the 50's. The Bass channel will use one of those at it's input. That channel won't need that much more gain. Bob O. and Bob B. were right...that's where the great bass sound comes from! ("UTC iron magic" is what we call it)

Thanks again, even if no one replies.

Dave O.
Let's build one!!
 
Let's build one!!
I like the way you think....And I love your avatar!!!

Over the years, I've saved many an old piece of tube broadcast gear from the dumpster. Of course, the Sta-Levels, and RCA BA-6a limiters are too valuable to tear-up as donor chassis and power supplies, but I likely have an old modulation or frequency monitor that could be stripped and put to good use. Loads of octal sockets in them too. Need to go digging through the shelves in the garage now.

Dave O.
 
I like the way you think....And I love your avatar!!!

Over the years, I've saved many an old piece of tube broadcast gear from the dumpster. Of course, the Sta-Levels, and RCA BA-6a limiters are too valuable to tear-up as donor chassis and power supplies, but I likely have an old modulation or frequency monitor that could be stripped and put to good use. Loads of octal sockets in them too. Need to go digging through the shelves in the garage now.

Dave O.
I'd be starting from ground zero here, but it sounds like a cool piece of kit to own.
 
Hi Dave, thanks for chiming in, glad you enjoyed reading it!

Unfortunately (despite everything discussed over the course of this thread!) I still haven't built one. I did procure most of the parts including power and output transformers (picked up a recreation of a UTC from the USA as the orginals are ridiculously expensive!), valves, filter caps, case etc. Still haven't found the 'right' vu meter. But a little while back I lost a hard drive containing all my research and all the schematics I drew up. I did have a habit of printing them and making notes and adjustments but I've also moved house since, so they're MIA along with most of my projects. Sure I'll get back round to it but in the meantime if you build something please share!

Cheers
 
I'm thinking perhaps I could get away with just one meter, and a switch to select which channel it's monitoring.

Actually, I'm thinking of just making a 2 channel version of this...One for guitar, and the other for bass.

One question might be, how much distortion does the un-buffered meter add to the output? Will the difference really be audible if it's not switched in on a channel? Don't really know till I try I guess. In most cases the meter has a pad before a +4dbm output. Otherwise it would be on the peg most of the time. The resistor pad cuts the distortion added by the meter, but doesn't completely eliminate it. Probably adds .1 or .2% depending on the meter and the pad.

Besides being a recording and broadcast engineer, I'm also a bass player...so I can test that channel really easily. Need to take it over to my guitarist's home studio to test that channel.

Step One... Dig an appropriate chassis w/power supply out of the garage.
Step Two... Remove all the original wiring and components.
Step Three... Start building.

I'm not at Step One just yet. :)
Dave O.
 
I love this...I have so far only built in appropriate salvage chassis cleaned up and painted black...baked in the summer sun. Mostly old test gear chassis from PGE ...alot of Stancor & Thoradson power and chokes waiting on the shelf. In and out xformers of what ever I can get or gut. Nice Weston VUs and old USN and Tripplets here and there. Prefer tube rectifiers and separate chassis for power & audio. My kind of cats You are.....
Thx
 
I have a couple of these Modulation Monitors (not the frequency monitors) laying around being useless.... https://worldradiohistory.com/Archive-Catalogs/Gates-Harris/Gates-Modulation-Monitor-1-1948.pdf
For some reason, this PDF has both the frequency and modulation monitor manuals together.

Looks like a nice, industrial strength power supply up near 400 volts, and plenty of octal sockets for a 2 channel version of this. I might even keep the tube rectifier. There's a Triad audio output transformer, but it likely wouldn't work for the cathode follower we seek. Looks like 7k to 8 ohms for driving a monitor speaker...so I'm back to figuring out what's the best output transformer to use for this Cathode Follower 6V6 output.

Dave O.
 
For what reason do you want the 6v6 as a cathode follower?
My old broadcast line amp uses 6v6 in pp as does SA 39 for example. Iam no expert to offer much help...lust curious. Your modulation unit chassis looks very very cool and so does the larger one. Stancor opt have worked well for me as they have 500 ohm taps ...many old tube pa units use pp 6L6 and have these opt...they can be had cheap and are not that uncommon. I am enjoying this rhread....thx
 
Well, there are several reasons for the CF 6V6 (thanks for asking!)...
Here's my thinking...

