Motown Direct Amplifier-inspired Preamp?

[untune]

I bet George Petersen knows a lot of Motown secrets ... his email address is in this article about some Motown secrets (who knows if it's still valid) but I think his LinkedIn is ok and I hope he is too.

Cheers
JM

Good info, thank you. For the moment I've contacted the Motown Museum to see if they keep any diagrams or records that they might be willing to share. I've also contacted someone from the AES regarding the Zoom meeting posted up earlier in the thread regarding contact for John Windt. There's also the possibility that @pucho812 might be able to shed some light in that regard too—I'll post if I learn anything new

 

[ruffrecords]

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.

Can we just for a moment go right back to the beginning because I would like to be clear about what you see as the goals of this project. From the above paragraph it seems you do not expect to be able to clone the original but you would like to pay homage to it in some way. I am guessing this does not extend to providing 5 channels but a single channel would presumably include the VU meter and expect the guitarist to set the VU level using his guitar volume control just as in the original. Would it need to also provide the front panel volume control? Internally would it need to closely follow the original topology if possible or simply mimic its functionality? Perhaps instead of a Mackintosh power amp it could include a tube headphones amp for monitoring - handy for control room guitarists.

Cheers

IAn

 

[untune]

Can we just for a moment go right back to the beginning because I would like to be clear about what you see as the goals of this project. From the above paragraph it seems you do not expect to be able to clone the original but you would like to pay homage to it in some way.

Hi Ian,

That was originally written on the presumption that we would likely never find out exactly how the original was designed, but as the discussion has progressed it has branched off a bit and now there's a chance we may get a clearer picture (Rick at uneeda audio has kindly agreed to get in touch with John Windt to see if he wishes to be contacted). Therefore that opening paragraph was mitigating expectations to some degree—I'd love to be able to build a clone (at least as far as component availability etc allows) if the resources were there; my more realistic assumption was that it'd be mostly guesswork based on the info we had, the function of the device, any clues from the Acme unit, and the experience of anyone who might want to contribute. I do think it's an important piece of recording history and feel that it'd be a shame for it to be lost, but better preserved through a community such as this.

I am guessing this does not extend to providing 5 channels but a single channel would presumably include the VU meter and expect the guitarist to set the VU level using his guitar volume control just as in the original. Would it need to also provide the front panel volume control?

I think it'd be best aiming for a single channel preamp section—instrument in, line out. There's nothing to say someone couldn't stick two (or five!) in the same enclosure—I suppose it would depend on details like the power supply etc. I like the philosophy of fixed gain and having the musician alter their instrument level based on the VU meter, but in this day and age we probably desire a bit more control over the gain staging and things like transformer saturation, overdrive etc before going into a analogue to digital converter as opposed to an Ampex tape machine. Given that from what we know it seemed to be more about 'clean' gain though, perhaps the best approach would be to keep it as simple as possible, and then expand with additional ideas such as variable gain etc? Maybe even switchable between fixed and variable—lots of options! The front panel volume control on the original unit would have been for controlling the mix of that particular channel going to the monitor amp, so it would not need to be replicated here but given the above, I think an output attenuator would be wise.

Internally would it need to closely follow the original topology if possible or simply mimic its functionality? Perhaps instead of a Mackintosh power amp it could include a tube headphones amp for monitoring - handy for control room guitarists.

As close to the original topology as we could get it, but again this depends on how much we can discover about the original. And if we can't discover any more than we already know, then it'll be more about mimicking the functionality. John Windt (in the AES video) revealed that there was a gain trimmer hidden behind the mixer knob so it could well be that there actually was some degree of gain control in the preamp after all—seemingly to compensate for differences in pickup output etc. And that does sound like a good idea with the headphone amp. Or just as well you could build a McIntosh power amp and a cabinet and replicate the live monitoring aspect too!

