Motown Direct Amplifier-inspired Preamp?

[ruffrecords]

To know exactly we need to connect a guitar with single coil pickups (lower level than humbuckers) to a sound card and measure the signal level using various positions of the volume potentiometer and different energy in the picking. REW should give us the answer. We must not forget that the impedance of the input must be about 500k ohm (I checked) 1 Mohm is too much.

Cheers
JM

Did you just volunteer to do that??

Cheers

Ian

 

[abbey road d enfer]

Mohm is too much.

Really? So Fender, Ampeg and Marshall are wrong?

 

[ruffrecords]

No problem Jaco.
2M2 is my "go to" for a DI input, that's why I mentioned that, but anything you like can go there of course.

I would not trust a pot at the input to act as a grid leak as well - just asking for scratchy noise. Instead use 2M2 across the grid then100nF to 470K pot.

We could also upgrade the PSU a bit. Up the 40uF smoothing to 220uF say. I notice the basic HT is 350V - these guys knew that more HT means less distortion - which suggests a 265V transformer secondary. HT drops from 375V to 350V over 2K which suggests HT current is just 12.5mA.

Edit: I would also suggest we keep the 6AQ5. Quisescent cannot be much more than 10mA so dissipation should not be an issue and it is still basically a 6V6 in a smaller bottle.

Cheers

Ian

 

[untune]

Did you just volunteer to do that??

I don't have a working guitar here but I do have a couple of basses, one is a Precision with a traditional split coil P pickup, the other has a custom made humbucker with a coil tap so won't really be much good for a 'standard' measurement. No scope here but if you can point me towards any ideal software (or if a DMM is enough) I can get some measurements from the Precision

Edit: I would also suggest we keep the 6AQ5. Quisescent cannot be much more than 10mA so dissipation should not be an issue and it is still basically a 6V6 in a smaller bottle.

Sounds good, out of interest are the two similar enough that the circuit wouldn't need too much modification to go with one or the other? Just curious as I've never used any octal valves, and might give a 6V6 a go

 

[jacomart]

Really? So Fender, Ampeg and Marshall are wrong?

No, they are absolutely not wrong but their amplifiers have a tone control that in the “flat” position is not really flat but tends to compensate the pickups natural midrange resonance. Since this DI has no tone control, a lower input impedance decreases the Q value, therefore tends to flatten the response and facilitate equalization.

EDIT: Otherwise we could add a "fixed" tone control like Fender did in the Champion 600 to compensate for this resonance?


Cheers
JM

 

[Stevie342000]

Agreed, and I'm struggling to find anything with a date. I have two versions of the MEQ5 schematic, the one posted earlier in this thread, another probably earlier version with a tube rectifier and minor component value differences (18K in the NFB rather than 20K, a couple of 20uF filter caps as opposed to 40uF) plus different power TX, nothing major

Can't remember where I found this but I *think* CJ might have posted it in another thread

I would think some point between 1951 and not later than 1962, most of the Pultec range was a complete by the late 50's. A quick google and I found that the earliest date I can find so far for the MEQ5 is 1962 pultec – Preservation Sound
Harveys ad about half way down.
Pultec were never good as far as I can recall in putting in a date on the schematic, it is not always there.

There stereo SP3 Panner is mentioned in the same article my best guess at a date for the SP3 and MEQ5 is about 1955 to coincide with 3 track Ampex tape machines as the 4th channel is listed as spare on the circuit diagram I have.

 

[1000tinyempires]

So, I actually built a pair of MB1's with some modifications, like a 10 turn pot for the gain control and, here's the fun part, a DI input. Frankly, they're great as instrument pres, perfect amount of gain. Not saying this was what Motown had, but, a 12ax7 and a 6V6 cathode follower is enough gain to drive a 10k/150ohm transformer with a vintage guitar. I used an old Heathkit donor.

 

[btown2009]

So, I actually built a pair of MB1's with some modifications, like a 10 turn pot for the gain control and, here's the fun part, a DI input. Frankly, they're great as instrument pres, perfect amount of gain. Not saying this was what Motown had, but, a 12ax7 and a 6V6 cathode follower is enough gain to drive a 10k/150ohm transformer with a vintage guitar. I used an old Heathkit donor.View attachment 81251

Plenty close enough

 

[abbey road d enfer]

No, they are absolutely not wrong but their amplifiers have a tone control that in the “flat” position is not really flat but tends to compensate the pickups natural midrange resonance.

That is true for most Fender (not all) and Marshall amps, but not Ampegs.

Since this DI has no tone control, a lower input impedance decreases the Q value, therefore tends to flatten the response and facilitate equalization.

A lower Z just kills treble (3-5k), not the 500-800Hz squawk.

Anyway, it would be silly to expect a DI going through a mixer and a HiFi speaker to sound like a guitar amp. The electric guitar sound is an acquired taste, shaped by distortion-laden amps and very imperfect speakers.