First one is that the original Motown DI box used a CF of some sort to drive the output transformer.

Second is that we really shouldn't need the extra gain of a normal plate loaded amp stage...but the low output impedance will be welcome driving the transformer. There can already be 3-4 stages of gain before it. Might not need even that much.

Third is, I can use a nice sounding UTC 600/600 A20 for the output transformer, which I have in stock. Output Z of the CF 6V6 should be fairly low. I think 600/600 will be ok, but I've never done a 6V6 in CF...so we'll see.

If you read back through all 18 pages of this, you will see quite a bit of very informative debate about this. The original Motown box may have been a 12AU7 in CF, with the sides in parallel. I like the idea of staying with octal's, since my chassis is full of them now. The ACME recreation of the Motown box apparently used a 6V6 in CF...That after consulting with some Motown folks, so I guess I'll go that route too.

I like the sound of 6V6 tubes in a lot of gear I have. Not the least of which is my pair of Sta-Level's, and pair of SA39's (actually one is an SA38 that I updated to be a 39). And I have plenty of NOS ones in my tube stash to keep those fed. And I forgot to mention my pair of Collins 26-U limiters. Again, 6V6 outputs. I'm not sure that cathode followers really have much of a "sound" since they aren't contributing any gain. But again, I have little experience with CF designs.

Dave O.
 
Thx Dave O...
I have not built anything with a Cf, so much to learn. I believe the old Ampeg B15 (portaflex)
Used a 6SL7 in the front end...I have steered away from this tube as it has a very high plate resistance...but I once had a great custom made unit that used a 7F7 (loctal 6SL7)as 1st
tube...I used it for bass DI...it was great...maybe the B15 schematic will help you
 
I have an Ampeg B-15n in mint condition, and I totally love the sound of it!
It uses the 6SL7 preamp as well.

My understanding is that the 6SL7 is kinda similar to the 12AX7...or actually even more like a 12AT7, but octal of course.
6SL7 has gain of 70, and 12AX7 is 100. 12AT7 is 60-70. Plate R is a little different too.
6SL7 seems to be fatter sounding than either of the others.

Similarly, the 6SN7 is sorta similar to the 12AU7...again, an octal.
In my experience, the 6SN7 is the best sounding dual triode out there. Better than the 12AU7. So, it will probably be stage 3 & 4. Very linear, and smooth sounding tube.

Of course the negative feedback will cancel a good bit of the gains. The original Motown DI used quite a bit of feedback...so we are told.

We will see.
Dave O.
 
I believe that the noval 5751 is VERY close to the octal 6SL7. Maybe the closest commonly used tube.
 
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Th x...I am jonsing to get a project going...taxes..foot tall grass and driveway auto repair are time eaters...thx guys for reminding me there is life over the horizon...I need to revisit the 6SL7 front end tube...over & out
 
This rack and box are chock o block of 6SN7 cathode followers. The instrument preamp with the meter in the wooden box is the Ampeg B15 front end feeding and 6SN7 gain stage and cathode follower, super text book. The output was crazy hot in a lot of situations so I ended up feeding the cathode follower into a mic pre transformer wired backwards to 150 ohm balanced out. This thing has been used in a studio setting for 10 years, for bass, a lot of Fender Rhodes and even guitar. Plug the instrument into the front and the balanced output direct to either tape machine or pro tools. Ignore the varimu compressor and under that is a rackmount stereo version of the above unit. With 15k to 15k Altec transformer inputs wired to the patchbay. Beneath that is a power supply for the Presto mixer at bottom. The mixer came to me stripped of the output transformer. So now all 3 channels have individual outputs to make 3 mic pres. And guess what, those all run into 6SN7 text book gain and cathode followers feeding 10K to 150ohm edcors.

All this is to say that cathode follower outputs are very very useful in the studio.


20230126_104317.jpg
 
I've done CF 600 Ohm outputs with different power tubes like EL84, ECL82, 807. Sounds excellent. The old tech knew their stuff.
 

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