Cheers

 

[pucho812]

Good info, thank you. For the moment I've contacted the Motown Museum to see if they keep any diagrams or records that they might be willing to share. I've also contacted someone from the AES regarding the Zoom meeting posted up earlier in the thread regarding contact for John Windt. There's also the possibility that @pucho812 might be able to shed some light in that regard too—I'll post if I learn anything new

Are you wanting to contact John windt?

 

[untune]

Are you wanting to contact John windt?

Hi pucho

I'm hoping he may be willing to clarify things for us, as I think he's probably the most knowledgeable person on the subject; Rick Chinn has kindly forwarded a message to John this afternoon, and from there it's upto him if he wants to respond. I certainly don't want to pester him so I'm going to leave it there and keep reading up on whatever I can find for now

 

[1000tinyempires]

I have never tried to make "the motown instrument pre" but I have made quite a few devices where a guitar or bass is plugged in goes through some tubes and transformers and the output is line level and balanced. Over the years the problem that would happen over and over again would be the bass player would take it to a session and the engineer would not understand that it was a preamplifier and treat it like a DI. Problem is that's 40dB difference. The version of this I enjoy making the most is just the front end of Portaflex, which already had a line out after the tone controls in like 1963. So something to consider is the end user.

 

[pucho812]

Hi pucho

I'm hoping he may be willing to clarify things for us, as I think he's probably the most knowledgeable person on the subject; Rick Chinn has kindly forwarded a message to John this afternoon, and from there it's upto him if he wants to respond. I certainly don't want to pester him so I'm going to leave it there and keep reading up on whatever I can find for now

I understand. No wukkas as they say in australia. But if you change your mind send me a DM with contact info.

 

[untune]

Portaflex

Those look fantastic! That's the basic idea yeah - not a DI box in the sense that it's going into a mic channel but a dedicated instrument amplifier. Do you find that the different circuits/tubes have a radically different effect on the sound or are they quite similar (when you take away tone controls etc)? I'll have a scan around for Portaflex circuits

I understand. No wukkas as they say in australia. But if you change your mind send me a DM with contact info.

Thanks i'll give him some time, see if he responds, and if not I'll send a DM over. Appreciate it!

 

[1000tinyempires]

My experience has been that keeping the voltages high, or at least 250vdc makes the most difference. In theory a 6SL7 vs 5751 vs 12ax7 if designed for the tube or lifted from a circuit for the tube things are generally clean and linear. The trick I found for people looking for tube break up is to run that B+ around 120vdc which gets you to distortion etc with the same amount of tubes but requires looking in the tube manual to adjust the circuit. But in general I try to keep things closer to what Ampeg and Motown were striving for, good clean gain at line level from an instrument.

 

[untune]

My experience has been that keeping the voltages high, or at least 250vdc makes the most difference. In theory a 6SL7 vs 5751 vs 12ax7 if designed for the tube or lifted from a circuit for the tube things are generally clean and linear. The trick I found for people looking for tube break up is to run that B+ around 120vdc which gets you to distortion etc with the same amount of tubes but requires looking in the tube manual to adjust the circuit. But in general I try to keep things closer to what Ampeg and Motown were striving for, good clean gain at line level from an instrument.

That's really useful information, thanks for sharing! If we don't learn anything more about the original then I think we at least have some good starting points. Lots of equipment around the early-mid 60's used 6SL7 at the front, some followed by a 6V6, loads of 12AX7s too. And given that we know which power TX and choke the Acme one uses, that gives a rough starting point with the power supply.

I keep following interesting little threads from tiny bits of info that pop up too—like yesterday I read that the 6AQ5 can be considered 'virtually identical' to a 6V6 when run at 250VDC. So I'm keeping an eye out for that tube showing up in earlier designs.

Another member earlier in the thread made a comment about the 6V6 being potentially used in a voltage regulation role. Not dug much up on this BUT in an interview from the late 90s, Mike McLean made a comment about getting hold of the Audio Cyclopedia (which would have been the 1959 1st edition) when he designed the Motown 3 channel recorder and other bits of studio tech he built in the early 60s. So if anyone has a copy of that, there could be some clues in there too.