 

[jacomart]

This is because the Ampegs (and some Fenders) are bass amps. Bass extension ranges from 41Hz to 392Hz ((fundamental freqs) and it doesn't have the same problem as the guitar (guitar ranges from 82 to about 1175 Hz and total range is about up to 6kHz considering harmonics). I don't think anyone here is so dull as to have thought a DI sounded like a guitar amp otherwise we would have talked about Fender Twins or J800s or similar stuff schematics. In any case mine was a proposal, everyone can choose the input impedance they prefer as long as democracy exists.

Cheers
JM

 

[ruffrecords]

Sounds good, out of interest are the two similar enough that the circuit wouldn't need too much modification to go with one or the other? Just curious as I've never used any octal valves, and might give a 6V6 a go

They should be pretty much identical apart from the bases and dissipation. I see no need to change any circuit values.

Cheers

Ian

 

[ruffrecords]

So, I actually built a pair of MB1's with some modifications, like a 10 turn pot for the gain control and, here's the fun part, a DI input. Frankly, they're great as instrument pres, perfect amount of gain. Not saying this was what Motown had, but, a 12ax7 and a 6V6 cathode follower is enough gain to drive a 10k/150ohm transformer with a vintage guitar. I used an old Heathkit donor.

Do you remember which mains transformer you used?

Cheers

Ian

 

[Winston OBoogie]

One size doesn't have to fit all. 2M2, 1M, or whatever suits you as a first grid value.
If Jaco likes lower, Jaco isn't wrong. If I like 2M2, I'm not necessarily wrong either. Use what works for you.**
I like 499K a lot of the time for bass DI so...

No need at all to stick to original power supply schemes. I'd feel masochistic using AC on the heaters for instance, but if it's easier for some, or they prefer it for some reason (can't think of one myself but...)
And as Ian suggested, upping your B+ cap values for filtering can only be a good thing.

Your MB-1 mic amp/instrument amp looks cool @1000tinyempires

** Abbey is correct in that a lower value will kill/roll off high end.

 

[jacomart]

At post # 129 of this thread I also suggested 1Mohm, then I thought that maybe a little less would be preferable, to dump that resonant peak at the cost of a little treble attenuation, but it could also be a good idea to switch between two or three different input impedances to be able to change from a twangy sound to a softer one and vice versa. If I'm not mistaken there is a Line6 stomp that does it, now there is little to invent!

Cheers
JM

 

[1000tinyempires]

Do you remember which mains transformer you used?

Cheers

Ian

The Heathkit ID 22 transformer that came in the donor unit. I think I kept the power supply fairly close to original for filtering.

 

[Winston OBoogie]

...a good idea to switch between two or three different input impedances to be able to change from a twangy sound to a softer one and vice versa. If I'm not mistaken there is a Line6 stomp that does it, now there is little to invent!

Yep. I don't see there being any big issue with switching noise if a guitar's plugged in the unit. Have your highest impedance be the default grid leak, and then a toggle with centre off position could parallel one of two other resistors with that to get a couple of lower impedance options.

If there isn't a guitar plugged in, maybe have the 1/4" jack be a shorting type which goes to ground, or even 10K - 15K (close to typical guitar impedance) if you want to check noise of the device itself without guitar or bass cable noise & such in the loop.

Lots of options.

 

[untune]

They should be pretty much identical apart from the bases and dissipation. I see no need to change any circuit values.

Cheers

Ian

Cheers Ian I thought as much; after more thought last night I agree sticking with the 6AQ5, stands to reason that designing for the lower dissipation/plate voltage means the two should be interchangable if people so wish. After a bit of reading the 6AQ5 seems to be regarded well enough in terms of sound, and there seems to be a plentiful supply of NOS for relatively cheap (for now at least!)

At post # 129 of this thread I also suggested 1Mohm, then I thought that maybe a little less would be preferable, to dump that resonant peak at the cost of a little treble attenuation, but it could also be a good idea to switch between two or three different input impedances to be able to change from a twangy sound to a softer one and vice versa. If I'm not mistaken there is a Line6 stomp that does it, now there is little to invent!

Cheers
JM

Was going to say it makes sense to have a few toggle switching options but I think Winston beat me to it!

So, I actually built a pair of MB1's with some modifications, like a 10 turn pot for the gain control and, here's the fun part, a DI input. Frankly, they're great as instrument pres, perfect amount of gain. Not saying this was what Motown had, but, a 12ax7 and a 6V6 cathode follower is enough gain to drive a 10k/150ohm transformer with a vintage guitar. I used an old Heathkit donor.

Another great looking project there—is the VU buffered or just strapped across the output?

 

[mjrippe]

Another great looking project there—is the VU buffered or just strapped across the output?

Same question about the Motown - how is the meter implemented? I would say from the info we have that it is strapped across the output of the preamp. As most of you probably know, this can cause a significant increase in distortion. In the Altec 1567 (I know, not the design here) there is a meter switch. Turning it ON brings distortion up to nearly 1%!

 

[Winston OBoogie]

Go for broke and shove another 6V6 as a follower in there to drive the meter, peg that sucker!




'tis a mere jest of course.

 

[btown2009]

Meter is across the output.