Motown Direct Amplifier-inspired Preamp?

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Winston OBoogie

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" I was really curious about the simplicity of the meq" ... "I was wondering if the 150 acts as cathode resistor for v1a, and the 20k for the 6aq5, or whether the DVR for the output transformer primary is more important there."

Yes the 150 R is V1a's cathode R, and the 20K is predominately the 6AQ5's cathode R. DCR of the OPT has no role in the biasing.

But we also have DC feedback from the 6AQ5's cathode to V1a's cathode which will act as a partial "forced bias".

It's a nice symbiotic arrangement in my opinion :)


Haha! And no way, literally up the road from me, I actually work down by the canal (at least, pre lockdown, still remote at the moment) so I know the area quite well

Yep, up/down the road. A stone's throw.


doesn't necessarily have to be continuously variable but I think having a few set gain settings could be useful to allow for variations in input. It could still be 'set and forget' for the most part but you could still choose an alternative gain if you remember to kill your monitors or do it when the power is off.

If you put the +tve of a capacitor (220uF/6V3 is probably close enough for rock 'n' roll) at the junction of the 120R and 1500R of the MB-1's schematic, you could then use various value resistors from the cap's -tve to ground in order to determine the amount of bypassing that the 1500R has.
Unless I've lost the plot, the ratio of 120R and 1500R gives, approx. a 20dB boost between an unbypassed 1500R (cap -tve left open, or with some arbitrary high value) and a fully 220uF bypassed 1500R (cap -tve shorted to ground).
Then there'd be no nasty DC on your switch to cause cracks and pops.
 

untune

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Yes the 150 R is V1a's cathode R, and the 20K is predominately the 6AQ5's cathode R. DCR of the OPT has no role in the biasing.

But we also have DC feedback from the 6AQ5's cathode to V1a's cathode which will act as a partial "forced bias".

It's a nice symbiotic arrangement in my opinion :)
It's making sense now, like you said pages back there's elegance in the simplicity; And yes DCR, looks like my phone autocorrected to DVR and I didn't catch it!

Yep, up/down the road. A stone's throw.
It really is a small world, did some foley recording for a work project in those woods behind Mill Brow not long ago actually! I also remember needing a very last minute replacement tube for a preamp years ago, and a guy from Swinton on another forum met me outside the office with one :LOL:

If you put the +tve of a capacitor (220uF/6V3 is probably close enough for rock 'n' roll) at the junction of the 120R and 1500R of the MB-1's schematic, you could then use various value resistors from the cap's -tve to ground in order to determine the amount of bypassing that the 1500R has.

Unless I've lost the plot, the ratio of 120R and 1500R gives, approx. a 20dB boost between an unbypassed 1500R (cap -tve left open, or with some arbitrary high value) and a fully 220uF bypassed 1500R (cap -tve shorted to ground).
Then there'd be no nasty DC on your switch to cause cracks and pops.
That's good to know, thanks; it seems that little spot at the V1a cathode has a lot of potential for different implementations, given the relative simplicity of it, it's probably best to experiment with the different options when building to find what sounds best. The MEQ approach is simple and fits as long as the gain works, but as Ian says the MB-1 approach is probably a bit more flexible and will allow for a limited (but sufficient, I'd think) range of gain without upsetting the bias. I found this thread earlier which has quite a comprehensive explanation of what is happening in that little network on the MB-1, useful reference for anyone who is looking for info on it

Hi untune

John Windt is on Linkedin. Windt Audio. If you send me Email [email protected] I will sent it to John.
Thanks Duke, never thought to look on LinkedIn, while since I last logged in on there! One of our members here has the schematic we were originally curious about and has given some helpful guidance, there's some sensitivity regarding the details so I decided a few weeks ago it was perhaps best not to pester John; regardless—I would think this kind of community might appeal to his interest so feel free to point him in this direction if you think he might want to join.

The Motown Museum did get back to me after a while too and confirmed that unfortunately they don't keep records of technical documents, but they wished us luck with the project :)
 

Winston OBoogie

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I found this thread earlier which has quite a comprehensive explanation of what is happening in that little network on the MB-1, useful reference for anyone who is looking for info on it


Thanks, PRR and Jonte explain it well. I'm certainly glad that my eyeball guesstimate yesterday of approx. 20dB gain increase by bypassing the 1500R isn't too far off PRR's properly calculated figure of 22dB (from gain of 11 to 145).
I guess being back in Swinton hasn't driven me completely insane then! ;)

Cheers.
 

untune

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Thanks, PRR and Jonte explain it well. I'm certainly glad that my eyeball guesstimate yesterday of approx. 20dB gain increase by bypassing the 1500R isn't too far off PRR's properly calculated figure of 22dB (from gain of 11 to 145).
You were right on the money there! :D

I guess being back in Swinton hasn't driven me completely insane then! ;)
Ha, give it time :LOL:

I'm going to update the schem later with the updated bootstrapped cathode/grid a la the MB-1 plus DC blocking cap;

I've not paid much mind to the output transformer yet—It has been suggested that due to the minimal design of the Pultec, the component choices make more of a significant difference, the original output transformer being an important contributor to the MB-1 "flavour" (if indeed the MB-1 even has a "flavour" to speak of)

In the UK the Sowter 1290 might be the closest 'vintage' style available to the HS-50. Not sure if there are any decent 15K:600 alternatives aside from the Edcors etc, as the HS-50 will be practically impossible to get hold of (in this country) I'd expect. Or there's the option of finding someone who can custom wind to original spec, CJ posted some teardown docs of the original, but that'd probably be mucho dinero
 

untune

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4:1 should be fine. I was not aware we needed a 5:1. Is that what the MB-1 uses?

