Motown Direct Amplifier-inspired Preamp?

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olhsson

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That’s certainly how it’s done now. This is 8 track recording rather than direct to stereo so there’s still a final vocal overdub and mixing stage to follow.
Live vocals were SOP for most major label records well into the early '70s. Most took tape generation loss very seriously and wanted a master tape that was a live mix off the floor. What we were doing was radical only we didn't realize it.
 

abbey road d enfer

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The organist was the only person isolated.
If I understand correctly, guitars, bass and drums were in the pit, only the organ was in iso. That makes a lot of sense. Very little spill from guitars into the drums, no spill from drums into the guitars cause DI'ed, and the organ on its own.
And headphones for the drummer (or studio speaker?) and organ player, and a speaker for the guitar players (aforementioned A7?)
 

untune

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Do you remember which mains transformer you used?

Cheers

Ian
@ruffrecords Just to roll back a few pages, I dug around and found that the power transformer for that Heathkit ID-22 (assuming you hadn't already found it!) was a 480V@60mA CT secondary, 6.3V@3.2A for the heaters. The Hammond 369JX from the first post is 500V@69mA with 6.3V@2.5A. Should be more than adequate I'd imagine, if the 6AQ5 wants 250V max? No idea what kind of rectification the original Motown might have used, the MEQ5 had both tube and SS depending on the schematic, can't imagine it would have a huge difference on the sound but I'm no expert!

There were no PA systems beyond a few vocal mikes! You had to play soft enough to hear each other. That way of playing was most of the vintage sound people mistakenly think tubes created.
It really was 'all in the fingers' after all... but we can't be having that kind of talk around here :LOL:;)
 

emrr

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There were no PA systems beyond a few vocal mikes! You had to play soft enough to hear each other. That way of playing was most of the vintage sound people mistakenly think tubes created.

I saw The Sonics first reunion gig (2007?), most members hadn't played a gig since about 1970. They were in the same hotel so we got to talk the next morning. They were freaked out by the stage wedges, they'd never experienced monitors.
 

ruffrecords

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So, bottom line seems to be we need maybe a PCB with the MEQ 12AX7/6AQ5 based amplifier and a power supply on it. To keep options open, the mains and output transformers could be external as could be the "preset rotary switch" and the in and through jacks and the VU meter.

Cheers

Ian
 

untune

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So, bottom line seems to be we need maybe a PCB with the MEQ 12AX7/6AQ5 based amplifier and a power supply on it. To keep options open, the mains and output transformers could be external as could be the "preset rotary switch" and the in and through jacks and the VU meter.

I think the 12AX7>6AQ5 CF is the right way to go, 15K:600 output tx. Going back to what btown said a few pages back, "the unit is 3 stages, input pre, then another pre /output tube driver, then the last stage is the transformer driving tube" is there need of an additional pre stage in front with a preset rotary/gain control or will the single 12ax7 be enough?

Also just curious what the 10pF cap from grid to plate on the 2nd half of the 12ax7 is for in the MEQ5, it's not in the MB-1 despite them being almost identical otherwise. Is it there to quell oscillation, or is it a feedback/frequency shaping thing?
 

ruffrecords

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It is possible that in the original another stage was needed just to buffer the guitar so its signal could then be split to the DI and also to the internal mixer pots that feed the Mackintosh amp. It might have been as simple as a cathode follower. From what others have said there should be more than enough gain available for almost any guitar with just the MEQ circuit.

Cheers

Ian
 

untune

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It is possible that in the original another stage was needed just to buffer the guitar so its signal could then be split to the DI and also to the internal mixer pots that feed the Mackintosh amp. It might have been as simple as a cathode follower. From what others have said there should be more than enough gain available for almost any guitar with just the MEQ circuit.

Cheers

Ian
Thanks Ian, my guess was that gain would be sufficient. Slow progress (thanks to the day job) but getting there—369jx mains transformer is pretty much identical to what you would find in an LA-2A so I've been studying that, and opting for the full wave rectifier that's shared between the two. Not sure of any of these old designs that use a choke, wondering if a CLC filter might be worth looking into or if it'd be overkill for this type of circuit?
 

ruffrecords

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Thanks Ian, my guess was that gain would be sufficient. Slow progress (thanks to the day job) but getting there—369jx mains transformer is pretty much identical to what you would find in an LA-2A so I've been studying that, and opting for the full wave rectifier that's shared between the two. Not sure of any of these old designs that use a choke, wondering if a CLC filter might be worth looking into or if it'd be overkill for this type of circuit?
The 369jx is probably overkill for this project but you are right, we do need to think about suitable mains transformers. We only need 0.75A at 6.3V and probably 30mA HT winding.

