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Marantz 7 Clone

thefragger

thefragger

Certified Crazy.
Hot on the heels of my RH84 amp, and partially in preparation for my Gainclone build, sparked by this thread by Sam Cogley (I hope I'm not stepping on your toes!), I want to replicate the core of the Marantz 7 preamp.

I found this;

marantz7c_style.jpg


Then I checked my stock of iron, and found this;

IMG_6323_sm.jpg


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AND it will fit perfectly into a recycled chassis I wanted to use for it, an old 2.4kW light dimmer...

IMG_6327_sm.jpg


​


:D

Step 1, design the PSU!

I think the big green potted transformer might be a bit much for this preamp (I've calculated it's 500VCT secondary current at 500mA, where the schematic calls for 30mA) but there are some 120 to 240 step-up transformers on eBay that I should be able to source cheaply. Should I go for the gusto with the big one, or buy two small ones?

:banana:
 
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Save that TX for a BIG stereo amp. You need something like this little potted Chicago.:yes: Hmmm:scratch2:
 
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jaymanaa said:
Save that TX for a BIG stereo amp. You need something like this little potted Chicago.:yes: Hmmm:scratch2:
Click to expand...

Hmmm indeed... I need to buy my way back into barter town! I might have to keep some extra eyes out for stuff like that... thanks, Jay :)

That's a sweet looking piece of iron, I've got to say... :drool:

I wish the local Hamfest was more than once/year... I scored a lot of iron for my $20. The other power TX I've got is 800VCT :sigh:
 
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No toe stepping here! Don't forget that you're only looking at one channel in that schematic.
 
Very cool. Just remember to de-rate your parts as much as possible. Use larger wattage resistors than actually required and higher voltage caps, for instance. That's another secret of the classic tube Marantz equipment.

That huge transformer is a little overkill, but with plenty of chassis real estate it would definitely be something to talk about!

I wonder what would happen if you used tube rectification or selenium rectifiers in your project. I'm sure the sound quality would be head and shoulders above your typical "Marantz 7C" clone.

Of the currently made stuff, I like Shindo construction a lot because it's all point to point wired. Hard to find nudies of Shindo gear on the web, but I've seen a few pics and liked what I saw. Great to attempt a DIY project along those lines...
 
I was thinking about posting the tube rectification question in my thread on the subject. I don't feel like messing around with selenium - new ones seem to be horribly expensive, and I have a spare 5U4 laying around.
 
FWIW, preamps are the only place I really use tube rectifiers. I think if I built it I would probably go tube, but if SS, I would use fancy diodes (0 recovery cree).
 
I've got some MUR860's kicking around from a bulk buy for my Gainclone, but tube rectification sounds like fun!
 
What would be the best tube to go with, I cant find any info on that M4C1
Would alterin the supply degrade anything?
 
I think I've found myself a suitable PTX--from a curbed Electrohome console--with a 6.3V secondary and a 600V CT'd secondary (300-0-300).

I gotta see what rectifier tubes I've got kicking around, although I do love the 6CA4.

Furthermore I've been eying Russian tubes as of late; think I could get away with using 6N2P-EV tubes in place of the 12AX7 tubes? (heater wiring changes)


:scratch2:




I shouldn't be thinking about all this, what with finals just around the corner...
 
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Although I always get hot and bothered by a monster power transformer, in this case I think I'd bite the bullet and find something more suitable in terms of voltage and current draw.

Dropping that much current and voltage will cause more problems than it will solve, create a lot of heat in the process and choke current delivery.

I've got a heavy--and I do mean HEAVY--duty military potted power transformer with a 6.3V, 2.1 amp tap, a 5V, 2 amp tap, a 435v, 50MA tap and a 335v, 125 amp tap.

Drop me a PM if you're interested--you pay shipping/packaging and it's yours.

Best,

John
 
golana said:
Add it up, playin'with the boys means a little math.
Click to expand...

I would if I knew what I was adding up :no: I mean, adding up the heaters is easy stuff-- here's there current draw at this voltage, bingo bango.

For the operating current, I think I have to use the graphs in the datasheets, find the operating point, and draw some lines (I've done it once, about a year ago) to find the current consumption.


heekma said:
Although I always get hot and bothered by a monster power transformer, in this case I think I'd bite the bullet and find something more suitable in terms of voltage and current draw.

Dropping that much current and voltage will cause more problems than it will solve, create a lot of heat in the process and choke current delivery.

I've got a heavy--and I do mean HEAVY--duty military potted power transformer with a 6.3V, 2.1 amp tap, a 5V, 2 amp tap, a 435v, 50MA tap and a 335v, 125 amp tap.

Drop me a PM if you're interested--you pay shipping/packaging and it's yours.

Best,

John
Click to expand...

John! I've been following your threads-- you've got some most-excellent projects!

Actually I solved the over-sized PTX problem in post 12-- this 'new' one I've got is a bit smaller than my clenched fist (3" x 4" x 3")... however your offer is tempting (can't really ever have too much iron... ). :scratch2:
 
Heya fragger!

No problem on the power transformer--always eager to help a fellow member and DIYer out if I can. :thmbsp:

Here's a great data sheet link for the 12ax7:


http://pdf1.alldatasheet.com/datasheet-pdf/view/121279/GEC/12AX7.html

At typical class A operation, with 250v on the plates, looks like 1.2ma current draw. Max plate dissipation is 1 watt with 300v on the plate, about 33ma.

As a rough starting point for determining if your transformer is up the the task, you've got four sections drawing between 1.2 and 1.5 ma. I'd guess maybe the sections running at 285v might draw 1.5-2ma?

So maybe figure on 8-10ma current draw for high voltage as a rough estimate...but you'll still need to know the grid voltages for each section being used to get absolute current figures.

Best,

John
 
Reviewing the schematic, it says right by the traffo;

260-0-260
30mA
Click to expand...

So it looks like 30mA for the HV wining, and for the filaments;

17V 12A
Click to expand...

The current rating seems kinda high on the filament winding, but I'll thumb through the datasheet and see what I can add up.

*edit* Alright, assuming the voltage is stepped down to 12.6 (only six heaters present, meaning none are halved) that's a draw of 0.150A each tube, or 0.900A for all six. This leads me to believe that the current capacity of the 17V secondary is 1.2A */edit*

Another question; subbing in 12AU7 tubes for the 12AX7 tubes would make the pre "quieter" (less gain), but I don't think I have to change any values of the passive components, do I?
 
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Picked up an old '60s-era lab chronometer that looks like it was stripped of its nixie tubes. It has a 300-0-300 traffo in it; potted and easily fits in the palm of my hand (but I'm a big guy... )

Time to tack on more parts for the latest digikey order! 1watt resistors all around?
 
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