Motown Direct Amplifier-inspired Preamp?

[jacomart]

This is an interesting thread, Regarding guitar pickups output levels they were not so different from today production from Gibson or Fender or other main brands.
Here a table but I think that Google can do tons of nformations

Cheers
JM

 

[Winston OBoogie]

Ian, it'll typically be about another 10dB on top of what a -10dBv signal would need. So around 22dB for Hi-Z input to +4dBu output should cover it. Add a bit for good measure maybe.
Plus whatever else you might want "in hand" to get some 'dial-able' clipping happening in the 2nd or whichever stage.

1 X 12AX7 + 1 X 12AU7 + 1 X 6V6 (as used in the ACME) certainly gives us enough electrons on hand to configure things for that

 

[ruffrecords]

The info I have is that is uses 2 x 12AX7 which seems far too much for me.

Cheers

Ian

 

[Winston OBoogie]

If that's the case then, yes!, far more than needed.

 

[emrr]

The big question in my mind right now is just how much gain do you need to get a typical 1960s electric guitar output up to 0dBm?

Passive humbucker '74 Tele bass is hitting steady 0dBm on my console with 12dB gain, peaks 6dB hotter.

What crazy thing would you do with two 12AX7? See phase split input on RCA BA-25A, go PP, then drop a side for SE 6V6 cathode output? Nah.....that's crazy!

 

[jacomart]

Considering the possible use of vintage single coil pickups I think a maximum gain of about 40 dB is correct

 

[abbey road d enfer]

It depends very much on how it's played and on the instrument..
I measured 4Vpp from a Les Paul bashed to death. In that case you need attenuation...
But most of the times the signal is between 50mV and 500mV.
Bass usually has higher average level, by about 6dB.

 

[ruffrecords]

Considering the possible use of vintage single coil pickups I think a maximum gain of about 40 dB is correct

In an earlier post the OP indicated the gain required was about 34dB (-30dBu input raised to +4dBu). That is well within the capability of a single 12AX7. However, with possibly a 4:1 output transformer you would need another 12dB bringing the total up to 44dB. But then again -30dBu is only 24mV which seems rather sensitive to me.

The other question is just how faithfull to the original topology do we need to be? Apparently the original used lots of NFB so its not tube tone we are looking for but more like transparency.

Cheers

ian

 

[jacomart]

In an earlier post the OP indicated the gain required was about 34dB (-30dBu input raised to +4dBu). That is well within the capability of a single 12AX7. However, with possibly a 4:1 output transformer you would need another 12dB bringing the total up to 44dB. But then again -30dBu is only 24mV which seems rather sensitive to me.

The other question is just how faithfull to the original topology do we need to be? Apparently the original used lots of NFB so its not tube tone we are looking for but more like transparency.

Cheers

ian

24mv is a reasonable value for a single coil pickup with volume not at maximum, with Motown we are mainly talking about funky music and r & b, many single coils and few humbuckers where the clean tone is a must as well as a lot of feedback.

Cheers
JM

 

[1000tinyempires]

Even with a ton of NFB it's hard to imagine 4 triode gain stages off of a guitar pickup not being just too much gain. My understanding is that the Motown device was a mixer, 5 in and one out. If that's the case odds are good that the VU meters are seeing cathode followers before the mixing and output stage. So 2 triodes of gain, one triode as cathode follower, one triode mixer and then the 6V6 output. But I only get to that topology because I am trying to eat up triodes to match the Acme tube compliment. This topology would keep the NFB to each channel rather than global. Seems like a very complicated way to pet this particular cat.

 

[jacomart]

In fact we are talking about the gain of the signal path and not the configuration of the whole device. It seems quite logical that it is a mixer and among other things, given the size of the cabinet, it seems strange to me that it is made with only two 12ax7s and one 6v6. If I lived near Detroit I would pay a visit to the Motown Museum Studio A to get a closer look and ask a few questions.

Cheers
JM

 

[pucho812]

for some more info on Motown check this out from John wind who was in their tech department

https://www.youtube.com/watch?v=RdCsig1b4G4&t=3255s

 

[abbey road d enfer]

This brings an interesting POV. So each channel has its own 600 ohm +4dBu output with gain control. I would think it takes a dual triode per channel.
Then the signals are mixed for sending to the McIntosh amp. I would think it takes two stages (maybe the 6V6 included). So that's a minimum of 6 or 7 tubes. Do we have proof of the existence of a 6V6 in the Motown guitar box? the plot thickens...

 

[ruffrecords]

This brings an interesting POV. So each channel has its own 600 ohm +4dBu output with gain control. I would think it takes a dual triode per channel.
Then the signals are mixed for sending to the McIntosh amp. I would think it takes two stages (maybe the 6V6 included). So that's a minimum of 6 or 7 tubes. Do we have proof of the existence of a 6V6 in the Motown guitar box? the plot thickens...

That is the way I read the info we have so far. The guitarists set the guitar volume controls for 0VU on their respective meters. They then used the front panel controls to set the mix that fed the Mackintosh amp they were listening to. So I concur there are 5 separate +4dBu outputs and a five input mixer feeding the Macintosh. There would be no point in having a 6V6 drive a Mackintosh so the only sensible alternative would be on each of the five 0VU outputs. Is there really five 6V6s in the original?

Cheers

Ian

 

[abbey road d enfer]

The problem is the picture is blurred by ACME presenting a new product as a re-creation of a vintage design.
There is simply not enough concrete info on this. Even the descriptions by McLean and Folcrum are subject to caution.

 

[jacomart]

For mixing purposes the guitar / bass levels cannot all be the same so probably 5 independent outputs were routed to the mixer and one, mixed, went to the monitoring amp? So maybe there was only one 6v6? But is it right that in monitoring all the guitars have the same level? I don't think a rhythm guitar should have the same monitoring level as a solo or a bass, it would just do chaos. The reality is that we have little information but very well confused and making inferences is likely to lead us very astray.

Cheers
JM

 

[abbey road d enfer]

For mixing purposes the guitar / bass levels cannot all be the same so probably 5 independent outputs were routed to the mixer and one, mixed, went to the monitoring amp? So maybe there was only one 6v6? But is it right that in monitoring all the guitars have the same level? I don't think a rhythm guitar should have the same monitoring level as a solo or a bass, it would just do chaos. The reality is that we have little information but very well confused and making inferences is likely to lead us very astray.

Cheers
JM

The link in Pucho's last posts explains a lot of things. The inputs had a hidden preset trimmer, set by on eof the techs. There was an input dedicated to "acoustic" guitar and one to bass, but in addition it seems each player had its designated input with their designated guitar and role.

 

[jacomart]

unfortunately at the moment I only have an iPad and I don't see neither the Pucho’s link nor your attachments (and I don't understand why)

Cheers
JM

 

[Winston OBoogie]

1000tinyempire's post (#15 in this thread) showing the Pultec MEQ line amplifier seems a decent starting point to me.
As drawn, with a Triad HS-50 output transformer, gain from the 1st 12AX7 grid to the 600 secondary winding is about 28dB. Using a Cinemag 4:1 instead gets you a couple more dB @ 30dB total. Circuit has some 30dB negative feedback.

Seems hard to justify needing another 12AX7 but, who knows?

 

[cyrano]

FWIW
Pucho's link doesn't work for me either. It resolves to "mown", which obviously isn't a URL.

That's on a Mac, using Brave (Chrome based).