Back in the day, high value, high voltage electrolytics were not available to the designers so CLC designs were common. Even so there was still a lot of ripple left on the HT (by today's standards). Probably a good reason for using a cathode follower output stage because it adds a bit of PSRR. And to further reduce hum the 12AX7 stage has further decoupling of its HT. These days 450V electrolytics are common and cheap because they are used in computer SMPS so we can use a couple or three of those in a simple low cost CRCRC network and obtain very low ripple HT. The HT current is very modest so we could even use a low cost DIL rectifier bridge.

Cheers

Ian
 

untune

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Hammonds are relatively easy to acquire which is a bonus; the 300 series are tapped to work with 120-240 on the primary, I think the 200 series are 120V only but otherwise identical. Many appear to have additional heater windings geared at valve rectifiers, of the ones that don't are the aforementioned 369JX, nearest neighbour is the 370AX which is 480 VAC CT @ 58mA, 6.3V @ 2.5A. VVT do a 500VAC @ 50mA and 6.3V @ 1.5A but it'll be UK only. There's no notable cost difference either, they all sit around the £70 mark. The Hammonds do look quite snazzy in all black too :LOL:

High Voltage (Plate) & Filament - 39 VA to 940 VA (300 Series)

VTH14670-1440 - HT Transformer 250-0-250v 50mA (500v 30mA) 6.3ct 1.5A

Back in the day, high value, high voltage electrolytics were not available to the designers so CLC designs were common. Even so there was still a lot of ripple left on the HT (by today's standards). Probably a good reason for using a cathode follower output stage because it adds a bit of PSRR. And to further reduce hum the 12AX7 stage has further decoupling of its HT. These days 450V electrolytics are common and cheap because they are used in computer SMPS so we can use a couple or three of those in a simple low cost CRCRC network and obtain very low ripple HT. The HT current is very modest so we could even use a low cost DIL rectifier bridge.

Of course, I'm thinking of the vintage approach :D CLC might be the more traditional/'authentic' approach but plenty more options with bigger electrolytics. There are dual 100uf+100uF 500V cans that would keep things tidy if space was at a premium, but easy enough to go bigger and lower the ripple
 

StevieG

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Live vocals were SOP for most major label records well into the early '70s. Most took tape generation loss very seriously and wanted a master tape that was a live mix off the floor. What we were doing was radical only we didn't realize it.
Agree entirely, but I was referring to the use of different mixes for control room and musicians. If you’re not recording direct to stereo, it’s not essential to constantly monitor the stereo mix in the control room?
 

olhsson

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There was only one mono monitor speaker in the control room! We had a headphone mixer, but most other musicians felt they gave better performances without headphones. The main monitoring concern was that nobody made any important mistakes that required another take or at least a portion to splice in.
 

untune

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There are plenty of other mains transformer alternatives. Carnhill do a nice toroid that could be perfect for this project:

240V/6.3V toroid power transformer – Amp Maker: Guitar amp kits and parts

Universal mains input, 6.3VAC at 1A and 250V at 30mA

And only £33

Cheers

Ian

Of course, again I was just looking at the stuff that might have been close to 'vintage' but as you say, probably a bit over spec for this application (although it looks like Pultec liked to overshoot a fair bit, IIRC the MB-1 draws <15mA from a tx rated at 600V/50mA :LOL: )

Looks like a good option that, much cheaper, efficient; I was working out to about 350V HT after rectification and this as stated gives us about 320V so not much of a drop and plenty to work with (y) can heater voltage be elevated without a CT or will a virtual one work?

I never thought to look at Carnhill actually; for some reason I had it in my head that Carnhill/Sowter didn't do mains transformers, at least not cheaply :unsure:
 

ruffrecords

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Heater voltage can easily be elevated without a centre tap.

The Don Audio GSSL transformer at about £40 would also do the job. Its 9V secondary can be used to make a dc heater supply for those who prefer it.

An alternative is to use a small SMPS for the heaters. Then there afre lots of small low cost HT only transformers to choose from which are available at Mouser/Digikey.

I just checked the 6V6 datasheet and its heater is also 0.45A just like the 6AQ5. So from the power supply point of view there is no disadvantage in using the 6V6 but it does give you the option of raising the CF current if you want to.

It is not a complicated circuit. Point to point would be perfectly feasible for DIY

Cheers

Ian
 
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