Cheers

Ian
Hi Ian, the MEQ-5 (and Pultec variations of that same circuit) plus the MB-1 all appear to use the Triad HS-50, here is CJ's teardown for reference

One of the MB-1 schematics has a Peerless S-449 but I've never found any info on it. Might be a bad scan and it's actually a 448, but the spec doesn't seem to match so it could well have been a custom one
 

Winston OBoogie

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So from C.J.'s teardown, the Triad HS-50 core is high nickel, flux saturation about the same as Supermalloy-80N.

So a 4:1 or 5:1 nickel from any of the usual suspects will be fine. Since it's fed from a fairly low source impedance with a decent amount of current, the difference in ratio won't matter, other than there being 2dB extra gain and tube amp headroom* with a 4:1. You may need to adjust the value of the 2.0uF output cap, erring on the going bigger side of things with a 4:1.

*Edit: technically, I suppose you won't get this extra amp headroom if your 4:1 is loaded with 600 ohms on the secondary since to looks like peak current from the follower is about 8mA, but for modern higher secondary loads, you would. It'll all come out in the wash anyway I'm sure :)
 
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untune

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So from C.J.'s teardown, the Triad HS-50 core is high nickel, flux saturation about the same as Supermalloy-80N.

So a 4:1 or 5:1 nickel from any of the usual suspects will be fine. Since it's fed from a fairly low source impedance with a decent amount of current, the difference in ratio won't matter, other than there being 2dB extra gain and tube amp headroom* with a 4:1. You may need to adjust the value of the 2.0uF output cap, erring on the going bigger side of things with a 4:1.

*Edit: technically, I suppose you won't get this extra amp headroom if your 4:1 is loaded with 600 ohms on the secondary since to looks like peak current from the follower is about 8mA, but for modern higher secondary loads, you would. It'll all come out in the wash anyway I'm sure :)
Makes sense that Acme went with the Cinemag for theirs, 50/50 nickel/steel and 4:1

I guess the differences would be negligible at the end of the day. Carnhill do a 15K:600 actually, never noticed it before, it's not in their design guide—they're always woefully short on details as it is so who knows how it might sound. I picked up in another thread that Sowter's 1290 has significantly lower primary DCR compared to the Triad/UTC it stands in for, i.e. the numbers don't align (and presumably neither does the construction) but people who have used it in their LA2As aren't complaining so perhaps it's moot anyway. Both the Carnhill 9K6:600 and 15K:600 options could be had for less than the cost of a single Sowter either way!

Edcors are popular with these build I believe, there was a thread on here with tips on the MB-1 from someone who had built quite a few and had experimented, annoyingly I have lost it (will post if I find it) but I did save his wisdom in another document :LOL: see below:

1. the edcor 15k/600 ohms is no good for MB1. they dont have the mojo of a proper made for the purpose trafo. the edcor lacks the bass response. they roll off significant below 50hz within the MB1 topology. a proper made trafo will perform right down to 10 hz. right up past 40 khz.

these MB1 have a terrific bass response with a good trafo as was designed by pultec way back then. the trafo design requires at least 3 layers (half prim- whole sec - half prim) and wound around the best grain orientated jap steel.

2. use wirewound resitors in the cathode circuits. 1k5. 1k8. and 18K. this helps the noise to stay way down and makes for fat forever stability.

3. use hi stab 1 watt for plate resistors. (non essential actually) . but for peace of mind only costing a couple of extra bucks.

4. use the best most expensive .047 coupler you can get. i use french solen ones. they are small of size and dont leak. use the biggest non leaky 2.2 uF trafo signal ground feeder cap - go bigger if u want but wont make any difference.

5. get the HT up to that 340 - 350v level. the amp will work and sound good with much lower HT (250 - 300) .but it really fires and sounds real HOT with the higher potential.

6. follow circuit detail and apply the dc (potential only - no current of course) to the heater circuit. should measure around 50 v or so. this not only makes right the cathode - heater spec for the 12ax7 - but also significantly cancels/negates any heater -cathode leakage current. it will completely cancel that type of induced hum.

7. use a 6AQ5 or 6BQ5 pentode as V2. in place of the parallel coupled 12AU7. change the feeder plate resistor to 220K (frig with that value to play with the pentode's bias) . that tube will make a nice difference to the sound of the amp. be careful tho. those tubes require twice as much heater current. the amp will sound HOT.