 

[mjrippe]

I have a .PDF of the second edition Audio Cyclopedia (Audio Cyclopedia 2nd.pdf), but my first edition is on plant-based media. I'll take a look and see if anything pops out at me.

 

[jacomart]

I found the schematics of the two submixers that were used in the Motown studios along with the Western Electric main console.
My spider-sense tells me that maybe Mike McLean may have somehow been inspired by the circuits of the Ampex MX-10 and Altec 1567A. Who knows?

Cheers
JM

 

[untune]

I have a .PDF of the second edition Audio Cyclopedia (Audio Cyclopedia 2nd.pdf), but my first edition is on plant-based media. I'll take a look and see if anything pops out at me.

Hey Mike!

That would be great, thanks! Couldn't track it down despite searching and the 1st edition is like golddust it seems (and about as expensive!)

I think I read (elsewhere on this forum probably, might have been PRR) that many of the tube circuits were dropped from the 2nd edition in favour of transistors so it might not paint a full picture, but surely there'll be something useful in there

I found the schematics of the two submixers that were used in the Motown studios along with the Western Electric main console.
My spider-sense tells me that maybe Mike McLean may have somehow been inspired by the circuits of the Ampex MX-10 and Altec 1567A. Who knows?

Cheers
JM

Good find! I think I may have gathered these schematics (along with many, many, many others ) and I did pick up a quote from Mike McLean regarding the original Motown 3-track recorder "I started to hand-draw the circuits; I got one circuit from the 351, another from an Ampex 600, and one from an Altec mixer, for the outputs" so the Altec was very likely the 1567a you've attached, as there are shots of it in the Motown studio. That was in the fall of 1961; I think the direct amplifier was 1966 (not 100% sure) so the question is whether he discovered or developed any others in the next five years—I'm guessing it's likely, but I'm not sure how fast the tech would have been moving then, how available parts were, costs etc etc.

 

[Winston OBoogie]

One thing to consider when talking about the possible/probable maybe? use of the 6V6 as output valve, however overkill it might seem to us...
I noted somewhere along the way in this thread - the output stage was configured so that engineers could double load it to send to two 600 ohm load destinations via the patching system. So it needed the grunt and low impedance to drive a transformed 300 ohms, not 600 ohms.

 

[jacomart]

AFAIK Western Electric and RCA mixer/consoles from the 40s used 6L6G valves, but unfortunately I haven't been able to find any schematics so far... may be I have something lost in my HDs somewhere... stay tuned

Cheers
JM

 

[mjrippe]

Well, the Audio Cyc. 1st Ed. has info and circuit snippets for nearly every type of amplifier stage I have heard of, and some I have not (gas tube coupled stages?). The only full mixer schematic is the one attached, which uses pentode input stages. There is also talk about the Langevin 5116 which is pentode/pentode. And of course MC-60 and other McIntosh pieces.

 

[jacomart]

Here it is! Manual and diagrams of the RCA console mod. BC-2B, complete with a program amplifier with a 6V6GT output stage driving a 600 ohm load but, given the valve used, certainly also much lower loads.

Cheer
JM

 

[ruffrecords]

That looks like a very good candidate for the output stage. Just need to identify the right transformer.

Cheers

Ian

 

[jacomart]

That looks like a very good candidate for the output stage. Just need to identify the right transformer.

Cheers

Ian

I seriously think @emrr could give us a hand

Cheers
JM

 

[emrr]

Same as the BA-3/13, but 6v6 instead of 6l6. You couldn’t pick a more expensive transformer approach. I’m all for it if anyone wants to spend the money. Output z is close to 100.

https://groupdiy.com/threads/variou...ress-and-experiments.41032/page-2#post-844805

That looks like a very good candidate for the output stage. Just need to identify the right transformer.

Cheers

Ian