8. when u take out the nfb by grounding the first cathode with a large electro in series with a pot. wired around that 1k5 cathode resistor - 220uF - 2k2. the MB1 pretty much behaves like any other non nfb amp tho. the gain roars up , but not so much as to be destructive. depends on the level u putting in.
 

Winston OBoogie

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Yep. Lots of the Sowter transformers aren't actually copies of what they're supposed to be "recreating", some are even wrong ratios. Whether this is relevant depends on the results or implementation I suppose.

The Carnhills are decent transformers for what they cost, especially for us in the UK/Europe without high shipping/import cost etc.. They're an all steel core though so different flavor to the HS-50 but again, depends on the results you want or you're looking for.
 

ruffrecords

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Hi Ian, the MEQ-5 (and Pultec variations of that same circuit) plus the MB-1 all appear to use the Triad HS-50, here is CJ's teardown for reference

One of the MB-1 schematics has a Peerless S-449 but I've never found any info on it. Might be a bad scan and it's actually a 448, but the spec doesn't seem to match so it could well have been a custom one
The HS-50 is an excellent transformer with a 5:1 ratio so I guess that is what we should aim for to be truly compatible. We are basically looking for a 15K:600 transformer. There should be plenty of choices both ancient and modern.

Cheers

Ian
 

untune

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Yep. Lots of the Sowter transformers aren't actually copies of what they're supposed to be "recreating", some are even wrong ratios. Whether this is relevant depends on the results or implementation I suppose.

The Carnhills are decent transformers for what they cost, especially for us in the UK/Europe without high shipping/import cost etc.. They're an all steel core though so different flavor to the HS-50 but again, depends on the results you want or you're looking for.
I had no complaints about the VTB2291 I used last time; of course I've no other to compare it against either :D I think that with outputs like these, 4:1 and 5:1, they're flexible enough to be used in a multitude of other projects so it wouldn't necessarily be a waste to get a few to try out

The HS-50 is an excellent transformer with a 5:1 ratio so I guess that is what we should aim for to be truly compatible. We are basically looking for a 15K:600 transformer. There should be plenty of choices both ancient and modern.

Cheers

Ian
Going off a 1963 UTC catalogue and the info I originally dug up, I expect the original was probably an A-24 (or possibly HA-113 or LS-50?) so the nearest modern option is the Sowter 1290 or the slightly bigger but unshielded 1010 for AC coupled cathode follower outputs. The Carnhill VTB2380, very little info on that though, perhaps Colin at AML will know more on those.

These ones are a relatively new addition but the cost of importing from the States might be a bit prohibitive—same goes for sourcing any vintage Triads etc. I doubt too many made it to Europe and any that do crop up will be a small fortune I'm sure!
 

Uk03878v2

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. The Carnhill VTB2380, very little info on that though, perhaps Colin at AML will know more on those.
From a post (not by me) 9 years ago
“Colin said it is the same format electrically and physically as VTB1847, so I wired it in->yellow,brown, out->blue,violet and joining orange+red and green+black.”
 

untune

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From a post (not by me) 9 years ago
“Colin said it is the same format electrically and physically as VTB1847, so I wired it in->yellow,brown, out->blue,violet and joining orange+red and green+black.”
Cheers for that info, good to know!

Did a little more investigating into the Tab-Funkenwerk reproductions and it sounds like they are likely the most accurate/authentic non-vintage offering, given that they dismantled originals and constructed to meet the same spec.

Shipping to the UK though, ouch... looking at a total cost of £144-£150, that's without considering any additional fees etc... I've never really done any importing from the US so I don't know if there's any tax/duty on top.

The Sowter equivalent is £115 before shipping though, so perhaps not that much more!
 

untune

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Forgot to post this sooner, mostly a rearranged blend of the Pultec variations. *Think* that input is right, unless I've misinterpreted and it needs a 1M+ prior to the input cap, also can't remember where I plucked the 820K value from at this point, last few weeks are a blur :LOL: HT dropping resistors are unchanged for now until I can figure that stuff out, original had a 2K prior to the first cap but I thought it'd be ok to drop that since the original had a much higher HT to start with thanks to the 600V power transformer. Also uncertain if that's the best place to elevate the heater voltage... I did an alternative with a different power transformer but left it off for now, just used the Hammond to work from. Changes/suggestions/additions welcome!
 

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solkatten

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VTB2291 is a solid performer. I recently tried a VTB2425 but it was much more prone to noise pick up, so I switch back to 2291. I have not tried the VTB2380.
 

untune

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I recently tried a VTB2425 but it was much more prone to noise pick up
That is good to know; I didn't see that they did that one (actually for an MEQ-5) until a couple of days ago but it sounds like it'd be better suited to a build that you can space out or shield a bit. Any notable sound difference between the two in your project?
 

solkatten

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I tried the VTB2425 with tertiary feedback to cathode of the push pull stage. It was cleaner than the VTB2291 without the tertiary feedback, but it had the noise problem. Sound wise they are simular... VTB2291 is well documented trafo. Primary inductance ca 100H.
 

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